Of all the triumphs of man's ingenuity for which this age is so remarkable, none, perhaps, has conducted more to the wellbeing and happiness of the human race than the art of Steam Navigation. This is apparent when we consider the vastly extended means of communication which we now enjoy with the most distant parts of the globe; the fresh impulse given to commercial undertakings by the rapidity, the safety, and the certainty of steamships, as compared with sailing-vessels; and the increasing spread of civilization and Christianity attendant upon our intercourse with distant and semi-barbarous nations. It is not wonderful, therefore, that the merit of having invented an art so pregnant with interest to mankind should be claimed for many different individuals; and we accordingly find the names of ingenious men of all countries associated, more or less, with its origin.
The use of paddle-wheels, propelled by manual or animal power, dates back to a very remote period, these having been employed by the ancient Egyptians, the Romans, and other nations of antiquity, for propelling their war-galleys; but it is doubtful whether any advantage was thus obtained in economy of labour, as compared with the use of oars. It is not, therefore, until the gradual development of the steam-engine, and its introduction as a motive power, that we can hope to find any advancement in the long-sought-for art of navigating ships against adverse winds and currents. The first historical notice we meet with of such a combination—a steam-engine placed on board a vessel to act as the propelling agent—occurs in the year 1543, when we are informed that Blasco de Garay, a sea-captain in the service of Charles V. of Spain, succeeded in propelling a ship of 200 tons burthen in the harbour of Barcelona, at the rate of a league (or three miles) an hour. No information is afforded us of the nature of his apparatus, except that it comprehended a boiler, which, it is stated, was liable to burst; that the power was transmitted through paddle-wheels, and that the vessel could be turned with much facility by means of the apparatus. We can only speculate as to the nature of this mysterious engine, but it seems probable that it owed its efficacy to the reaction of a jet of high-pressure steam, on the same principle as that famous classical toy, the Æolipile of Hero, invented B.C. 120. Notwithstanding that the scheme was commended by the emperor and his ministry, and its author promoted, we do not read of any second experiment being made, or of any further notice being taken of the invention. We may assume, therefore, that in this case the propelling power was found to be insufficient and unsatisfactory, and the experiment was worthless in its result.
In the year 1630, David Ramsey, "page of the king's bed-chamber," obtained a patent "To make boats, ships, and barges goe against the wind and tyde," but we do not hear of any experiments having been made by him. The patent office contains records of various similar suggestions made between the years 1630 and 1681, but nothing of any practical value appears to have been effected. At the latter date the ingenious Dr Papin, a Frenchman, described a method of propelling a vessel by steam. The only engine then known, however, being itself so crude and imperfect, the doctor experienced so much difficulty in reducing his scheme to practice, that it is believed no actual trial of it ever took place. His principal difficulty lay in obtaining the required rotatory motion from the reciprocating one of the piston, for which purpose he proposed to employ two cylinders, the piston of one of which should be ascending, while that of the other should be descending, the continuous rotative motion being obtained by means of racks attached to the extremities of the piston-rods, working alternately into a pinion on the paddle shaft. Although Dr Papin's schemes can only be viewed in the light of theoretical suggestions, he still deserves much credit both for his idea of the atmospheric engine, and for his proposal to employ it for working the paddles of a boat. Savery, on the other hand (who published his Miner's Friend in the year 1698), although a great actual improver of the steam-engine, and famous in his day as a clever mechanician, appears to have doubted the applicability of his engine to the propulsion of ships, since he only alludes cursorily to the possibility of such a thing.
In the year 1705 Newcomen, having adopted Papin's suggestions of the cylinder and piston, and Savery's method of condensation, first completed the atmospheric engine, and made it capable of becoming, in practical hands, an efficient propelling power; and it is worthy of remark, that even at the present day, we have several excellent paddle-wheel steamers which are most satisfactorily propelled by modern atmospheric engines, constructed by the late Mr Seward. The great engineering difficulty at this period was how to convert the reciprocating motion of the piston into the rotary motion of the shaft; for although, to our eyes, the crank may appear a very simple and almost self-evident expedient for this purpose, it was not till long afterwards that we find it introduced.
In the year 1730, Dr John Allen proposed to propel a vessel by the re-action of a jet of water forcibly expelled from the stern—a scheme which has been repeatedly revived since his time, and which has recently been attended with a considerable amount of success in the hands of Mr Ruthven of Leith. Six years after Dr Allen's proposal, Jonathan Hulls obtained a patent for his "Invention of a machine for carrying ships and vessels out of or into any harbour or river against wind and tide, or in a calm." His idea of a steam-boat was as follows; and however we may now be inclined to smile at his rude mechanism, in comparison with the beautiful machinery of a steamship of our own times, Jonathan Hulls undoubtedly deserves much credit for his ingenuity. In his boat two paddle-wheels were suspended in a frame projecting from the stern. In the body of the boat were two steam cylinders, whose pistons acted on the atmospheric principle; that is to say, they were impelled in one direction only, by the pressure of the atmosphere acting against a vacuum. To each piston one end of a rope was fastened; the rope was then carried round a grooved wheel or pulley on the corresponding paddle-wheel, the other end of the rope being allowed to hang free, with a weight attached to it. When one of the pistons descended in its cylinder by the pressure of the atmosphere, it pulled its rope, and consequently moved round the paddle-wheel in a degree due to the length of the stroke and the diameter of the pulley. While the piston was ascending in the cylinder, on the re-admission of the steam, the counter-balance weight at the end of the rope dragged the pulley round in the contrary direction; but the pulley being attached to the paddle-wheel by
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1 The historical portion of this article is indebted to Professor B. Woodcroft's Sketch of the Origin and Progress of Steam Navigation for some valuable facts. Steam Navigation.
It was so arranged that the paddle-wheel remained stationary during the retrograde motion of the pulley. There being two cylinders and two paddle-wheels in the boat, one would be in motion whilst the other was stationary, and thus a continuous progressive movement was given to the boat. It is uncertain whether this plan was ever put in practice.
We now arrive at the era of James Watt, whose inventive genius removed most of the obstacles which had hitherto prevented the steam-engine from being effectively employed for propelling vessels. His main improvement, after his invention of the separate condenser, was the substitution of the double-acting engine in place of the single-acting or atmospheric engine, by which means the power of an engine of given size and weight was at once doubled, while the motion was at the same time rendered more uniform. About this time also (1780) the crank and fly-wheel were first patented by James Pickard. Although Watt's improvements rapidly paved the way for the successful adaptation of the steam-engine to the purposes of navigation, we do not find that he himself devoted much attention at first to this subject, confining his views to perfecting the rotatory-engine, and increasing its economy. Accordingly, we find that it was not till after the expiry of their patent in 1800 that Boulton and Watt's engines were applied to this use.
In the year 1781, the Marquis de Jouffroy constructed a steamboat at Lyons of the following dimensions:—140 feet long, 15 feet beam, and 3½ feet draught of water. His experiments, which were made in the river Saone, were probably unsuccessful, as the subject was allowed to drop.
Leaving undescribed some abortive attempts of Ramsay and Fitch in America, and Serrati in Italy, which were attended with no practical result, we pass on to the first really successful attempts at steam navigation, which were made in 1788 by a Scottish gentleman, Patrick Miller of Dalswinton, in Dumfriesshire. Having previously experimented with boats propelled by the power of men and horses applied to paddle-wheels, he resolved to make the steam-engine do this work; but neither he nor Mr James Taylor, who resided in his family as tutor, and assisted him in his experiments, could devise a plan for applying the engine. In this dilemma Taylor suggested that they should call to their assistance an old schoolfellow of his, Mr William Symington, an engineer, at that time employed in endeavouring to adapt the steam-engine to wheeled carriages. Mr Miller accordingly saw Symington in Edinburgh, and, after examining the model of his locomotive carriage, was convinced of the perfect applicability of a similar engine to drive the paddle-wheels of a boat, and gave orders for one to be made under the direction of Symington and Taylor. This engine was accordingly made in Edinburgh, sent to Dalswinton, and put together by them in October 1788. The engine, in a strong oak frame, was placed on one side of a twin, or double pleasure-boat, on Dalswinton loch; the boiler was placed on the opposite side, and the paddle-wheels in the middle. In the same month of October the machine was put in motion, and the inventors had the gratification of witnessing the perfect success of their efforts. Although the cylinders of their engine were but 4 inches in diameter, this first steamboat attained a speed of 5 miles an hour on the waters of the lake.
Mr Miller, being now desirous of trying the experiment on a larger scale, commissioned Mr Symington to purchase one of the canal-boats employed on the Forth and Clyde Canal, and to have suitable engines constructed for her at Carron Ironworks. When this new machinery was ready, a trial took place on a straight reach of the canal of about 4 miles in length, on the 26th of December 1789, when the vessel moved at the rate of about 7 miles an hour.
Many other experiments followed with a similar result, and the following notice of them was sent by Lord Cullen to several of the Edinburgh newspapers:
"It is with great pleasure I inform you that the experiment which some time ago was made upon the great canal here by Mr Miller of Dalswinton, for ascertaining the powers of the steam-engine when applied to sailing, has lately been repeated with great success. Although these experiments have been repeated under a variety of disadvantages, and with a vessel built formerly for a different purpose, yet the experiment acquired was such as to show that, with vessels properly constructed, a velocity of 8, 9, or even 10 miles an hour may be easily accomplished, and the advantages of so great a velocity in rivers, straits, &c., and in cases of emergency, will be sufficiently evident, as there can be few winds, tides, or currents which can easily impede or resist it; and it must be evident that, even with slower motion, the utmost advantages must result to inland navigation."
Although these experiments were thus partially successful, and their value well understood and appreciated, we find that Mr Miller's boat was soon afterwards dismantled and laid up at Carron, and nothing further was at that time attempted. This apparent apathy can only be accounted for by the fact (which was afterwards acknowledged by Mr Miller himself), that Symington's machinery at this time was not equal to the task of propelling a boat with the degree of certainty and regularity necessary to insure commercial success. Hence, although the great principle of the possibility of steam navigation was thus apparently settled by Mr Miller's experiments in 1788 and 1789, it was not till the year 1801 that a really practical steamboat was first produced in Scotland. In this year Thomas Lord Dundas, who was well acquainted with Miller's experiments, and who was a large proprietor in the Forth and Clyde Canal, engaged Mr Symington to undertake a series of experiments on this subject, with the view of employing steamboats for towing on the canal in place of horses.
The result was the production of the Charlotte Dundas, named after his lordship's daughter, and which, from the lotte Dundas simplicity and practical nature of its machinery, may be justly considered as the "first practical steamboat." The superiority of this boat over its predecessors lay in Symington's more judicious arrangement of the machinery, which consisted of Watt's double-acting engine, working a connecting-rod and crank, which turned a single paddle-wheel, revolving in a well-hole near the stern of the vessel. This engine had one horizontal cylinder, 22 inches in diameter and 4 feet stroke. In March 1802 Lord Dundas, Mr Speirs of Elderslie, and several other gentlemen, being on board, the Charlotte Dundas "took in drag," says Mr Symington, "two loaded vessels, each upwards of 70 tons burden, and with great ease carried them through the Long Reach of the Forth and Clyde Canal to Port-Dundas, a distance of 19½ miles in six hours (being at the rate of 3½ miles per hour), although it blew so strong a gale right ahead that no other vessel on the canal that day attempted to move to windward." Notwithstanding this favourable result, the scheme was doomed a second time to disappointment, in consequence of some of the proprietors of the canal becoming alarmed at the destructive effects of the wash of the steamboat upon the banks. The boat was therefore laid up in a creek of the canal, where it remained an object of curiosity merely for several years. It may be remarked, that this production of Symington's possessed every necessary qualification which is considered requisite, even at the present day, to make a good and useful steamer; and in proof of the confidence it inspired in its own time, we may observe that the Duke of Bridgewater actually ordered eight steamers from Symington for use on the Bridgewater Canal, to be built on the model of the Charlotte Dundas. His grace dying, however, shortly afterwards, this order was never executed. We now arrive at the period when American enterprise stepped in to avail itself of the painful and laborious results of these costly experiments, which although made and perfected in this country, had not yet been turned to good account. About a year after Symington's experiment with the Charlotte Dundas, Fulton, the American engineer, made a similar though less successful experiment on the Seine, for the weight of his engine broke the vessel in two, and the whole went to the bottom. He persevered, however, and in August 1803 he completed another vessel with its machinery. This boat was 66 feet long and 8 feet wide, and moved so slowly that his experiment is described as having been a failure. He afterwards came to Scotland, and saw Symington's steamboat on the Forth and Clyde Canal; his visit being thus recorded by Mr Symington:
"When engaged in these experiments I was called upon by Mr Fulton, who told me he was lately from North America, and intended returning thither in a few months, but having heard of our steamboat operations, could not think of leaving this country without first waiting upon me, in expectation of seeing the boat, and procuring such information regarding it as I might be pleased to communicate, observing that, however advantageous such an invention might be to Great Britain, it would be still more valuable in America, where there were so many great navigable rivers. In compliance with his earnest request, therefore, I caused the engine-fire to be lighted up, and, in a short time thereafter, put the steamboat in motion, and carried him 4 miles west on the canal, returning again to the point from which he started, in one hour and twenty minutes (being at that rate about 6 miles an hour), to the great astonishment of Mr Fulton and several gentlemen, who, at its outset, chanced to come on board. During the trip Mr Fulton asked if I had any objection to his taking notes regarding the steamboat, to which I made no objection, as I considered the more publicity that was given to any discovery, intended for general good, so much the better; and, having the privilege secured by letters-patent, I was not afraid of his making any encroachment upon my right in the British dominions, though in the United States I was well aware I had no power of control. In consequence, he pulled out a memorandum-book, and, after putting several pointed questions respecting the general construction and effect of the machine, which I answered in a most explicit manner, he jotted down particularly every thing then described, with his own observations upon the boat during the trip."
Fulton having thus obtained what information he could, returned shortly afterwards to America, and, in conjunction with Mr Livingstone, obtained a patent securing to them the prospective advantages of steam navigation in America, by what they were pleased to call "their invention of steamboats." They very wisely got all their machinery from England; so that in the year 1807 the first steamboat in America was launched, and fitted with a pair of engines constructed by Boulton and Watt.
This vessel, called the Clermont, though probably fitted with superior machinery to that in Symington's boat, was barely as fast, making under five miles an hour. Her dimensions were 130 feet long, 16½ feet beam, and 7 feet deep; the boiler 20 feet long, 7 feet deep, and 8 feet broad; the steam-cylinder (one only) was 24 inches in diameter, and 4 feet stroke; burthen 160 tons. Her paddle shaft was of cast-iron, with no outer support beyond the sides of the ship. The diameter of the paddle-wheels was 15 feet, the boards being 4 feet long, and dipping two feet in the water. She was subsequently lengthened to the extent of 140 feet keel. In the beginning of the year 1808 the Clermont was placed for regular work on the Hudson River, between New York and Albany, a distance of 125 geographical miles, and was crowded with passengers, her speed after the alteration being at the rate of 5 miles an hour. This was, therefore, the first steamboat that ever ran continuously for the accommodation of passengers, and the first that ever remunerated her owners, and to this the Americans may justly lay claim; but that Fulton was the "inventor" of the present system of steam navigation, as asserted by some American authors, cannot be admitted; nor, indeed, did he "invent" any single improvement in the construction either of the machinery or the vessel. The success of their first steamer induced Messrs Fulton and Livingstone to build two other vessels, the Car of Neptune, of 300 tons, and the Paragon, of 350 tons, also supplied with Boulton and Watt's engines.
The first person who ever took a steamer to sea was also Stevens, an American, R. L. Stevens of Hoboken, who had been 1808, associated with Livingstone previously to the connection of the latter with Fulton, and had brought his experiments to a successful issue nearly as soon. As Fulton, however, had secured to himself the exclusive privilege of navigating by steam in the state of New York, Stevens boldly took his vessel round by sea from the Hudson to the Delaware. To him are due many of the present peculiarities of American steamers. He it was who first adopted the long stroke; the upright guides for the piston-rod; the beam overhead, raised on a high framework of wood, working above the deck; and the connecting-rod, descending thence to the paddle-shaft, all characteristic of American steamers to the present time. He also improved the form of the American boats, by substituting a fine entrance and run for the old bluff bow and stern, as well as by increasing their relative length to eight or ten times the beam. Stevens is believed to have been the first engineer who constructed a "tubular" boiler, though these did not come into general use till long after his time.
Although steam navigation had been thus early introduced on the American waters, it was not till the year 1812 that the first regular passenger-steamer made its appearance in this country on the Clyde. This was the Comet, built Comet, for Mr Henry Bell, the proprietor of the Helensburgh 1812, Baths on the Clyde, and who had long been a most zealous advocate of steam propulsion. This little vessel was 40 feet long on the keel, and 10 feet 6 inches beam, propelled by a steam-engine of three or four horse-power, with a vertical cylinder, and working on the bell-crank principle—the engine being placed on one side of the vessel, and the boiler (of wrought iron) on the other. The Comet made her first voyage in January 1812, and continued to ply regularly between Glasgow and Greenock, at a speed of about 5 miles an hour. She was propelled by two small paddle-wheels on each side, each wheel having four boards only. She was afterwards transferred to the Forth, where she ran for many years between the extremity of the Forth and Clyde Canal and Newhaven, near Edinburgh. The distance is 27 miles, which is stated by Mr Bell to have been performed, on the average, in 3½ hours, being at the rate of above 7½ miles an hour.
Mr Bell had on several occasions brought his projects for heavy steam navigation under the notice of the British government, but always without success; and it was not till the year 1819 that the admiralty of the day became impressed with the importance of steam-power for towing men-of-war, chiefly through the representations of Lord Melville and Sir George Cockburn. The first steam-vessel in the royal navy was then built, and was also named the Comet. She is still in existence, and measures 115 feet in length, 21 feet in breadth, and draws 9 feet water, being propelled by a pair of engines, by Boulton and Watt, of 40 horse-power each.
But to return to Mr Bell's steamers on the Clyde. The Comet was so successful, that two other steamers, of increased size and power, were constructed; and, in 1814, Mr Cook, of Glasgow, built a fourth, called the "Glasgow," which, in point of power and efficiency, became the standard at that time for river-steamers. The marine engines hitherto constructed had all been applied singly in the vessel; but in 1814 Messrs Boulton and Watt first applied two condensing engines, connected by cranks set at right-angles on the shaft, to propel a steamer on the Clyde. This was found to be a great improvement, and thenceforward almost all steamers have been fitted with two engines.
In the year 1815 a small vessel, with a side lever-engine of 14 horse-power, by Cook of Glasgow, made a voyage from Glasgow to Dublin, and thence round the Land's End to London. It then ran with passengers between London and Margate with some success, though encountering great opposition from the Thames watermen.
In 1818 Mr David Napier, to whom we owe the introduction of British coasting steamers, as well as of steam-packets for our post-office service, first established between Greenock and Belfast a regular steam communication by means of the Rob Roy, a vessel of about 90 tons burthen and 30 horse-power, built by Mr William Denny of Dumbarton. For two winters she plied with great regularity and success between these ports, and was afterwards transferred to the English Channel, to serve as a packet-boat between Dover and Calais. Soon after this Mr Napier had the Talbot built for him by Messrs Wood. She was 120 tons burthen; and when fitted with two of Mr Napier's engines, of 30 horse-power each, this vessel was in all respects the most perfect of her day. She was the first steamer that ran between Holyhead and Dublin. About the same time, also, he established the line of steamships between Liverpool, Greenock, and Glasgow, for which traffic he built the Robert Bruce, of 150 tons, with two engines, of 30 horse-power each; the Superb, of 240 tons, with two engines of 35 horse-power each; and the Eclipse, of 240 tons, with two engines of 30 horse-power each. All these were established as regular coasting traders before the year 1822.
In the latter year the steamer James Watt was built by Messrs Wood, to ply between Leith and London. She was the largest steamer that had yet been built, being 448 tons measurement, and fitted with two engines of 50 horse-power each, by Messrs Boulton and Watt. The Soho followed on the same line, and was equally successful. The next great advance made was in 1826, when the United Kingdom was constructed, this vessel having been regarded in her day with as much wonder and interest, from her (so-called) gigantic proportions, as were afterwards the Great Western, the Great Britain, and, more recently, the Great Eastern. The United Kingdom was 160 feet long, 26½ feet beam, and 200 horse-power; the ship being built by Mr Steele of Greenock, and the machinery by Mr David Napier. Prior to this time many improvements had been made in the arrangement and construction of the marine engine by Boulton and Watt, Maudslay and Field, Penn, and others of our eminent mechanical engineers; the expansive action of steam in the cylinder having already been taken advantage of by Messrs Maudslay and Field in their engines, which were also fitted with escape-valves on the cylinders, and other improvements.
The first steamer which crossed the Atlantic was the "Savannah," an American vessel, of 300 tons burthen, which arrived at Liverpool in the year 1819, direct from the United States, in 26 days, partly steaming and partly sailing. Being fitted with engines of small power, and the vessel being otherwise unsuited for ocean navigation, this must be regarded rather as a bold experiment (and not a very successful one) than as establishing the practicability of a rapid and regular steam communication between this country and America; for it is only in the combination of these two qualities that the steamship excels the sailing vessel. In 1829 the Curacoa, an English built vessel, of 350 tons and 100 horse-power, made several successful runs between Holland and the Dutch West Indies. Men of science, however, plainly demonstrated, to their own satisfaction, that the navigation of the Atlantic by steam-power alone was impracticable; and it was not till the Sirius and the Great Western had shown the fallacy of their reasoning, that the public mind was disabused of this idea. The Sirius was not built expressly for transatlantic navigation; she belonged to the St George Steam-Packet Company, and the Great Western had run with a good reputation between London and Cork. Her tonnage was about 700 tons, and her horse-power 320. She started from London on the morning of the 4th of April 1838, with 94 passengers. Though first in the race, she was only three days in advance; for on the 7th of the same month the Great Western, built and fitted at Bristol expressly for the purpose, followed her. The Sirius arrived at New York on the 22d, being 17 days clear on the passage, and the Great Western (sailing from Bristol) on the 23d, being 15 days. The Sirius again sailed on her homeward passage on the 1st of May, and the Great Western on the 7th of May, and they arrived, the first on the 18th, and the second on the 22d, being 16 and 13½ days respectively. The average speed of the Great Western on this voyage was thus 8·2 knots on her outward passage, and nearly 9 knots on her homeward, reckoning the distance at 3125½ knots for the one, and 3192 for the other. She consumed 655 tons of coal going out, having still 205 tons remaining in her coal-boxes upon her arrival at New York. Coming home her consumption was 392, having 178 tons remaining on her arrival at Bristol. Her average daily consumption varied from 27 tons, with expansive gear in action, to 32 tons without it. As the Great Western possesses considerable historical interest, some of her principal dimensions are here subjoined. She was designed and built by Mr Paterson of Bristol, and fitted with machinery by Messrs Maudslay, Sons, and Field of London. She is 212 feet long between the perpendiculars, 35 feet 6 inches beam, and 23 feet 3 inches depth of hold, drawing from 16 to 18 feet of water. Her tonnage is 1340 (builders' o.m.), and her engines (on the side-lever construction) are 440 horse-power. Her cylinders are 73½ inches in diameter, and 7 feet stroke, making 12 to 15 revolutions per minute. Her complete success was doubtless mainly attributable to the fact, that she was especially fortunate both in her designer and in her engineers, who are still, perhaps, the most eminent of the present day in their respective departments.
The practicability of transatlantic steam navigation being thus triumphantly established, the British Queen, the President, and other large steamships, were built in rapid succession, as well as many steam-vessels of war.
Up to this time the paddle-wheel was the only propelling agent employed; but in 1837 the rival system of propelling ships by means of the screw-propeller first came prominently introduced, into notice, through the successful experiments of Captain Ericsson and Mr F. P. Smith. Captain Ericsson's small vessel, of 45 feet in length, 8 feet beam, and but 2 feet 3 inches draught of water, towed the American ship Toronto, of 630 tons, on the Thames, on the 25th of May 1837, at the rate of 4½ knots an hour, against the tide, as authenticated by the pilot; and also towed the admiralty barge, with their lordships on board, from Somerset House to Blackwall and back, at the rate of about 10 miles an hour. Later in the same year Mr Smith made some very successful trips with his small boat and screw-propeller between Margate and Ramsgate. The next screw-vessel was the Robert Stockton, built in 1839 by Messrs Laird for an American gentleman, who had witnessed Captain Ericsson's experiments. This boat was also perfectly successful; but the Board of Admiralty still failed to recognise the peculiar applicability of this means of propulsion for vessels of war. The next year, however, in 1840, Mr F. P. Smith, having obtained the support of some influential mercantile men, brought out the Archimedes, a screw-vessel of 232 tons, The Archiburthen and 80 horse-power. The success of this vessel made so complete, that the Admiralty were at length induced 1840. to make a trial of the screw in the royal navy, and the Rattler was ordered to be built on the same lines as the Alecto paddle-wheel steamer, and to be fitted with engines of the same nominal power. The next screw-steamer worthy of notice was the Dove, an iron boat, constructed under Mr Smith's direction. Her speed, however, proved so unsatisfactory to her owners, that they ordered her to be changed into a paddle-wheel boat; and as it happened that she had been built with very fine after-lines, her constructor unfortunately charged her deficiency of speed to this circumstance, and adopted the theory that full stern-lines were the most advantageous for the action of the screw. The Rattler was now tried; and her trials having fully satisfied the Board of Admiralty, they ordered the construction of several screw-vessels, which were all built with full sterns. This idea having at length been proved, by further experiment, to be erroneous, and that, on the contrary, fine after-lines were absolutely required for the proper efficiency of the screw-propeller, so as to allow of a ready access and escape of the water, the whole of these vessels were deficient in speed, and some of them were altered at great cost.
The screw had meanwhile advanced rapidly into favour as an auxiliary power for fast sailing vessels in the merchant service; and more recently it has been extensively employed for full powered steamers of the very largest class (in preference to the paddle-wheel) by several of our great mail-packet companies, the Peninsular and Oriental Company taking the lead in this respect.
The requirements of the great navigable rivers of America have naturally led to the supremacy of that nation in the art of river navigation. The description of the large American river-steamer, The New World, given in another part of this article, as a type of her class, will be found both novel and interesting.
This rapid sketch of the rise and progress of steam navigation would not be complete without referring specially to the wonderful development it has lately received in the construction of the Great Eastern, whose giant bulk is even now struggling into life under peculiarly depressing circumstances. In addition to the interest naturally excited by the immense size of this vessel (whose proportions will be given hereafter), she is destined to solve another problem in marine engineering, namely, the desirability of combining screw-propeller and paddle-wheels in the same steamship.
A statistical table is subjoined, showing the progress of steam navigation in the British Empire, from its first introduction in 1814, down to the most recent times for which returns have been received.
Table showing the progress of Steam Navigation in the British Empire, by the Registrar-General of the Board of Trade.
| Year | Steamer | Reg. Tons. | Steamer | Reg. Tons. | Year | Steamer | Reg. Tons. | Steamer | Reg. Tons. | |------|---------|-----------|---------|-----------|------|---------|-----------|---------|-----------| | 1814 | 6 | 672 | 2 | 456 | 1836 | 69 | 9,700 | 600 | 67,999 | | 1815 | 10 | 1,394 | 10 | 1,633 | 1837 | 82 | 12,147 | 668 | 78,288 | | 1816 | 9 | 1,238 | 15 | 2,612 | 1838 | 87 | 9,857 | 722 | 82,716 | | 1817 | 10 | 2,054 | 19 | 3,950 | 1839 | 65 | 6,522 | 770 | 80,731 | | 1818 | 9 | 2,538 | 27 | 6,441 | 1840 | 77 | 10,639 | 824 | 90,860 | | 1819 | 4 | 342 | 32 | 6,637 | 1841 | 54 | 12,391 | 856 | 104,845 | | 1820 | 9 | 771 | 43 | 7,243 | 1842 | 67 | 14,203 | 906 | 115,320 | | 1821 | 23 | 3,266 | 69 | 10,534 | 1843 | 53 | 17,739 | 942 | 121,455 | | 1822 | 28 | 2,634 | 96 | 13,125 | 1844 | 73 | 6,930 | 988 | 125,675 | | 1823 | 20 | 2,521 | 111 | 11,553 | 1845 | 73 | 11,950 | 1012 | 131,202 | | 1824 | 17 | 2,234 | 126 | 15,739 | 1846 | 88 | 17,172 | 1070 | 144,784 | | 1825 | 29 | 4,192 | 168 | 20,297 | 1847 | 115 | 17,333 | 1154 | 146,557 | | 1826 | 76 | 942 | 248 | 22,958 | 1848 | 128 | 16,476 | 1253 | 158,078 | | 1827 | 30 | 3,784 | 275 | 32,490 | 1849 | 50 | 13,480 | 1296 | 167,310 | | 1828 | 31 | 2,285 | 293 | 32,632 | 1850 | 81 | 15,527 | 1350 | 187,631 | | 1829 | 16 | 1,761 | 304 | 32,283 | 1851 | 88 | 23,527 | 1386 | 204,654 | | 1830 | 19 | 2,228 | 315 | 33,444 | 1852 | 112 | 31,792 | 1414 | 229,616 | | 1831 | 36 | 4,430 | 447 | 37,445 | 1853 | 162 | 49,008 | 1534 | 264,336 | | 1832 | 38 | 4,090 | 380 | 41,669 | 1854 | 189 | 66,446 | 1708 | 326,462 | | 1833 | 36 | 3,95 | 415 | 45,017 | 1855 | 263 | 84,862 | 1910 | 408,290 | | 1834 | 39 | 5,756 | 462 | 50,735 | 1856 | 245 | 58,621 | 1950 | 417,717 | | 1835 | 88 | 11,281 | 538 | 60,520 | | | | | |
It should be observed that the "register" tonnage here given is exclusive of the tonnage of the engine-room, which in a well-powered steamer generally amounts to one-half, or more, of the registered tonnage. An addition of one-third should therefore be added for the gross tonnage of this table. To take an example. The Shannon, West India mail packet, has
Register tonnage........... 2187-24 Engine-room do............. 1284-57 Gross do.................... 3471-81
The horse-power (nominal) averages about one-third of the register tonnage; so it may be fairly assumed that this country now possesses a fleet of 2150 merchant-steamers, having an aggregate gross tonnage of 670,000 tons, and a nominal horse-power of 165,000 horses.
The steam navy of this country consists at present of about 468 vessels, having an aggregate tonnage of 470,000 tons, and a nominal horse-power of 110,000 horses.
| Affloat (1859) | Building or Covering | Total | |---------------|----------------------|-------| | Ships of the Line | Screw | Paddle | Screw | 16 | 49 | | Frigates | 19 | 9 | 6 | 34 | | Block Ships | 9 | | | 9 | | Mortar Ships | 4 | | | 4 | | Corvettes and Sloops | 38 | 35 | 9 | 82 | | Small Vessels | 3 | 24 | | 27 | | Gun Vessels | 25 | | | 25 | | Gunboats | 161 | | | 162| | Floating Batteries | 8 | | | 8 | | Tenders, &c. | 4 | 38 | | 42 | | Troops & Store Ships | 13 | 2 | | 15 | | Yachts | 1 | 4 | | 5 | | Total | 319 | 112 | 32 | 463|
Explanation of statistical table. In constructing a steam-vessel three things require to be specially considered, each of which is a sufficiently complex study in itself; namely, the ship, the engines and boilers, and the propeller. To combine these in such a manner as to produce a perfect whole is one of the most difficult problems of modern engineering, demanding at once the theoretical attainments of the natural philosopher, and the laboriously acquired knowledge and shrewd sagacity of the practical mechanician. As the limits of this article must preclude the pursuit of theoretical investigation, it is proposed to confine it almost exclusively to the practical part of the subject, and to a record of the results of actual and approved performance.
The marine steam-engine, although acting on the very same principles as the ordinary land condensing-engine, and provided with the same integral parts, differs from it essentially in the particulars of weight and form, being necessarily made as light, and as compact, as possible. These requirements throw many obstacles in the way of the marine engineer which are not encountered on shore, both as regards the engines and boilers; and his difficulties are increased by the stern necessity which exists on board ship for the utmost economy in the consumption of fuel, the value of which is there immensely enhanced.
The oldest type of the marine-engine is the side-lever variety, which, till within the last ten or twelve years, was almost universally employed in steamers; and which is, indeed, still preferred for paddle-wheel steamers, by at least two large mail-packet companies—viz.: Cunard's and the West India Mail Company (fig. 2). There are several good reasons why this form of engine should be thus favoured. In the first place, the working parts are well balanced on either side of the main centre (c, fig. 2), so that the engine will stand in any position without the piston having a tendency to fall by its own weight, thus enabling the crank to be kept in the most advantageous position for giving prompt motion to the shaft immediately that the engine is started. Direct-acting engines are often very troublesome in this respect. Another advantage of the parts being nicely balanced is, that the engine works with little friction, and consequently less strain, and tear and wear of the brasses and moving parts of the machinery. Hence the side-lever engine is very economical in maintenance and repairs, as well as in the quantity of oil and tallow required for lubrication, no mean item in the expenditure of some engines. Again, this form of engine admits of a good long stroke and connecting-rod, by which means the steam may be used to best advantage in the cylinder, while, at the same time, the thrust of the piston is transmitted to the crank in the most equable and effective manner. Many pairs of side-lever engines are still doing their work well, after more than twenty years' service, and have cost less for repairs than most direct-acting varieties in half that time.
But although it may be quite true that side-lever engines are thus economical in their working, it does not necessarily follow that they are the best form of engine for passenger-steamers. On the contrary, when we consider the value of space and weight in a first-class merchant-steamer, it appears probable that they are really more expensive Steam Navigation.
than the lighter and more compact forms of direct acting engines which have now so generally come into use, and by employing which we may save at least 20 feet in the length of the engine-room, and 100 tons of displacement, in engines of 500 nominal horse-power. Direct-acting engines, being susceptible of great variety of form, have assumed as many different shapes as there are manufacturing engineers ready to invent, adapt, or distort them, as the case may be; and the now very general use of the screw-propeller has, of course, varied and modified these forms still more. In the merchant service, the height of the machinery is not of much moment, provided only that it does not raise the centre of gravity of the vessel too high; but in the steam-navy it is considered essential that the whole of the engines and boilers should (if possible) be kept under the water-line of the ship, as a protection from shot; which, in the case of screw-vessels, is now generally accomplished (see fig. 15).
Direct-acting paddle-wheel engines may be classed under Varieties four heads; namely, those which preserve the parallelism of the piston-rod by means of the system of jointed rods called a parallel motion (figs. 3 and 4); those which use guides or sliding surfaces for this purpose (figs. 5 and 7); oscillating engines (fig. 6), in which the cylinders are hung upon pivots, and follow the oscillations of the crank; and those denominated "trunk-engines" (fig. 8), in which a hollow cylindrical trunk is attached to the piston, and passes, steam-tight, through the cylinder cover. Several of these varieties have their distinctive appellations, being known as the "steeple-engine" (which is a favourite form on the Clyde); Maudslay's double-cylinder engine (fig. 7); the annular-piston engine (with two piston-rods); the atmospheric engine; the combined-cylinder engine (Plates XXXIV. and XXXV.); and several others. As verbal description is of little value in making these forms intelligible to the reader, sketches of the more characteristic of them are subjoined, as well as detailed plates of approved examples (see Plates). These engines are generally made with two cylinders, but in the case of screw-engines there are sometimes three, and sometimes four cylinders. placed in all possible positions; being found upright, inverted, horizontal, and inclined.
Screw-engines are made either with or without gearing. The use of geared wheels intervening between the engine and the propeller admits of a slow speed of piston with a high velocity of screw, and is so far beneficial, but in practice there are several disadvantages attending it.
The driving-wheel is necessarily very large and cumbrous, while the wooden teeth with which it is fitted are subject to unequal wear, and are liable to be "stripped," or broken off, by a sudden stroke of the sea upon the screw. Their revolution is also attended with a loud rumbling noise, from which there is no escape on board-ship. In steam-vessels of war it is difficult to keep the top of the large wheel sufficiently low, while at the same time their draught of water admits of the use of a screw of great diameter and pitch, by which means the necessary speed may be obtained for the ship without unduly increasing the velocity of the piston. Hence there are comparatively few geared screw-engines in the Royal Navy. In the case of full-powered screw-engines in the merchant service, the use of gearing is generally found to be necessary (fig. 10), but it may be advantageously dispensed with wherever the power of the engines is not calculated to give a very high speed to the ship. The velocity of piston in actual use in different classes of steamers will be hereafter noted.
The following (Figs. 11 to 16) are other examples of Screw-engines:
Fig. 9.—Horizontal Screw-engine (Direct), adapted for the Royal Navy, &c.
Fig. 10.—Vertical Screw-engine (Geared), adapted for the Merchant Service.
Fig. 11.—"Trunk" Screw-engine, Direct.
Fig. 12.—Oscillating-Cylinder Screw-engine, Direct.
Fig. 13.—Double Piston-rod Screw-engine, Direct.
Fig. 14.—Horizontal Cylinder Screw-engine, Geared.
Fig. 15.—Screw-engines in the Royal Navy, Direct. The cylinder of the steam-engine, being that portion of the machine in which the power is developed, must be considered as its principal member. Upon its dimensions depend, in some degree, the size of all the other parts of the engine, as well as its reputed powers, being called an engine of 100 or of 200 horse-power according to the diameter of the cylinder, modified to a small extent by the length of the stroke. This, called the nominal horse-power, is obtained by the formula—
\[ \text{H.P.} = \frac{\text{Area of cylinder} \times \text{effective pressure} \times \text{speed of piston}}{33,000}. \]
In this formula the area of the cylinder is taken in square inches; the "effective pressure" is assumed at 7 lb. (by some makers at \( \frac{7}{4} \) lb.) to the square inch; and the speed of the piston (according to the arbitrary rule adopted by the admiralty), is presumed to vary with the length of stroke, as shown in the following table:
| Stroke | Speed of piston | Stroke | Speed of piston | |--------|----------------|--------|----------------| | Ft. In.| Ft. per min. | Ft. In.| Ft. per min. | | 3 | 180 | 6 | 221 | | 3 | 188 | 6 | 226 | | 4 | 196 | 7 | 231 | | 4 | 204 | 7 | 236 | | 5 | 210 | 8 | 240 | | 5 | 216 | 9 | 248 |
It is at once apparent that the power thus calculated cannot be the real power of the engine, since it is wholly irrespective of the pressure of steam in the boiler, the perfection of the vacuum in the condenser, the actual number of reciprocations of the piston, and the varying loss by friction depending upon good or bad workmanship, and the general plan of the engine. For the sake of convenience, however, the nominal horse-power is still retained, since it defines, with tolerable accuracy, the actual size of the engine, and its commercial value, in so far as the latter is dependent upon the dimensions of the cylinder. To remedy, in some degree, the uncertainty attending the use of this term, it is now becoming usual for the purchaser of a steam-engine to insert a clause in his contract, binding the manufacturer to show a certain specified amount of indicated horse-power.
The indicated horse-power of an engine is obtained by the aid of a valuable little instrument called an indicator, consisting mainly of a small cylinder placed in connection with the cylinder of the engine, both above and below the piston. This little cylinder is open at the top, and is fitted with a piston which presses against a spiral spring. The cock which connects the indicator with the cylinder of the engine being opened, steam is admitted under the piston of the indicator during the one stroke, and vacuum during the other, precisely as in the large cylinders; thus causing the little piston to push and pull alternately against the spiral spring. If the pressure were uniform throughout the stroke, the indicator-piston would start at once from top to bottom, and vice versa, remaining stationary until acted upon by the opposite pressure; but since the pressure within the cylinder of a steam-engine is constantly varying during every portion of the stroke, it follows that the pressure on the spiral spring of the indicator, and the corresponding movement of the indicator-piston, must be variable too. If a pencil be fixed to the piston-rod of the instrument, it will register the fluctuations of pressure upon a piece of paper held close to it; but unless some provision be made for allowing the pencil a clear space on the paper at each successive instant of time, it will only move up and down in the same vertical line, and the markings due to fluctuation of pressure will be indistinguishable. To obviate this, the paper receives a circular motion in one direction during the down-stroke of the piston, and a reversed circular motion during the return-stroke, the result being that, as the pencil moves vertically up and down, a continuous curved or sloping line is traced on the paper. By this line an oblong space is inclosed, called the indicator-figure, card, or diagram, the vertical ordinates of which will then represent the effective pressures at the corresponding portions of the stroke, and their mean length will therefore indicate the average pressure in the cylinder during the whole period of the stroke.
To find the indicated horse-power, therefore, we must take the area of the cylinder in square inches, multiply it by the average pressure as found from the indicator-figure, and again by the actual number of feet through which the piston is travelling per minute; when the product, divided by 33,000, is the indicator or gross horse-power of the Steam Navigation.
This must not be confounded, however, with the effective power of the engine, or that actually available for the purpose for which the engine is used. To obtain this, a considerable deduction (about 25 per cent., it is believed) must be made for the friction of the moving parts, and for the power required to work the valves, air-pump, feed and bilge pump, &c.; but as this would be nearly alike for all well-constructed engines of equal power, and no ready means exist for testing it, the gross or indicated horse-power is taken as the measure of the power in all ordinary cases.
Indicator-Diagrams taken from Screw-engines of 500 horse-power, by Ravenhill, Salteld, and Company.
(Steam in Boilers 20 lb.)
| Steam | 117 (13.5) | |-------|------------| | Vacuum | 9.2 (13.5) |
Top stroke... 20 ft. Bottom... (23 ft.)
Diameter of cylinder... 71 inches. Length of stroke... 3 feet. Indicated pressure (mean of 6 experiments)... 21,904 lb. Mean number of revolutions per minute... 63½.
\[ \frac{3559 \times 204 \times 381 \times 21,904}{33,000} = 1001,248 \text{ h.p.} \times 2 = 2002,496. \]
Speed of the vessel at a mean draught of 24 ft. 9½ in... 10¾ knots.
The size of the boiler is obviously a very important element in determining the indicated horse-power of an engine, inasmuch as the speed of the piston (or the number of revolutions per minute) depends mainly upon the supply of steam from the boiler. The power of an engine may thus always be increased by adding to the size of the boiler, provided the steam-passages are large enough to admit of the increased flow of steam without its becoming throttled or "wire-drawn." A large boiler, however, implies a large consumption of coal as a necessary attendant upon any increase of power in the engines, or velocity in the ship; so that in practice it is generally found inconvenient for seagoing steamers to urge their engines to the utmost duty of which they are capable, as tending to limit the distance which it is possible to run with a definite weight of coals. Hence it follows, that while vessels making short runs (such as the Holyhead packets) will show an indicated horse-power of four, or even five times their nominal, a transatlantic steamer cannot afford to do so, although her engines may be equally efficient.
It will be understood, from what has been already said, that the speed at which marine engines are driven is very various, and also that it is liable to vary (even in the same vessel) according to circumstances; such as the steaming capacity of the boilers, the necessity for economizing fuel, and the dimensions of the paddles or screw. Apart from the proper or "calculated" speed, there is of course the additional consideration of the variable trim of the vessel, and the undulations of the sea, which affect the speed of the engines by throwing more or less work upon them, in proportion as the propelling agent is deeply or lightly immersed. The subjoined tables will convey some idea of the velocity at which pistons are driven (under the most favourable circumstances of trim) by some of the principal marine engineers of the day:— ### Speed of Pistons in Merchant-Steamers (Paddle and Screw)
| Name of Vessel | Makers of the Machinery | Diameter of Cylinder | Length of Stroke | Revolutions per Minute | Speed of Pistons in ft. per min. | How propelled | |----------------|------------------------|----------------------|-----------------|-----------------------|----------------------------------|--------------| | Great Eastern | Watt | 84 | 4 | 50 | 400 | Screw, direct. | | Delta | Penn | 72 | 7 | 25 | 350 | Paddle, feathering floats. | | Great Eastern | Scott Russell | 74 | 14 | 12 | 336 | Do, common. | | Shannon | R. Napier | 97 | 14 | 18 | 326 | Do, " | | Mersey | Maudslay and Field | 60 | 5 | 30 | 300 | Do, " | | Ceylon | Humphrys | 72 | 3 | 50 | 300 | Screw, direct. | | Colombo | R. Napier | 72 | 5 | 6 | 264 | Do, " | | Atrato | Caird | 96 | 9 | 15 | 270 | Paddle, feathering. | | Pera | Rennie | 75 | 4 | 32 | 256 | Screw, geared. | | Oneida | Inglis | 82 | 4 | 25 | 234 | Do, " | | Tamar | Penn | 72 | 7 | 16½ | 231 | Paddle, feathering. | | Prince Consort | Scott Russell | 30 | 2 | 6 | 45 | Do, " |
### Speed of Pistons in Government Screw-Steamers
| Name of Vessel | Makers of the Machinery | Diameter of Cylinder | Length of Stroke | Revolutions per Minute | Speed of Pistons in ft. per min. | How propelled | |----------------|------------------------|----------------------|-----------------|-----------------------|----------------------------------|--------------| | Agamemnon | Penn | 70½ | 3 | 6 | 60 | Trunk, direct. | | Mohawk | Humphrys | 42½ | 2 | 2 | 88 | Horizontal, direct. | | Esk | Scott Russell | 50 | 2 | 9 | 68 | Oscillating, direct. | | Arrogant | Penn | 55 | 3 | 0 | 61 | Trunk, direct. | | Prince-Royal | Maudslay and Field | 64 | 3 | 0 | 58 | Horizontal, direct. | | Simoon | Watt | 43½ | 3 | 0 | 55 | Oscillating, direct. | | Conflict | Seaward | 46½ | 2 | 0 | 70 | Horizontal, direct. | | Duke of Wellington | R. Napier | 94 | 4 | 6 | 30 | Horizontal, geared. | | Highflyer | Maudslay and Field | 56½ | 2 | 6 | 53 | Horizontal, direct. | | Dauntless | R. Napier | 84 | 4 | 0 | 31 | Horizontal, geared. | | Fairy | Penn | 42 | 3 | 0 | 46 | Oscillating, geared. | | Sharpshooter | Miller and Ravenhill | 46 | 3 | 0 | 38 | Horizontal, geared. | | Riflemen | Ravenhill | 34 | 2 | 2 | 36 | Oscillating, geared. | | Rattler | Maudslay and Field | 40½ | 4 | 0 | 25½ | Double cylinder, geared. |
All the merchant-steamers in this table have a speed of above 13 knots, and the government steamers of 10 knots.
Some of these speeds are nearly twice as great as would be sanctioned by the table previously quoted as embodying the practice of James Watt; and although, theoretically speaking, there may be no objection to such high velocities, they are inconvenient in practice, from the tendency of the working parts to heat by the friction, from the rapid wear of the parts, and their increased liability to accident or derangement.
Although engineers are perfectly agreed as to the superior advantages of a long stroke for their engines, it will be seen by the preceding table how rarely in the case of screw-engines this desirable object can be accomplished. The cause of this is, that the pitch of the screw-propeller (by which term is implied the linear advance made by the screw during one complete revolution, supposing it to be working in a solid), cannot be effectively increased beyond a certain proportion, depending upon the diameter of the screw; and as this is necessarily limited by the draught of water, it follows that the only available means for augmenting the linear advance of the screw is by increasing the number of revolutions. For each revolution of the screw, two journeys of the piston (in a direct engine) are required, and to enable this to be done within the required time, the strokes must be short. The chief disadvantages attending a short stroke are the more frequent recurrence of the "dead points" of the crank (when the piston arrives at the top and bottom of the cylinder), at which times much of the momentum of the moving parts is destroyed; and the loss of a certain quantity of steam contained within the cylinder ports or passages at each stroke, which does not exert a direct pressure on the piston. It is natural to suppose, also, that short-stroke engines do not derive so much benefit from expanding in the cylinders as those having longer strokes.
Another desideratum for all kinds of steam-engines is a long connecting-rod, as tending to diminish the angular strain thrown upon the main crank, and thus avoid the loss Long and short stroke engines.
made apparent by the accompanying sketch (fig. 18), in which \(a b\) represents a long connecting-rod, and \(a' b'\) a short one, their relative efficiency varying as the angles \(a, b, c,\) and \(a', b', c'.\). The defects of a short connecting-rod become sensible in practice by greater liability of the bearings to heat, by an increased wear of "brasses" and packings, and a larger consumption of oil for lubricating.
The cylinder of a steam-engine is never allowed a full measure of steam from the boiler, this being shut off at some part of the stroke according to the power it may be desirable to exert. A certain quantity of steam, varying generally from \(\frac{1}{3}\) to \(\frac{1}{2}\) of the contents of the cylinder, is always excluded by the slide-valve, which is made to close the steam-port before the end of the stroke. In most engines a further amount of steam is excluded by means of a separate valve, called the expansion-valve, which is so arranged that it may "cut off" the steam, or prevent a further supply, at any desired point, according as it may be wished to economize fuel, more or less, at the expense of velocity. Thus, some engines are worked with \(\frac{1}{3}\) of a cylinder full of steam to each stroke, some with \(\frac{1}{2}\), and others with only \(\frac{1}{4}\); or the same engine may be worked successively at the different grades of expansion corresponding to these quantities. This is called "working expansively," because the portion of steam thus shut in continues to expand in volume, and to give out elastic force, to the end of the stroke. Two advantages arise from cutting off the steam in this way. Firstly, it allows the stroke to be completed under a diminished pressure, so that the piston comes gently to rest at the top and bottom of the cylinder, without imparting a destructive jar to the machinery; and, secondly, it is economical of power (or, which is the same thing, of fuel), since it is found that the force actually exerted upon the piston by the isolated steam, during its expansion into the increased volume as the piston descends in the cylinder, is considerably greater than that due to the simple pressure of the same weight of steam acting at a uniform density.
It is found by calculation that when the steam is cut off at \( \frac{1}{2} \) stroke, seven-tenths of the power already exerted in the cylinder is added by the subsequent expansion of the steam; when cut off at \( \frac{3}{4} \), \( 2\cdot1 \) times the power is added; and when cut off at \( \frac{1}{4} \), \( 2\cdot4 \) times nearly. According to the usually-received natural law regulating the pressure and elasticity of steam, it is assumed that the pressure is inversely proportional to the volume of the steam after it has expanded into the increased bulk, or, in other words, that when the steam has expanded to twice its original volume, its pressure will be reduced one-half; when it has expanded four times its volume, the pressure will be \( \frac{1}{4} \), and so on. The pumping-engines in Cornwall, which do their work so very economically, use steam of about 40 lb. pressure, cutting it off in the cylinder after \( \frac{1}{4} \)th or even \( \frac{1}{8} \)th part of the stroke has been made, the remaining \( \frac{1}{2} \)th being performed wholly by expansion.
It is very seldom, however, and that only when special means are provided for this purpose, that the principle of expansion can be beneficially carried out in marine engines to an extent nearly approaching that just mentioned. It is a well-known property of all gaseous fluids, steam of course included, that any expansion of volume is necessarily accompanied with the loss of sensible heat, which is taken up in the latent form by the expanded gas or vapour. Hence, when the steam expands under ordinary circumstances within the cylinder of a steam-engine, a portion of it is compelled to part with its latent heat, to enable the rest to retain the gaseous form. This portion of steam, therefore, condenses into water of the same temperature, which forms a thin film over the interior surface of the cylinder. When the return stroke begins, and the watery lining of the cylinder is brought into connection with the condenser, it rapidly evaporates into steam of low tension. This steam, besides vitiating the vacuum, acts still more injuriously by robbing the cylinder of the heat which it required for evaporation; when the metal of the cylinder, being thus lowered in temperature, condenses the steam, upon its re-admission, to a serious extent. Thus it happens that the principle of expansion, when carried out to any great extent in cylinders which are only "clothed" in the usual way, has so frequently failed to realize the expected economy of fuel; and this has been most unjustly charged to a defect in the principle of expansion.
In the case of the Cornish engines already mentioned, where the steam is expanded to eight times its volume with known advantage, the cylinder is invariably surrounded with a "jacket" kept well supplied with dense hot steam from the boiler, by which means it is retained at a high and nearly uniform temperature during the entire stroke; and to this steam-jacket it is mainly due that so remarkable an economy attends the use of expansion in Cornwall. The cylinders of a marine engine, on the other hand, are protected from radiation by a clothing of felt and wood only; but in the few instances where a steam-jacket has been applied, the most beneficial results have followed.
Another mode by which the expanded steam may be protected from condensation in the cylinder is by previously imparting to it an extra dose of heat beyond that due to its pressure, or, in other words, by "superheating" it. It is apparent that this extra heat becomes available for the supply of the latent heat demanded by the expanding steam, which is thus saved from premature condensation.
In order to derive the utmost benefit of which the principle of expansion is capable, it is necessary that the initial pressure of steam should be considerable, that it should have plenty of space to expand into, and that the cylinder of expansion should be maintained at a high temperature. These conditions would seem to imply the use of a large jacketed cylinder of sufficient strength to bear the high initial pressure. As such a cylinder, however, would be very heavy and cumbrous, the plan has been occasionally adopted of using two cylinders, in which to utilize the steam, namely, a large and a small one. In this case the high-pressed steam from the boiler is admitted into the small cylinder only, and after expanding in that to twice or three times its volume (by which its pressure is reduced to one-half, or one-third), it is then admitted from the small cylinder into the large one, where the expansive process is finally completed under the most favourable circumstances.
This combination, called the combined-cylinder engine, has of late been brought prominently forward by the engineering firms of cylinder Randolph Elder and Co., of Glasgow. Plates XXXV. and XXXVI. engines represent the engines of the steamers Callao, Lima, and Bogotá, made on this principle, and which have attracted much notice by their remarkable economy of fuel. Their principal dimensions will be afterwards given with the description of the plates. They are also described by Elder at the late meetings of the British Association. These engines are constructed with the view of getting the greatest amount of power from a given quantity of steam at a given pressure, with less total weight of engines, boilers, and water, and occupying less total space than is found in the ordinary class of steam-engines on board of steamships. To accomplish these objects the following construction of engine has been adopted—The cylinder capacity is so great as to admit of the steam being expanded to within 2 lb. of the pressure in the condenser at the end of the stroke, while the engines are working full power. In order to reduce the violent shock of steam at 42 lb. pressure on such a large piston, a cylinder with a piston one-third of the size is placed beside it. This small cylinder admits steam directly from the boiler during the middle of its stroke, after which it is cut off. This steam is consequently reduced to one-third of its original pressure, or to 14 lb., at the end of the stroke, and it then enters the second or larger cylinder. Here it is expanded three times more, or down to 4\( \frac{1}{2} \) lb. Thus, the steam at 42 lb. is expanded to 14 lb. in the first cylinder, at which pressure it enters the second cylinder, and is further expanded down to 4\( \frac{1}{2} \) lb.; but as the second piston has three times the area of the first, the load will be the same on both pistons, and the piston-rods, cross-heads, and connecting-rods may be the duplicates of each other. The steam is super-heated in the boilers to about 400°, and the cylinders are steam-jacketed and clothed with felt and wood. The feed-water is heated before entering the cylinders. It is stated that although the single-jacketed or super-heated steam, this was found inadequate to prevent condensation in the cylinders without the use of the steam-jackets; in addition, the indicator diagrams taken from these engines showing a marked increase of power resulting from a free use of the steam-jackets, the supply of steam to which may be modified at pleasure.
These vessels have all shown a minimum consumption of from 2\( \frac{1}{2} \) lb. of best Welsh coal per indicated horse-power per hour, of fuel in their speed being at the same time 12\( \frac{1}{2} \) to 13 knots, which must be Lima, Bogotá considered a very satisfactory result. Their consumption of coal gots, &c., at their usual working trim is about 3 lb. per indicated horse-power, the vessel making 11 knots; whereas the more usual consumption of modern marine engines varies from 4 to 5 lb. per indicated horse-power per hour, and the average consumption of all classes cannot be less than 6 lb.
It is not contended, however, that the system of expanding in two cylinders is essentially requisite towards the attainment of a great economy in the consumption of fuel, and there are many considered instances of single-cylinder engines in which the same beneficial results have followed a like judicious combination of means and appliances for this purpose. A case in point is supplied by the recent performances of the steamship Thunder, a vessel fitted with machinery of much the usual kind, by Messrs. Dudgeon of London. Although supplied with steam of only 14 lb. pressure, her engines do not consume more than 2\( \frac{1}{2} \) lb. of coal per indicated horse-power per hour, the vessel making 13 knots. Her machinery Steam Navigation.
It may be stated that a consumption of 2½ lb. of coal per indicated horse-power per hour would represent in Cornwall a "duty" of about 90,000,000 pounds raised 1 foot high in an hour by a bushel (or 94 lb.) of coal, which is considered economical working even for a Cornish engine. The success achieved in the case of the Thunder appears to be due to the conjunction of the following good qualities in the machinery—viz., a perfect command of steam in the boilers, the super-heater, and expansion of steam in "belted" or jacketed cylinders, and the allowance of an unusually large inlet for the steam by the main valves.
Another example of unusual economy in the consumption of fuel has been recently shown in the auxiliary screw-steamer Omeo, fitted with engines of 100 nominal horse-power, by Messrs Morrison of Newcastle. These engines use steam at 60 lb. pressure, which is expanded to a large extent in single cylinders, and afterwards condensed in the usual way, the cylinders being surrounded with steam-jackets. The engines work up to 425 indicated horse-power, while driving the ship at 9 knots, and burning only 2¼ lb. of coal per indicated horse-power per hour. As the use of high pressure steam necessarily implies larger boilers of corresponding strength, the Omeo's boilers are cylindrical with "coned" furnaces and upright "coned flues," fitted with a superheating chamber on the top. The use of this high tension, however, is not to be recommended for passenger-steamers.
The question of economy of fuel is of vital importance even in a national point of view, as affecting the maintenance and extension of some of our great postal lines of ocean steamers, and it is now receiving a large share of attention both from steamship owners and engineers. The subject naturally divides itself into two heads—the production of steam in the boiler, and its subsequent employment in the engine.
The boiler.—It is a material point towards economical working that the boiler should be large enough to ensure a constant command of steam without the necessity for "forcing" the fires, or constantly stirring them. This may be prejudicially in more ways than one; in the first place, each time that the fire-door is opened the cold air rushes in through it, and mixing with the hot gases in the furnace, checks their perfect combustion, at the same time that it robs the interior of the boiler of much valuable heat. Again, if the boiler be deficient in heating surface, the fires must be kept thin, to promote rapid combustion; and as these fires are specially liable to "burn into holes," a quantity of cold air enters the furnace through them, and the same cooling effect is produced in the flues and passages. It may be also remarked that, however desirable it may be to "burn smoke" by admitting air into the furnace above the bars, it is seldom an economical process, and if not managed with great caution, is apt to become very much the steam Navigation, the natural consequence of stirring the fire too much, is, that a large quantity of small coal and cinder falls through the bars into the ash-pit, and as the boilers cannot supply the constant demand for steam unless the fires are kept bright and active, these cinders cannot be re-burned, probably, for fear of checking the formation of steam. They are thrown overboard, therefore, with the ashes, and a heavy expense is incurred.
It may be thought by some persons that stoking is a mere mechanical operation, easily acquired by the commonest labourer; not a mere but this is a great and vital error, which generally costs steamship mechanical owners many thousand pounds before they find it out. The stokers, operation, in fact, may be wasting coals by the ton at the furnaces of the boilers for want of proper ventilation, while the engineer is straining his utmost nerves to save a few pounds weight by economizing steam in the engines, and possibly congratulating himself, at the same time, upon his able management. It is no unusual case for a difference of 20 per cent. in the consumption of fuel to arise simply from good or bad stoking, by which is meant the whole management of the fires and the draught. The quality of the coals is another important item in estimating the consumption per horse-power, and some remarks on this subject will be made hereafter. In large ships the mere labour of passing the coals along the com to the front of the fires is very severe, and some contrivance of fort of the slides or rails to enable the buckets to be easily run down the stokers firing stage is recommended. The stoker's duty is, at the best, a should most certainly be carried out in such a manner as to be consulted, which adds to their comfort and convenience, whether it be by reducing the heat of the stoke-place, supplying them with a tap of cold distilled water, &c., is amply repaid by the increased attention bestowed on the fires. A marked reduction in the heat of the stoke-place in many of her Majesty's ships has attended the use of the double smoke-box doors, shown in the subjoined sketch (fig. 20).
Ventilating Smoke box Door.
Diameter of holes, 1¾".
Use of saltnet water being necessarily used in the boilers of sea-going marine steamers, this is liable to become more and more saturated with boilers, Steam Na- salt and earthy impurities in proportion as the steam passes off to the engines. A twofold evil thus arises. The super-salted water, as it increases in density, demands more heat before it will part with its steam; and the insoluble ingredients it contains (chiefly the carbonates of lime and magnesia, and the sulphate of lime), gaining strength with the abstraction of the steam, are deposited inside the boiler, thus forming a non-conducting skin, which greatly impairs its efficacy, and subjects the plates to risk of injury from the fire. To remedy this, a certain portion of the water in the boiler is "blown off" into the sea, its place being supplied by the feed-pumps with a corresponding portion of the hot-water, which results from the condensation of the steam by a jet of sea-water. As the temperature of the "feed," however, does not exceed 100°, while that of the brine it replaces is probably about 230°, it is evident that much heat is thus lost, more especially as a good deal of steam is believed to escape with the water that is blown off.
According to Dr Ure's experiments, the least possible amount of salt-hum solution in the open sea is 38 parts in 1000 by weight, and the sea-water used in the boilers of vessels from the Red Sea, 43 parts were found, the specific gravity of the water being 1·035. The Mediterranean contains about 38 parts in 1000, the British Channel 35·5, the Arctic Ocean 28·5, the Black Sea about 21, and the Baltic only 6·6.
The same authority states, that deep sea-water from the ocean (from whatever locality) holds nearly the same ingredients in solution, containing, on an average, in 1000 parts—
- Chloride of sodium or common salt. - Sulphate of magnesium. - Chloride of magnesium. - Carbonates of lime and magnesia. - Sulphate of lime.
Also a little salphate and muriate of potash, iodide of sodium, and bromide of magnesium.
It is now the usual practice to "blow-off" the requisite quantity of brine continuously, in the proper proportion to the amount of feed admitted, so as to keep the water in the boiler at a certain regular degree of saturation, at which it is found by experience that little or no deposition of scale will take place. Till within the last few years, boilers were always blown-off from the bottom only, it being not unnaturally supposed that the heaviest and most saturated water would be found there; but experience has now proved that the greater portion of the impurities from which the scale is formed are to be found on the surface of the water of the boiler (being carried upwards by the steam), and should be abstracted from thence. Mr Lamb, the superintending engineer of the Peninsular and Oriental Steam Navigation Company, was the first to introduce "surface blow-off," which is now very generally used in addition to blowing-off from the bottom, and is attended with a considerable improvement in the condition of the boiler surfaces.
The proper degree of saturation of water in the boiler may be always at the degree of saturation marked 17 on the scale of their hydrometer, which represents a saturation of between \( \frac{3}{4} \) and \( \frac{4}{5} \) parts of salt.
The following table shows the boiling point and specific gravity of sea-water (at 60° Fahr.) of different degrees of saturation, expressed in parts of salt contained therein, the barometer indicating 30 inches of mercury:
| Saltiness | Boiling Point | Specific Gravity | |-----------|---------------|-----------------| | Pure water | 212° | 1 | | Common sea-water | 212·2° | 1·029 | | Up to this point but little deposit will be formed | 214·4° | 1·058 | | 215·5° | 1·087 | | 216·7° | 1·116 | | 217·9° | 1·145 | | 219·1° | 1·174 | | 220·3° | 1·203 | | 221·5° | 1·232 | | 222·7° | 1·261 | | 223·8° | 1·290 | | 225·0° | 1·319 | | 226·1° | 1·348 saturated solution |
As a general rule, the atmospheric boiling point of the water should never be allowed to exceed 216°, when the barometer stands at 30 inches. The temperature must be ascertained by drawing off a small quantity of the brine, and boiling it in a deep copper vessel in the engine-room, a correction being made, as nearly as possible, for the state of the barometer.
The following table shows the height of the boiling point of steam pure water at different heights of the barometer:
| Barometer Inches | Boiling Point | |------------------|--------------| | 27 | 207·84° | | 28 | 208·69° | | 28½ | 209·55° | | 29 | 210·38° | | 29½ | 211·29° | | 30 | 212° | | 30½ | 212·76° | | 31 | 213·47° |
In testing brine by the hydrometer, care must be taken that it has the particular temperature for which the hydrometer scale was calibrated. This is usually 60° Fahr. About 1° of temperature makes a difference of 0·001 of the specific gravity, or 0·03 of the usual hydrometer degree, or 0·003 of the density of seawater. The steam raised from salt-water and fresh is precisely the same in every respect; but it has been found by experiment that water of the density which it usually acquires in marine boilers, demands about one-tenth more of heat to convert it into steam than if it were fresh-water, its "capacity for heat" being greater to this extent. It is needless to say, that salt itself will not be deposited until the brine arrives at its point of greatest saturation, or three times the density which the water should ever be allowed to acquire; but what the engineer has to guard against is, the deposition of a scale due to the insensible decomposition of the sulphate and carbonates of lime, and the carbonates of magnesia. These are at first held in solution by the water, but are subsequently rendered insoluble, and become deposited on the plates and tubes of the boiler, partly from the free carbonic acid being expelled by the boiling of the water, and partly by its continued saturation.
Many, though hitherto unsuccessful, attempts have been made to obviate the necessity for this expensive process of blowing off, condensation. The only effectual remedy is the employment of fresh water in the place of salt in the boilers; but this can only be accomplished by the adoption of "surface condensation." By this term is understood the condensation of the steam by contact with a large extent of cool metallic surface, instead of the ordinary method of condensing by a jet of sea-water. This principle, though repeatedly tried, has hitherto proved more or less inefficient, and the invention of an effective method of surface condensation is still a problem in marine engineering. It is believed that an economy of about 15 per cent. in consumption of fuel would result from the use of fresh-water in the boilers of marine engines, with a longer duration of the boiler, and the saving of much valuable time consumed in cleaning. The average duration of boilers using salt-water does not exceed six years, while those using fresh-water last eight or nine; but the life of a boiler is very uncertain, depending so much on the care and attention bestowed upon it.
The process of "scaling," a boiler, or removing the deposit from the internal surfaces, is a very tedious and troublesome one, the "scaling" scale being detached by hammers and chisels, after being loosened as much as possible by lighting fires in the furnaces of the empty boiler. In some recent experiments on this subject made at Portsmouth by Mr Lindsay, the boiler was filled with hot air at a temperature of 400°, which acted most successfully in detaching the scale by the rapid expansion induced. The boiler was afterwards filled for service, and so soon as a pressure of steam was obtained, the bottom blow-off cocks were opened, and most of the scale previously detached was "blown off" into the sea.
Almost all boilers are now fitted with an auxiliary or "donkey" Donkey-engine, for the purpose of keeping up a continuous supply of feed, engine, while the regular feed-pumps attached to the large engines are not working. The "donkey" is also made useful for pumping water either from the sea or the bilge, and is an invaluable aid in case of fire.
In many steamers the feed-water is heated to a point considerably above the temperature of the condenser, by means of the waste heaters, heat of the boiler itself; being brought into contact either with the brine which is blown off, or with the hot air at the foot of the chimney. By this means its temperature may be raised from about 100° to 180° or 200°; and as modern practice shows the advantage of freshening the boiler by a plentiful circulation of feed, it is very desirable that its temperature should be thus previously raised. Various modes of feeding the boiler will be found mentioned in the paragraphs to the plate accompanying this article. The feed-water heater of the Great Eastern has acquired an unfortunate notoriety from the sad consequences attending its explosion, though there is no inherent danger in the arrangement there adopted, which has been safely and successfully applied in many other vessels.
When the ebullition inside a boiler is so rapid and violent that "priming" the water rises with the steam in considerable quantity, and is carried over with it to the engines, or is blown up the waste steam-pipe, the boiler is then said to "prime." This is one of the most Steam Navigation.
Dangerous and troublesome propensities to which a boiler can be subject, as it may occasion a back-down in the engines by the shock of the piston upon the incompressible fluid (if escape-valves of sufficient capacity are not fitted), and in all cases it entails a great loss of heat carried off by the hot-water which boils over. Priming may arise from a variety of causes, but the prevalent one, more especially in the government service, is a too contracted steam space over the water of the boiler. For where the reservoir of steam from which the engines are supplied is very small, there must be constant pulsations of pressure in the boiler; and each time that the surface of the boiling water is relieved of a certain amount of pressure by the rapid withdrawal of a cylinder full of steam, it boils up with great violence, and possibly overflows into the steam-pipe. The only remedy for this is an addition to the size of the steam-chest, and an increased height above the surface of the water to the steam-pipe orifice. Priming, however, is frequently the result of accidental causes, apart from the construction of the boiler. Water charged with mud or mucilage, which forms a viscid scum on the surface, is sure to induce it; also while the ship is passing from fresh-water into salt, and vice versa. A new boiler with clean "raw" surfaces, is more liable to prime than after it has contracted a coating of scale, in consequence of the brisker ebullition going on, as well as from the dirt and grease left in a new boiler by the workmen. It is a usual practice to put talc low in a boiler as a preventive of priming, but this is not always attended with the desired effect. When the boiler primes very much it is necessary to blow the fires, so as to prevent the too rapid formation of steam.
All boilers are subject to the loss of a certain quantity of water, which rises with the steam in the shape of fine spray, and passes over with it into the cylinders of the engines. When much of this is present, the steam is said to be "wet;" but it is believed that all steam raised in the ordinary way is more or less charged with water in a state of fine subdivision. To evaporate and utilize this water is one of the principal incentives to the use of surcharged or "superheated" steam. The other advantage arising from its use, namely, the prevention of condensation in the cylinders, has been already referred to while treating of expansive working.
The steam in the boiler may be superheated in a variety of ways, but these methods seem preferable which use for this purpose the spare heat at the bottom of the chimney, which would otherwise be almost entirely lost. The accompanying sketch (fig. 21) explains the method
Fig. 21.
Lamb and Summers' Superheating Apparatus.
which has been already largely employed in the steamships of the Peninsular and Oriental Steam Navigation Company, those of the Union Steam Packet Company (carrying the Cape mails), and many others at Southampton, and which has been attended with the most successful results. It will be observed, that the steam in its way from the boilers to the engines passes through the superheating chest A, at the foot of the chimney, the steam occupying the narrow spaces between the sheet-flues through which the smoke and hot air pass.
BB are stop-valves for admitting the steam to the apparatus, or excluding it if necessary.
CCC are stop-valves for passing the steam direct from the boilers to the engines without going through the apparatus.
DD are stop-valves for admitting the superheated steam to the engines, or shutting it off when common steam only is used.
EE is a square casing enclosing the apparatus, and forming the foot of the chimney, the flues and hearth of which entirely surround the superheating chest. Other casings of thin iron are fitted outside this to prevent the radiation of heat.
F is a door for getting into the chimney, and examining the flues of the apparatus.
The chimney is not rigidly fastened to the square casing, but ships over the projecting part HH, the space between being filled with clay. This mode of carrying the chimney is adopted, so that, in the event of collision, the loss of the chimney should not entail the destruction of the apparatus and its connections.
It is found, from experience, that a heating surface of about 4 square feet per nominal horse-power of boiler is required to superheat the steam under ordinary circumstances. The temperature of the steam after leaving the superheating chest is generally found to be about 350° to 360°, and in the slide-jacket, from 290° to 300° less, according to the length of the steam-pipe.
The saving of fuel in the steamships of the Peninsular and Oriental Company, by the use of this apparatus, is stated to vary from 20 to 33 per cent., without any injurious effects resulting to the piston-packings, &c. By this simple and inexpensive process the whole A high steam given off by the boiler is "superheated" from the temperature due to its pressure (which for steam of 15 pounds pressure would be ture not 250°) to a temperature of from 350° to 360°, which has been proved required, to be amply sufficient for obtaining all the benefit derivable from the process. That much of the heat of superheated steam is really expended in evaporating the particles of water held in suspension seems to be proved by the fact, that its temperature will fall as much as 40 or 50 degrees, in some cases, during its passage from the boiler to the engine, though there is no perceptible escape of heat by radiation from the surface of the well-protected steam-pipe. The heat thus apparently lost is undoubtedly taken up (in the latent form) by the steam resulting from the vaporization of these watery particles, by which means the heat already contained in the water is turned to good account, and the evaporative power of the boiler is virtually increased.
A great many experiments have been made to test the actual economy of the process by comparison with the existing consumption of coal before the superheating apparatus was fitted, and in cases every instance there has been a perceptible improvement. This sometimes takes the shape of a saving in speed in the propelled vessel, sometimes a saving of fuel alone is effected, and in other instances both of these are combined in the same vessel in variable proportions. Where the speed of the vessel has been kept a constant quantity, there would appear to be an actual saving of from 15 to 25 per cent. of fuel, according to the nature and qualities of the boiler to which the process has been applied, and the amount of expansion in the cylinders. The high rates of economy are naturally shown by those boilers which were previously the worst to keep steam with, and which required very hard firing to do so. Those added to priming, and wet steam rank next in apparent economy, while those boilers which show the least were originally the best economies of their class. There is no question, however, but that the process is beneficial in all cases, though not equally so, and enables the steam to be raised in the boilers without "hard firing," being resorted to, being in this respect a great boon to the stokers.
It is believed that not the slightest advantage over the ordinary Wethered's methods of superheating the steam is due to Mr Wethered's system suited to mixing superheated and ordinary steam together at the point where they enter the valve-jacket. To this gentleman's patent, steam, however, we owe, in a great measure, the general awakening of marine engineers to the undoubted advantages of the process, which Conserva have been till now so unreasonably overlooked. The plan adopted them by the government of combining for the steam machinery with marine only a few favoured and old-established houses, has undoubtedly engines, tended to promote conservatism in marine engineering. Innovations and improvements, the wholesome though often unpala table principle of competition being, in their case, scarcely roused into action. These lordly manufacturers have nothing to gain, in fact, by breaking new ground, being well assured of their accustomed orders from the Admiralty, and not caring to raise the ques- There are three principal kinds of marine boilers in use in this country, namely, the rectangular-flue boiler (which is now very generally discarded); the multitubular boiler, or, as it is more usually called, the tubular boiler; and the The tubular sheet-flue boiler. The tubular boiler (as shown in Plates XXXIII. and XXXIV.) is that in most general use. This construction enables a very large quantity of heating surface to be crowded into comparatively small space; while the form of the tubes, which vary from 2½ to 4 in. diameter, affords great strength, at the same time that the thinness of the metal composing them offers little impediment to the conduction of heat. They are attended with this inconvenience, however, that the flame arising from the combustion of the inflammable gases in the furnace is prematurely extinguished by the minute subdivision and rapid reduction of temperature to which it is exposed in passing through these small tubes.
It is well known that flame requires a very high temperature for its maintenance, and is easily extinguished by contact with a comparatively cool surface; as for instance, in passing through the wire-gauze of the miner's safety-lamp. A precisely similar effect is produced by the boiler-tubes, whose temperature, from their being surrounded with water, must be considered low when compared with that of the flame and hot gases passing through them.
The Americans have adopted a different form of tubular boiler (as shown in the accompanying wood-cut, fig. 22), in which the tubes are disposed vertically, the smoke and flame passing round the outside of the tubes, and the water being contained inside. These vertical-tube boilers are very effective in generating steam, and partly for this reason, that the flame reaches further amongst their tubes than in the case of a horizontal boiler, in consequence of the greater space outside the tubes in which the flame may develop itself. The importance of this, while using the bituminous flaming coal of the northern coal-fields, is very great. The absorbent surface of the vertical-tube boilers is, of course, greater than that of the horizontal in proportion as the external diameter of the tubes exceeds their internal diameter, and the weight of water it is necessary to carry is much less.
Sheet-flue boilers are constructed with numerous flat, narrow water-spaces, alternating with flues of the same form in place of tubes. The width of the water-spaces in Steam Na- "Lamb and Summers' patent sheet-flue boilers" varies vigation. from 1½ to 2 inches, and the flues from 2½ to 3 inches. They are extensively used in the steamers of the Penin- sular and Oriental Company, where they give much satis- faction from their durability, and economy in repairs.
When a marine boiler explodes, the presumption is, either Explosions that the safety-valve has not acted properly, or has been of boilers over-weighted, and the boiler has burst simply from excess of pressure; or that the water has been allowed to fall too low, and thus expose the tops of the flues or furnaces, or the boiler-tubes, which, getting red-hot by the action of the flame, have suddenly generated such a rush of steam, upon the re-admission of the feed, as to cause a rupture of the weakened plates. Explosions most frequently happen at the moment of opening or shutting a safety-valve or com- munication-valve, which shows that so long as the steam remains undisturbed within the boiler, it will sustain a very high pressure without bursting; but should a wave or pul- sation be carried through it, the equilibrium is instantly destroyed, and a rupture takes place. The very act of suddenly opening a safety-valve, or a communication-valve to the engines, would cause the water to boil up with great violence, and an immense volume of steam to be instantly liberated, in consequence of the water being relieved from a certain amount of pressure. In the event, therefore, of Precau- the discovery being made that any portion of the boiler has tions, &c. become overheated from want of water, the engineer should neither open the safety-valve nor admit the feed, but throw open the fire-doors, close the dampers, and draw the fires, after which the safety-valves may be cautiously relieved, and the feed gradually admitted, until the overheated sur- faces are covered with water.
In those parts of the boiler where the heat is most intense (as at the backs of the furnaces) the plates will gradu- ally become oxidated and weakened by the fire, even although kept constantly in contact with water. This is probably owing to the rapid disengagement of steam from the surface, which interposes a non-conducting film of steam between the iron and the water, and thus permits the former to get overheated. Thick plates, or overlapped joints, in such a position, "burn out" quicker than thin ones, from the imperfect conduction of the heat through the metal, and this is of course much aggravated when the plates are coated with scale. Plenty of steam room is a safeguard to a boiler, as tending to diffuse and neutralize any dangerous oscillation or sudden accession of steam. The immense rush of steam which always follows an ex- plosion is satisfactorily explained, when we consider that the instant the water contained in the boiler is relieved of pres- sure it throws off steam with great rapidity, and continues to do so until the whole mass of the water is reduced to the atmospheric condition of 212° Fahr. To make matters worse, the steam-chests of all the boilers are usually in com- munication.
It is gratifying to find that while explosions in Ame- Rarity of rica are so frequent, they rarely occur on board of steam explosions vessels in this country—a result which is doubtless to be insteamers. attributed, in a great measure, to the supervision of the Board of Trade. It is worthy of remark, also, that the majority of such accidents have happened to tug-boats, which, from not carrying passengers, are exempt from government interference; and it will be remembered that the Great Eastern, when her feed-water heater exploded with such fatal consequences, had not yet received a cer- tificate of sea-worthiness. By the Merchant Shipping Act Require- ments of ment of 1854 the Board of Trade are empowered to enforce cer- tain provisions of equipment of the vessel and her machinery, supposed to conduce to the safety of the passengers and ship. The principal points to which attention is directed by this act are, that the masters and mates of steamers shall have proper certificates of competency; that the hull and machinery generally shall be of sufficient strength; that the number of passengers carried shall be limited by the accommodation; that a sufficient number of boats be carried; that proper water-tight bulkheads be fitted, as well as pumps, fire-pumps and hose, life-buoys, lights, compasses, &c., &c.
Each boiler is required to have one safety-valve, and recommended for further security to have two, the weights upon which have been sanctioned by the Board through their surveyor. One of these valves (called the government safety-valve) is left locked beyond the control of the engineer of the boat, the key being placed under the master's charge. Every passenger-steamer is required by this act to renew her certificate of efficiency or sea-worthiness twice a-year, after periodical surveys have been held upon her hull and machinery; and if such certificate is not granted, she is debarred from carrying passengers until the required provisions are complied with.
It will now be desirable to convey some practical information regarding the coals used in steam-vessels. The qualities it is most desirable for steam-coals to possess may be summed up as follows:—1. They should have a high evaporative power, or, in other words, they should be capable of converting much water into steam with a small consumption of fuel. 2. They should not be highly bituminous, as such coals produce a dense black smoke which it is difficult to consume in the furnace, and the soot and tarry matter evolved are found to clog the tubes and flues, and detract from the evaporative power of the boiler. 3. The coal should light quickly, and be capable of a rapid combustion. 4. It should be sufficiently cohesive in its nature to bear the constant attrition it is subjected to without becoming broken into small fragments. 5. It should combine a considerable density with such a mechanical structure as may admit of its being stowed in the smallest possible space, this involving a difference of 20 per cent. 6. It should be as free as possible from sulphur, which induces progressive decay and spontaneous combustion.
### Table, showing an Abstract of the Principal Results obtained from the Best Coals of the United Kingdom, collated from the Admiralty Reports on Coals suited to the Steam Navy.
| Name of Fuel | Evaporative power No. of lbs. of Water converted into Steam by 1 lb. of Coal. | Weight of cubic feet in lbs. | Cubic feet occupied by 1 ton in cubic feet. | Cohesive Power percentage of large Coals. | Evaporative Power after Combustion of Combustible Matter in residua. | Evaporative Power per Hour per Square Foot of Grate surface. | Lbs. of Clinker per ton. | |-------------|---------------------------------|-----------------|---------------------------------|---------------------------------|---------------------------------|---------------------------------|-----------------| | Craigola | 9-35 | 60-17 | 37-23 | 49-3 | 9-65 | ... | 30-6 | | Anthracite (James and Awbrey) | 9-46 | 58-25 | 38-45 | 68-5 | 9-7 | ... | 0 | | Pentrefelin | 6-36 | 65-17 | 33-65 | 62-7 | 7-4 | 40-6 | 22-7 | | Duffryn | 10-14 | 53-22 | 42-09 | 50-2 | 11-8 | 69-8 | 0 | | Oldcastle Fiery Vein | 8-94 | 50-92 | 43-99 | 57-7 | ... | 71-0 | 0 | | Ward's Fiery Vein | 9-40 | 67-43 | 39-90 | 46-5 | 10-6 | 87-8 | 54-5 | | Binea | 9-94 | 57-08 | 39-24 | 51-2 | 10-3 | ... | 0 | | Llangennech | 8-86 | 56-93 | 39-94 | 53-5 | 9-2 | ... | 68-6 | | Pentrepoth | 8-72 | 57-72 | 38-80 | 46-5 | 8-98 | 61-5 | 80-2 | | Mynydd Newydd | 9-52 | 60-33 | 39-76 | 53-7 | 10-39 | 79-6 | 59-1 | | Three-quarter Rock Vein | 8-84 | 56-39 | 39-72 | 52-7 | ... | 88-3 | 42-8 | | Own Prood Rock Vein | 8-70 | 55-28 | 40-62 | 72-5 | 9-35 | ... | 40-8 | | Own Nanny-gros | 8-42 | 56-00 | 40-60 | 65-7 | 8-82 | 71-3 | 23-7 | | Reed | 9-53 | 58-66 | 38-19 | 35-0 | 10-44 | 71-4 | 0 | | Pontypool | 7-47 | 55-70 | 40-22 | 57-50 | 8-04 | 55-0 | 20-9 | | Bedwas | 9-79 | 50-50 | 44-32 | 54-00 | 9-29 | 90-5 | 25-2 | | Ebbw Vale | 10-21 | 53-30 | 42-26 | 45-00 | 10-64 | 90-5 | 9-3 | | Porth-Mawr | 7-53 | 53-30 | 42-02 | 62-00 | 7-75 | 77-3 | 27-0 | | Colehill | 8-00 | 53-00 | 42-26 | 62-00 | 8-34 | 75-7 | 39-5 | | Neath Abbey | 9-38 | 59-30 | 37-77 | 50-00 | 9-65 | 116-0 | 19-2 | | Lilyvi | 9-19 | 53-30 | 42-02 | ... | 9-58 | 89-0 | 36-0 | | Rock Vater | 7-68 | 55-00 | 40-72 | 65-5 | 7-88 | 91-0 | 33-0 | | Aberdare Company's Merthyr | 9-73 | 49-30 | 45-43 | 74-5 | 10-27 | 92-4 | 9-8 | | Thomas's Merthyr | 10-16 | 53-00 | 42-25 | 57-5 | 10-72 | 111-8 | 3-9 | | Nixon's Merthyr | 9-96 | 51-70 | 43-32 | 64-5 | 10-70 | 103-2 | 5-7 | | Hill's Pyleton Works | 9-75 | 51-20 | 43-74 | 64-0 | 10-18 | 119-8 | 7-5 | | Slivervught (Irish Anthracite) | 9-85 | 62-80 | 35-66 | 74-0 | 10-49 | 84-5 | 18-0 | | Dalhousie Jewel Stone | 7-08 | 49-8 | 44-98 | 85-7 | 7-10 | 63-0 | 62-2 | | Wallsend Elgin | 8-46 | 54-6 | 41-02 | 64-0 | 8-67 | 91-0 | 14-6 | | Grangemouth | 7-40 | 54-25 | 40-13 | 69-7 | 7-91 | 71-4 | 16-4 | | Eglinton | 7-37 | 52-0 | 43-07 | 79-5 | 7-48 | 90-0 | 8-2 | | Newcastle Hartley | 8-23 | 50-5 | 44-35 | 78-5 | 8-65 | 62-0 | 17-0 | | Carr's Hartley | 7-71 | 47-8 | 46-85 | 77-5 | 8-13 | 84-6 | 5-0 | | North Percy Hartley | 7-37 | 49-1 | 45-62 | 60-0 | 7-72 | 94-0 | 7-8 | | Hasting's Hartley | 7-77 | 48-5 | 46-18 | 75-5 | 7-96 | 104-0 | 1-7 | | Hedley's Hartley | 8-16 | 52-0 | 43-07 | 89-5 | 8-71 | 74-8 | 14-4 | | Original Hartley | 6-82 | 47-1 | 44-62 | 80-0 | 6-98 | 106-0 | 10-1 | | Derwentwater's Hartley | 7-42 | 50-4 | 44-48 | 63-5 | 6-65 | 95-0 | 28-0 | | Galley's West Seam | 9-22 | 51-6 | 49-41 | 68-5 | 10-73 | 96-5 | 11-6 | | Hart's Coal Company's Steambot | 7-48 | 49-5 | 45-25 | 79-5 | 7-83 | 61-0 | 9-8 | | Davison's West Hartley | 7-61 | 47-7 | 40-96 | 70-5 | 7-83 | 96-5 | 2-1 | | Cowpen and Sydney Hartley | 6-79 | 47-9 | 46-76 | 74-0 | 7-02 | 84-0 | 3-7 | | Balcarres Lindsay Mine | 7-44 | 51-1 | 43-83 | 70-0 | 7-58 | 93-5 | 22-3 | | Haigh Yard | 7-90 | 50-8 | 44-13 | 89-0 | 8-23 | 79-0 | 26-4 | | Johnson and Wirthington's Sir John | 6-32 | 51-6 | 43-41 | 82-0 | 6-62 | 80-5 | 34-4 | | Wylam's Patent Fuel | 8-92 | 65-08 | 34-41 | ... | 9-74 | 72-4 | 61-6 | | Bell's | 8-53 | 65-3 | 34-30 | ... | 8-65 | 91-5 | 76-1 | | Warlich's | 10-38 | 69-05 | 32-44 | ... | 10-60 | 96-5 | 29-7 | | Lyon's | 9-53 | 61-10 | 36-66 | ... | 9-77 | 93-0 | 38-7 | | Watney's Anthracite | 11-08 | 67-0 | 33-43 | 87-5 | 11-40 | 127-4 | 24-6 |
Admiralty experiments on coals for the Royal Navy. Steam Navigation.
Average properties of coal.
| Properties | Average of Seventeen Samples of Welsh Coal | Average of Six Samples of Newcastle Coal | |-----------------------------|-------------------------------------------|-----------------------------------------| | Theoretical evaporative power | 15785 | 14208 | | Specific gravity | 1325 | 1259 | | Coke | 8687 | 661 | | Moisture | 088 | 597 | | Frangibility, large | 792 | 859 | | " small | 208 | 150 |
Average Chemical Analysis of 100 parts of Dried Coal.
| Ash | 224 | 432 | | Carbon | 8913 | 7845 | | Hydrogen | 423 | 511 | | Nitrogen | 127 | 179 | | Sulphur | 101 | 136 | | Oxygen | 212 | 897 |
In the foregoing tables the "theoretical evaporative power" is deduced from the composition of each coal as determined by chemical analysis. It gives the maximum amount of heat which each coal could produce, calculated in terms of the number of pounds of water at 212°, which would be converted into steam at 212° by the complete combustion of 1 lb. of each variety of coal.
1 pound of pure carbon (according to the most accurate experiments) emits, by its combustion, an amount of heat sufficient to evaporate 14-88 lbs. of water at 212° into steam at 212°; and 1 lb. of hydrogen, when burned, emits heat enough to convert 63-56 lbs. of water at 212° into steam of the same temperature. It is found experimentally, that the quantity of water capable of being evaporated by any coal is (as nearly as possible) directly as the quantity of coke which can be produced from that coal; the fact being, that the heat required for evaporating water in the boiler furnace, as much heat is required for liberating the volatile products of the coal as is afterwards produced by the combustion of these volatile products, taking into account the cooling effect of the air admitted to maintain their combustion. Hence the very high evaporative power of anthracite coal, which, unfortunately, has certain counterbalancing disadvantages, that preclude its use in the boilers of a steam-vessel under ordinary circumstances. It is not only very difficult to light, but when lighted can be maintained in active combustion only by the aid of artificial draught, when the heat evolved is so intense as rapidly to destroy the fire-bars, as well as the material of the boiler itself.
Welsh coal, bars, as well as the material of the boiler itself, Welsh coal, which holds an intermediate rank as to its evaporative power between anthracite and the bituminous coals of the northern district, is considered the most suitable for steamers in general, and is much more easily stocked than bituminous coal.
Treatment As Newcastle and other bituminous coals demand careful and peculiar treatment in the furnace, it may not be out of place here to give some directions for stoking it. The fires should be kept at a uniform thickness of from 12 to 14 inches. When the furnaces of one boiler are being charged, the fresh coal should be thrown upon the right-hand half of each fire in succession for one charge, and then upon the left-hand half of each fire during the next charge, and so on alternately, so that the whole fires may never be covered with green coals at once. The green coal is to be thrown upon the front half of the fire only, and never at the back of the fire, but when necessary the red burning fuel must be pushed back by the shovel, to keep up the proper thickness of the fires at the bridge. Where no means are provided for admitting air through the fire-doors, these must be left slightly open, after charging with fresh coal. By a due observance of the three last directions, the formation of black smoke with north country coal may be prevented. The cinders, as they fall through the spaces between the fire-bars, are to be raked forward in the ash-pits, and at every fresh charge a portion of them is to be thrown upon the fires after the manner of that which is thrown from the stoke-hole but clinkers and ashes. The spaces between the fires must be all times to be kept clear of clinkers and ashes, so that the air may have free access to the burning fuel. When the coals cake on the bars, the poker must be gently used to raise and open them for the admission of air to the mass of the burning fuel.
Some of the "patent fuels" have a very high evaporative power, but they are all, more or less, difficult to manage in the furnace, and should never be used where the stokers are unaccustomed to their peculiarities. They are very valuable in special cases, from Steam Navigation.
We shall now advert to a few particulars having reference to the general construction and management of the Marine Engines of steam-vessels. The first valve through which engines the steam passes after leaving the boiler is the throttle-valve, by means of which the flow of steam to the engines is regulated or shut off entirely by hand. In the event of a ship pitching very much in a heavy sea, it is often necessary to station a man at the throttle-valve to shut off the steam from the engine whenever it begins to "race," or fly off at a high velocity, according as the resistance is removed by the propeller becoming raised out of the water. Both paddle and screw engines are subject to this dangerous action, but particularly the latter, on account of the screw being, from its position in the ship, more exposed to sudden variations of "dip" or immersion. To mitigate this (in some measure), the contrivance called a "governor" has been successfully applied in many cases of screw-steamer, governor whose consumption of fuel in bad weather has been thereby much diminished, as well as the working of the machinery rendered more regular. Indeed, the commander of a screw-steamer has often found it practicable, after this little instrument had been fitted to his machinery, to keep his vessel head to wind in such weather as would have formerly necessitated his laying to.
The annexed figure (23) represents the best, and indeed description almost the only, species of marine governor that has yet been applied. It is called "Silver's momentum-wheel governor," constructed by Messrs J. Hamilton and Company of Glasgow, who have purchased the patent. "It consists of a momentum-wheel A, fixed on the boss of a pinion B, which works loosely on the spindle C, and gears into the two-toothed sectors DD. These two sectors, being supported on a crosshead E, made fast to and carried with the spindle C, work in opposite directions on the pinion B; and as they are linked by the rods FF to the sliding collar G, which receives and works the forked lever H, communicate motion to the throttle-valve T. MM are vanes, and N is a spiral spring, both of which are adjustable."
"The action of the above instrument is as follows:—When the spindle of the governor is turned by the engine to which it is attached, the two toothed sectors, which are carried on the fixed crosshead, being geared into the pinion on the momentum-wheel, have the tendency to turn round on this pinion; but as they are linked to the sliding collar, they necessarily pull inwards this collar, and so compress the spiral spring, and this spring reacting on the collar, and consequently on the toothed sectors, serves to turn round the momentum-wheel, while the vanes on the momentum-wheel balance the action of this spring by the resistance the atmosphere offers to their progress through it. As the leverage action of the toothed sectors upon the momentum- wheel pinion increases (as the spring becomes distended, and vice versa), it will be seen that the reaction of the spring in propelling the momentum-wheel will at all times be uniform, and as much only is required as will carry round the momentum-wheel with its vanes at its proper speed, and overcome the friction of working the throttle-valve and throttle-valve connections. When the momentum-wheel is in motion, it will rotate with the engine to which it is attached at a velocity proportioned to that at which it is fixed by the connecting gear; and while the engine, from the usual causes, may attempt to vary this velocity, it cannot affect the momentum-wheel, but leaves it free to act upon the sliding collar, and consequently upon the throttle-valve—at one time closing the throttle-valve by its action in resisting any increase of velocity, and at another time opening the throttle-valve by its action in resisting any decrease of velocity on the part of the engine.
A momentum-wheel of 2 feet 8 inches in diameter, and 2 inches breadth of periphery, running at a speed of 180 revolutions per minute, is found to be sufficient to work with promptness and ease the largest throttle-valve."
The same engineer have also introduced, for this purpose, an ingenious modification of the ordinary Watt's centrifugal governor, called "Silver's four-ball governor," in which the action of a spiral-spring is substituted for that of gravity, and the whole apparatus (like the preceding one) is balanced, so as to remain undisturbed in its action by the pitching or rolling of the vessel.
It is evident that the action of all such governors is so far imperfect, that they do not shut off the steam before the speed of the engine has begun to be sensibly affected by the very evil it is desired to remedy, and it has been therefore attempted (by Mr Jensen of Copenhagen) to construct a marine governor, which may make use of the cause of the evil as the remedy against it; or, in other words, to employ the irregular immersion of the vessel as the means of regulating the engines, in preference to letting the engines regulate themselves. This he effects in a very simple and ingenious manner, by employing two small cylinders, which communicate with the sea below their pistons, whose motion is then transmitted, by rods and bell-cranks, to the throttle-valves of the engines. These cylinders are placed inside the vessel (one on each side), and as near as possible to the position occupied by the propeller. As the sea has free admission under the pistons, the latter are subjected to a varying pressure of water, nearly corresponding with that to which the propeller itself is exposed. This varying pressure (modified by the action of a spiral spring) is therefore communicated immediately to the throttle-valves through the cranked rods, and thus shuts off or admits the steam before the engines have felt the variation in the load about to be transmitted to them from the propeller.
The steam, after passing the throttle-valve, next enters the expansion-valve, where it is cut off at any desired portion of the stroke, by the action of an eccentric, or cam, on the main shaft. Such an arrangement is shown in Plate XXIX. The valve usually employed for this purpose is the "equilibrium" or "double-beat" valve, as shown in the annexed engraving. This kind of valve has the advantage of being opened and shut with great facility, since, from its construction, the pressure of the steam has no tendency to jam it against its seat, the objection to which all single flat valves are subject. It is also apparent that a slight rise of this valve gives a large opening for the steam to pass. In the engraving, the valves \(a\) are made of brass, and the valve-box, and the spindles connecting the valves, are of iron. In this instance the valves are purposely connected by iron spindles, in order that the linear expansion of the sides of the box containing the valves, and of the spindle connecting them, may be equal in amount, and therefore have no tendency to raise the upper valve off its seat, which would certainly ensue were the valves connected by a brass spindle, in consequence of the greater expansion of that metal by heat. This arrangement of the metals will be seen to be of special importance when superheated steam is used, and the temperature thereby increased.
Having passed the expansion-valves, the steam now enters the jacket of the cylinder slide-valves. These are usually valves so constructed and arranged as to fulfil the following conditions for the admission and exclusion of the steam, independently of the action of the expansion-valves:—1st, The steam is shut off a little before the end of the stroke, by the valve prematurely closing the steam-port aperture. The use of this is to check the velocity of the piston, by causing it to finish the stroke by the expansion of the enclosed steam only. This is effected by giving "lap" to the valve. 2d, The eduction-port, or the passage to the condenser, is closed a little before the end of the stroke, which is called cushioning the piston, because it then completes the stroke against an elastic cushion of vapour shut up between it and the top or bottom of the cylinder. 3d, The port is opened for the admission of steam to the cylinder a very little before the piston begins the return-stroke, in order that the steam may have filled the passages and the "clearance" of the piston, and have acquired its full pressure, by the time that the crank shall have turned the centre. This is effected by giving what is called "lead" to the valve. 4th, The communication with the condenser is opened a little before the end of the stroke, so as to have a vacuum ready made in the cylinder so soon as the return stroke begins. In this way each operation which takes place in the cylinder is slightly anticipated by the mode of setting the valves. In the case of screw-engines especially (which run at a high velocity), it is of the greatest importance that the steam-passages and valves should be of ample size, and those valves only should be used which give a large opening for the steam, with a short "travel" of the valve.
As the nature and limits of this article preclude a minute description of the details of the marine engine (which indeed are very similar to those of the stationary condensing engine, already given in the article Steam-Engine), we will not attempt this, but at once follow the steam from the cylinder into the condenser. In this magical little chamber the whole of those perplexing processes we have been considering are at once reversed, and all the labour and expense incurred in generating the steam in the boilers (themselves about twenty times larger than the condenser), are, as it were, instantly annihilated. The condensation of the steam is usually effected by the dispersion of a jet of cold sea-water amongst it, which is the most effectual means yet known for producing that instantaneous condensation, upon which the efficacy of the process is entirely dependent. Many attempts have been made, as we have before stated, to condense the steam by contact with cold metallic surfaces without the use of the water-jet, but they have all, more or less, failed, from the condensation not being sufficiently sudden. The plan known as "Hall's Condensers" is, indeed, partially successful; but, owing probably to their weight, bulk, complexity, and expense, they are very little used, although it is now twenty years since their first introduction. In most surface-condensers the steam is passed through a great many small copper pipes, contained in a cistern of cold water, through which a current from the sea is made to flow by means of a force-pump. In such an arrangement, the loss of water arising from leakage, or from blowing-off at the valves, is compensated to the boiler by employing a small apparatus to distil sea-water, by the aid of which the boilers are kept constantly supplied with fresh water. A close connection exists between the temperature of the condenser and the vacuum, the latter being of course more complete as the temperature is reduced. There is a limit, however, beyond which any further reduction of temperature, by injecting more sea-water, is attended by a loss of power. It is found in practice, that a temperature of from 95° to 105° (depending upon the pressure of the steam), is the most economical, with which a vacuum of from 27½ to 26 inches of mercury is obtained when the weather barometer stands at 29¼ inches, the standard of this country. It is a curious fact, and contrary to what might at first sight have been anticipated, that a better vacuum and a lower temperature of the condenser, is obtained with superheated steam, than with common steam, being probably owing to the more perfect condensation of the steam, when not mixed with particles of hot water held in mechanical suspension. This fact appears also to indicate, that the extra dose of heat contained in the superheated steam has been all previously and usefully expended in the cylinder (by supplying the expanding steam with latent heat), and that no portion of it survives to enter the condenser. Whatever the cause may be, the result is, that considerably less injection-water is required when superheated steam is used, much to the surprise of the engineer in charge.
According to Dr Ure's experiments, uncondensed watery vapour at a temperature of 100° balances 1½ inch of mercury; at 110°, 2½ inches; at 120°, 3½ inches; at 130°, 4¾ inches; at 140°, 5½ inches; and at 150°, 7½ inches of mercury, or exerts a pressure of 3½ pounds per square inch. In addition to the uncondensed vapour, a considerable quantity of atmospheric air is always present in the condenser, having entered it in combination with the condensing water. The contents of the condenser, therefore, are sea-water used for condensation, condensed steam, uncondensed watery vapour, and atmospheric air. To remove these is the duty of the air-pump. It has a capacity of about 8 gals. of that of the cylinder, and is furnished with a "bucket" and valves, which are now usually formed of a stout circular disc of vulcanized India-rubber. The air-pump draws its contents from the condenser through the foot-valve, and then passes them on through the delivery-tube and the discharge-pipe into the sea, a small portion of the hot water being abstracted by the feed-pumps to supply the boilers.
The machinery of a sea-going steamer should be as simple in design and possess as few moving parts as possible. In vessels designed for long voyages more especially, as well as in those which are intended for foreign stations, it is far preferable to dispense with those clever and ingenious contrivances for saving infinitesimal quantities of fuel which are in vogue with some manufacturing engineers of the present day, and more especially in Scotland. During a long run, as from Aden to Australia, for instance, the chances of derangement of the machinery are much increased by the mere inability to make the usual adjustments demanded by ordinary wear and tear; and it is surely wise to avoid the additional risk attending a great multiplicity of parts, the failure of any of which may cause the stoppage of the engines. When we consider that a large pair of engines may very possibly have five hundred different parts all in motion at once, and that each of these parts is making a thousand rotations, or double-oscillations, each half-hour, for twenty days consecutively, it can scarcely be wondered at, that accidents should occasionally happen. But allowing that everything goes well with this complicated machinery, it is not by the use of such finical refinements of mechanism that any great saving of fuel can be effected (for this is the main point aimed at), but rather by the careful and judicious management of the boilers and engines. The fortunate selection of a good chief-engineer for the vessel will generally effect more saving in fuel than the most ingenious and expensive "modern improvements." These remarks are not intended to apply to the obvious advantages obtained by superheating the steam, large expansion, careful clothing (or jacketing) of the cylinders and steam-pipes, &c., which do not add much to the complexity of the engines.
The tendency of modern practice is to run the pistons of steam-engines at a much higher speed than formerly. This is more especially the case with screw-engines, whose pistons frequently run at the rate of 400 feet per minute, in place of Watt's old rule of 220 as a maximum. Although theory does not impose larger dimensions on the moving parts of a machine on this account, it is found in practice that the shafts of screw-engines running at a high velocity must be considerably increased in size to avoid accident. This arises partly from the increased momentum of the parts in motion; partly from the greater tendency of the bearings to heat from friction; and partly from the more rapid wear and tear of the brasses and sockets, by which the accurate fitting of the parts is destroyed, and these are consequently subjected to unequal jerks and strains. The simple remedy for such disorders is to enlarge the main-shafts and bearings, the latter being also made unusually long, so as to diminish the effects of friction and wear and tear.
The iron shafts of marine engines revolve in sockets or linings of bearings lined with brass or gun-metal. These give rise to little friction, but, as their wear is rapid, they require frequent attention to keep the lining screwed up to the neck of the shaft, and they must be renewed when much worn. In the case of screw-ships, where the bearings of the screw-shaft are not readily accessible, this used to give much trouble, until it was found out (partly by accident) that bearings lined with lignum vitae instead of brass are subject to exceedingly little wear or friction. A plan has been therefore adopted of fitting these bearings with rings of lignum vitae alternating with rings of gun-metal, which answers very satisfactorily.
Before leaving the subject of shafts, it may be remarked that these, whether paddle or screw, appear liable to deterioration by continued use, and finally to give way, sometimes suddenly, but oftener gradually. It is contended by some that the iron of which they are formed has a tendency to lose its toughness, and assume a crystalline texture, from long exposure to the shocks and vibrations to which all such shafts are subject. Having had many opportunities of observing broken shafts, the author does not hold with this theory, but thinks the following explanation to be more probable. It is allowed to be a difficult operation to make large shafts perfectly sound in the centre, where the bars of iron of which they are built up are not always thoroughly welded into one homogeneous mass. These imperfections, when they exist, are of course not visible on the outside, nor do they seriously affect the strength of the shaft at first; but as the continued jarring and twisting goes on from year to year, they become more and more developed, and the shaft becomes loose in the centre, acquiring a "reedy" structure which gradually extends to the surface. A fracture then takes place, if the crack be not observed and the shaft renewed.
The efficient lubrication of the bearings and other working parts of the engine with oil or melted tallow is a matter of the utmost importance to be attended to, both as regards the smooth working of the machinery and its preservation from injury. From want of this simple precaution the bearings get strongly heated by the friction, and may either be damaged by the consequent expansion which takes place, or else... Steam-vessels are propelled either by paddle-wheels or screws.
1. **Paddle-Wheels**.—There are two kinds of paddle-wheels in general use in this country, namely, the common wheel, and that with feathering floats. The common paddle-wheel, notwithstanding many attempts to supersede it, still maintains a high place as a simple and efficient propelling agent; the faults which have been attributed to it being, it is believed, more apparent than real. When a steam-vessel is moored in a harbour and prevented from moving, or when first commencing motion after having been at rest, the defects of the common paddle-wheel appear to be very great. The paddle-boards, on entering the water, press obliquely down into it, tending to raise or lift the vessel up out of the water with a force which produces no useful effect. Again, when the paddle-board is leaving the water, it seems to do little more than raise or drive the water upwards in the form of back-water. It is only, therefore, in the middle of its path that the propulsion of the paddle seems to be exerted in forwarding the boat, and that only for a short time. A large part of the force of the steam-engine seems thus to be expended in raising the vessel, and in elevating the back-water, and only a small portion in carrying the ship forward.
This is the case of a vessel at rest, or not in rapid motion; but the phenomena of a paddle-wheel revolving when the vessel is in motion differ essentially from the phenomena of a wheel revolving on a vessel at rest. When it is just starting, or as yet moving very slowly, the evils here mentioned do in some degree take place; but by the motion of the vessel forwards (which is the result of the revolution of the paddles), the faults complained of are at once remedied, and the paddle-float of a common wheel in a quick vessel is virtually "feathered" as perfectly as the most practised rower could feather his oar. A little study of the geometrical conditions of a paddle moving forwards and in a circle at its actual speed renders this plain. The paths described by motion, the boards are trochoidal curves, being of the family of the cycloid; and from the study of the motion actually performed by the paddle-board of the common wheel, it is seen, first, that the board is inserted into the water in an angular position resembling closely the entrance of an oar into the water; secondly, that it is then made to act horizontally on the water during a short interval; and thirdly, that it is withdrawn from the water edgewise, with an easy and graceful movement.
When the paddle-wheel is either badly proportioned, immersed too deep in the water, or attached to a very slow boat, its action becomes much impaired or impeded. Hence much attention has been devoted to the construction of a paddle that should be more effective in these unfavourable circumstances than the common wheel. Some of the contrivances invented for this purpose have failed for want of perception of the precise motion it was necessary to give to the paddle-board; others from the complexity of the mechanism employed. In the year 1829 a patent was granted to Elijah Galloway for a paddle-wheel with moveable boards, which patent was purchased by Mr William Morgan, who made some unimportant alterations in the mechanism. This, called the feathering paddle-wheel, is represented by the above wood-engraving (fig. 26), by Steam Navigation.
Inspecting which it will be seen that a distinct feathering movement is imparted to the boards on entering and leaving the water. This movement, it will be observed, is derived from the eccentric motion of the periphery of the second paddle-centre, to which are hinged the long rods that communicate the desired movement to the boards turning on pivots. Wheels made on this principle, though considerably heavier and more expensive than the common paddle-wheels, are frequently preferred for sea-going steamers subject to much variation of draught. They have been known to improve the average speed of a steamer by more than a knot an hour, and they are always accompanied with less vibration than the common paddles.
The "slip" of the paddle-wheel, by which is meant the excess of its velocity above that of the vessel, is usually reckoned at 4th (or 20 per cent.) of the vessel's speed when the wheel is well proportioned, and the vessel tolerably fast. Feathering wheels have less slip.
The captains of steamers are frequently both surprised and disappointed to find how powerless their vessel is to drag a stranded ship off the shore, even when the whole power of their engines is exerted for this purpose. A slight consideration of the subject will show that the requirements of such a case are very unfavourable to the proper development of the power of a steamer. We will suppose a vessel fitted with a pair of paddle-wheel engines of 500 horse-power collectively. The diameter of each cylinder will then be, say, 80 inches, and the stroke 6 feet. The length of the crank will therefore be 3 feet, driving a paddle-wheel of, we will suppose, 28 feet effective diameter, reckoned at one-third of the depth of the boards from their extreme edge. When the piston of each engine alternately arrives at the top or bottom of its stroke, that engine is then powerless, and the whole of the work devolves upon the other engine, which is then at half-stroke, with the crank nearly at right angles to the thrust, and therefore in the most advantageous position for transferring the power. By bringing the pistons of both engines to a stroke, we obviously improve upon this, for now both engines are assisting to turn the shaft, though acting at a reduced leverage in the proportion of 3 feet to 2½ feet nearly. By calculating the pressure upon the two pistons, we find the statical power exerted by the engines (the one pushing and the other pulling); but as the thrust thus found is transmitted by a lever of the second order, the short arm of which is the crank, and the long arm the radius of the paddle-wheel, it is necessarily reduced in the inverse ratio which these bear to each other, or as 14 : 2½. The calculation would then be as follows:
\[ \text{Sq. ins.} = 5020 \times \frac{\text{area of cyl.}}{\text{per sq. inch of piston.}} \times 22 \times 2 \text{ (for both cylinders)} = 221,166 \text{ lb.} = 88.75 \text{ tons total pressure on pistons.} \]
Then as \( 14 : 2\frac{1}{2} :: 98.75 : 15.16 \text{ tons.} \)
This being further reduced by 20 or 25 per cent. for the friction of the machinery, working the air-pumps, &c., leaves scarcely eleven tons of thrust available for starting a weight, or dragging a stranded vessel off the shore, by a steamer of 500 nominal horse-power.
A similar calculation made for screw-engines shows a like result.
2. Screw-Propeller.—A screw as used for propelling vessels may be defined as a metal plate wound, edgewise, round a cylinder or spindle, as shown in the accompanying sketch (fig. 27a), which represents one full turn of the common screw.
This would be a single-threaded screw, but it is evident that two, three, or more threads, if kept uniformly parallel to each other, may in the same way be wound round the spindle without interfering with each other. We should thus have a two-threaded or a three-threaded screw, the former being chiefly used for propelling in the navy, and the latter in the merchant service. The whole length of one complete turn of the screw, measured in a straight line along the spindle, is called the pitch of the screw. In the preceding engraving, therefore, the pitch is measured by the length of the spindle (fig. 27a), since the thread makes one complete turn upon it. It is also apparent, that if this screw were turned once round in a piece of soft wood (in the same manner as a carpenter's screw), it would advance through the wood the exact distance between the cut ends of the thread, which (we have seen) is the pitch. Hence, by the pitch of a screw, we understand always its linear progression for one revolution, and the speed of the screw is measured by multiplying the pitch into the number of revolutions. If the screw were working in a solid, the speed thus found would give its actual linear advance; but as it revolves in water, which is a yielding medium, the water gives way to some extent, and the screw does not advance the full amount of its pitch, this deficiency in its progress being called the slip of the screw.
Now, if the screw represented by fig. 27a be cut into several portions by planes passing across it at right angles to the axis, each of these sections would have the appearance of the vane of a windmill. If the screw were two-threaded, the vanes or "blades," as they are called, would be exactly opposite each other, as shown in fig. 27b, or as in the annexed sketch, fig. 28b, which represents a two-bladed screw as used in propelling. The screw here represented is about one-sixth part only of the whole length, or pitch, of the full turn of the screw shown by fig. 27a, this small fraction of the pitch being found sufficient to absorb the whole power of the engines, so that any greater length of screw would only be hurtful by causing unnecessary friction, as well as by increasing the size of the aperture in which it works. By the length of the screw, therefore, is meant the fraction of the pitch employed, measured along the axis of the screw. By the diameter of the screw is meant the diameter of the circle described by the extremities of the blades during their revolution.
The effect produced in propelling the ship will be best understood by supposing the screw represented by fig. 27a, the screw to be revolving rapidly in a trough full of water. It would in propelling then send the water away from it with great force; but as action and reaction are equal, it would be itself, at the same time, urged in the opposite direction with exactly the same degree of force. If we suppose it, then, to be fixed in a ship, the ship will be pushed forward with the same force that is exerted by the screw in pushing back against the water. If the screw is made to revolve in the opposite direction, the converse of this takes place, and the ship is then pushed backwards by the reaction of the screw. The screw-propeller has been subjected by would-be inventors to an endless variety of forms; but these have generally shown themselves more or less inefficient according as they may have departed from the principle of the true screw. The first patent of any interest connected with this subject is that of Mr B. Woodcroft, taken out in 1832, for an "increasing pitch" screw-propeller. His specification describes "A spiral worm-blade or screw coiled round a shaft or cylinder of any convenient length and diameter, in such form that the angle of inclination which the worm makes with the axis of the cylinder continually increases, and the pitch or distance between the coils or revolutions of the spiral continually increases throughout the whole length of the shaft or cylinder upon which the spiral is formed." Mr Woodcroft's idea, that the after-part of the screw would thus be made to act with increased efficiency upon the water which had been previously acted upon by the foremost part, is undoubtedly correct in principle, and had a full turn of the thread been found necessary for propelling (as was at first thought), this plan would probably have been found practically advantageous; but when the length of the screw was cut down by Lowe to one-sixth part of the pitch, very little scope was afforded for Mr Woodcroft's refinement, and it has proved to be really of little or no value.
Mr F. P. Smith's patent was secured in 1836 for "a sort of screw or worm made to revolve rapidly under water in a recess or open space formed in that part of the after-part of the vessel commonly called the dead rising or deadwood of the run." Mr Smith's original drawings showed a screw with two whole turns of the thread, which was afterwards altered in 1839 to one whole turn.
Mr James Lowe obtained a patent in 1838 for a screw-propeller formed of "curved blades, each a portion of a curve, which, if continued, would form a screw." The drawings attached to his specification show a shaft with one blade, a shaft with two blades, and a shaft with four blades. The screw-propeller now generally used (see fig. 27 b, and fig. 28 b), may be considered as a combination of Smith's screw and Lowe's blades, its present form having been in a great measure determined by the series of experiments with the Rattler in 1844. (See page 660.)
Griffith's screw-propeller, first patented in 1849, is probably the best modification of the common screw which has yet been produced. Its principal feature consists in the employment of a large sphere occupying the central portion of the screw. The second peculiarity of Griffith's screw consists in the peculiar form of the blades, which, unlike those of the common screw, are larger towards the centre, and tapering towards the extremities. The extremities of the blades are curved from the front or propelling side towards the vessel, which causes the screw to take a greater hold of the water, and drive it towards the inner or central portion, which, in Griffith's screw, is the most effective part.
This propeller is represented in its simplest form by the wood-engraving (fig. 28 a), and as recently improved by the annexed engraving (fig. 29). It will be seen that this propeller consists of three main parts, viz., the boss which is keyed on to the screw-shaft in the usual manner; and the two blades, which have turned shanks fitting into bored recesses in the boss. Each blade is retained in its position by a key, which is adjusted into its place after the blade has been inserted and turned in its socket about ninety degrees, or until the arrow marked on the flange points to the pitch which it is desired the screw shall have, of which several have been previously measured, and marked upon the screw.
When Griffith's screw was first introduced, it was expected that great advantages would result from an arrangement in its construction (which it shared with Maudslay's feathering screw), by which the pitch or angle of the blades could with facility either be increased, diminished, or "feathered" during the voyage, to suit the varying exigencies of a steam-vessel at sea. Experience, however, has approved that the risk of derangement incident to the machinery requisite for this purpose is too great to admit of practical success, and also that the advantages to be obtained by such an arrangement are far less than was supposed. The use of screws to feather at sea has, therefore, been very generally abandoned. It will be observed, by looking at the engraving, that the blades of Griffith's screw tages of are quite distinct from the boss, into which they are inserted and keyed in such a manner that their angle or pitch may be altered and fixed before the voyage, though not at sea. The use of this arrangement is, that the engineer may find out experimentally the particular pitch of his screw which is most suitable to the engines and ship, experience having shown how very difficult a thing it is to hit upon the right pitch by previous calculation alone. Another advantage resulting from this arrangement is, that when a blade is accidentally broken, it can be replaced without having to remove the centre part, which in Griffith's form of screw is tolerably safe from injury. It is unnecessary, therefore, to carry a spare screw, but only a couple of blades. When the ship is placed under canvass alone, the screw is brought into a position with the blades vertical, in a line with the stern-post, when little resistance is offered to the water. Although Griffith's screw cannot be said to have shown any very decided superiority in speed over a common screw of the best form, it is certainly not inferior in this respect, while it is attended with less vibration, is less affected by a rough sea, and is more manageable under canvass from offering less resistance to the water, and less obstruction to the free action of the rudder.
When the common screw is employed in merchant-common steamers, a three-bladed screw is usually preferred, since this causes less vibration, and gives a steadier motion in a rough sea than the two-bladed screw. The resistance which such a screw occasions to the vessel, when sailing to sailing under canvass alone, is very serious, in addition to the difficulty experienced in steering; and it is found in practice, that but little advantage is gained by disconnecting the screw from the engines, and letting it revolve in its bear- Steam Navigation, in preference to dragging it through the water. Hence, in the case of steamships which depend much upon their canvass, one of three remedies must be adopted: namely, the screw must either be hoisted bodily out of the water; it must be feathered; or, thirdly, such a form must be employed (as Griffith's two-bladed screw, for instance), which will not interfere much with the sailing and steering of the ship, when the blades are placed vertical, and the screw left down in its place. The hoisting screw has been adopted generally for war-steamers, which are supposed to make great use of their sails, and which have a larger number of men available for quickly hoisting and lowering it. The annexed engraving shows the manner in which this is effected in the royal navy. A is the screw (of gun-metal); B is the hoisting-frame (also of gun-metal) which lifts the screw, with its bearings, bodily out of the stern-frame of the ship; C is a gun-metal rack, to hold the hoisting-frame at any portion of its ascent; D is the chain and pulley used in hoisting; E is a clutch upon the screw-shaft, to enable the screw to be disconnected, and rise when brought into a vertical position; F is the gun-metal lining of the screw-shaft, which passes water-tight through the inner stern-post I; G is the iron screw-shaft; H is the outer stern-post; K is the "trunk" through which the screw is raised to the main-deck, when the blades are brought vertical.
Having now seen what are the principal forms of screw-propellers in general use in this country, let us briefly examine some of the qualities inherent in the screw itself.
1. Pitch. The question between the relative values of fine and coarsely pitched screws still remains, in a great measure, undecided. In fact, our experience hitherto has only tended to show, that nothing but actual trial of different pitches can satisfactorily establish the best pitch of screw for any particular vessel. The points to be considered in reference to this inquiry are so numerous and complicated in their bearings upon each other, that they utterly defy previous calculation of their effects; some vessels giving the best results with coarsely-pitched screws running at a low speed, while other vessels, not very dissimilar, attain their highest velocity with a finely-pitched screw running fast. It is generally acknowledged, however, that a coarsely-pitched screw is the best for a vessel with fine after-lines, and a finely-pitched screw for vessels with full stems. The form of the after-lines has undoubtedly a very great influence on the most advantageous pitch of screw for that particular ship, depending on the amount of "back-water" in which the screw works, and the velocity with which it follows the ship. It is by no means an uncommon thing for one vessel to gain a knot an hour by an alteration of the pitch; while in the case of another vessel, perhaps, no improvement is effected by a similar alteration.
2. Diameter. This is made simply as great as the draught of water will admit. In sea-going steamers the top of the screw should be submerged about 18 inches or 2 feet at the average trim, to allow for the undulations of the sea.
3. Area and Length. By the area of the screw is generally understood the plane projection of the resisting surface of the blades. In the experiments made with the Dwarf, it was found that the speed of the vessel remained almost a constant quantity, although the length of her screw was successively diminished from 2 feet 6 inches to 1 foot, the area corresponding to each of these lengths being respectively 22.2 and 8.96 square feet. The slight improvement which did take place in the speed of the boat attended the diminished area. It seems at first sight extraordinary that so great a variation in the resisting surface should cause so little disturbance either in the speed of the engines or of the vessel, thus showing plainly how small a segment of the whole pitch is required to absorb all the power which the reaction of the water is capable of imparting, any extra length of screw beyond this point only retarding by friction. The Rattler's experiments were in the same way commenced with a screw 5 feet 9 inches long, which was gradually shortened until it reached its point of maximum effect at 15 inches only. It is now a common practice to make the length of the screw 1/4th of the pitch.
4. Slip. The apparent slip of the screw depends upon a slip, great variety of circumstances. It is modified by the diameter and by the speed, being generally found to diminish as these increase. Thus, the diameter of the Rattler's screw, during her experiments, was 10 feet, and her average slip 15 per cent.; while the Dwarf and Fairy, with screws of 5 or 6 feet diameter, show an average slip of about 35 per cent. The form of the after-lines of the vessel has a very notable effect on the apparent slip of the screw, which must not be regarded as a measure of the efficiency with which the propeller is acting. On the contrary, many vessels whose lines are most unfavourable for speed show an exceedingly small slip of the propeller; and in some instances of this kind there is not only no slip apparent, but the screw has actually what is called negative slip, which implies that the vessel is going faster than the rate at which the screw which propels it would advance if working in a solid. This curious and paradoxical result is due to the negative current which all ships, more or less, but especially those slip, with full stems, carry in their wake; and since the screw acts in this current, the apparent slip will be positive or negative in proportion as the real slip, or the velocity of the current, may preponderate; but in every case the screw must have some slip relatively to the water in which it acts. Suppose, for instance, that a badly formed ship has a current of water following in its wake, and closing in upon the screw at a velocity of 4 miles an hour, while the real slip of the screw is but 3 miles an hour, the result will be that the screw will show an apparent negative slip of 1 mile an hour. It must not be supposed that in such a case the power of the engines is economically applied, for, in fact, much power is useless consumed in dragging this current of water after the ship. The same apparent diminution of slip is always found when the vessel is advancing with a tide or current. Anomalies of this kind most frequently occur in auxiliary screw-steamers, where the vessel, after attaining a high velocity by sails alone, still continues to receive a propelling thrust from the screw, even after the speed of the latter appears to be less than that of the vessel.
In order to give the reader some perception of what really are the conditions of the screw most conducive to screws in speed in the vessel, I have selected the trials of twelve different screws made in the same vessel, the Rattler, arranging them in the order of their relative efficiency, beginning with the lowest. STEAM NAVIGATION.
1. With a four-threaded Woodcroft's increasing pitch screw, 9 feet diameter, 1 foot 7 inches long, and the pitch varying from 11 feet to 11 feet 6 inches (mean 11-275), the speed of the vessel was 8-159 knots; the engines making 24-15 revolutions per minute, and the screw 96—slip, 22-1 per cent.
2. With a three-threaded common screw, 9 feet diameter, 3 feet long and 11 feet pitch, the speed of the vessel was 8-23 knots; the engines making 24-2 revolutions, and the screw 94-3—slip 19-66 per cent.
3. With Sunderland's propeller, 8 feet in diameter, the speed of the ship was 8-38 knots; the engines making 17-49 revolutions, and the screw-shaft 69-97.
4. With the same screw as No. 2, reduced in length to 1 foot 7 inches, the speed of the vessel was 8-57 knots; the engines making 24-8 revolutions, and the screw 98-4—slip, 19-7 per cent.
5. With the same screw as No. 1, but with two of the blades cut off, the vessel's speed advanced to 9-06 knots; the engines making 29-6 revolutions, and the screw 107-3—slip, 25-97 per cent.
6. With a two-threaded common screw, 10 feet diameter, 3 feet long, and 11 feet pitch, the speed of the vessel was 8-958 knots; the engines making 24 revolutions, and the screw 95—slip, 13-8 per cent.
7. With a four-threaded common screw, 9 feet diameter, 1 foot 7 inches long, and 11 feet pitch, the speed of the vessel was 9-18 knots; the engines making 26-3 revolutions, and the screw 104-4—slip, 27-7 per cent.
8. With a two-threaded common screw, 9 feet diameter, 3 feet long, and 11 feet pitch, the speed of the vessel was 9-25 knots; the engines making 26-8 revolutions, and the screw 106—slip, 19-5 per cent.
9. With the same screw as No. 6, shortened to 2 feet, the vessel's speed increased to 9-448 knots; the engines making 25-5 revolutions, and the screw 107—slip, 19-5 per cent.
10. With the same screw as No. 6 further reduced in length to 1 foot 6 inches, the speed of the vessel was 9-811 knots; the engines making 27-92 revolutions, and the screw 110-7—slip, 18-3 per cent.
11. With the same screw as No. 2 further reduced in length to 1 foot 2 inches, the speed of the vessel was 9-88 knots; the engines making 27-39 revolutions, and the screw 108-4—slip, 15-97 per cent.
12. With the same screw as No. 6 further reduced in length to 1 foot 3 inches, the speed of the vessel increased to 10-74 knots; the engines making 26-19 revolutions, and the screw 103-97—slip, 10-42 per cent.
Trials made with her Majesty's Steamer Flying Fish.
| Screw | Diameter | Pitch | Revolutions of Engines | Speed of Ship | Indicated Horse-power of Engines | |-------|----------|-------|------------------------|--------------|--------------------------------| | | Ft. | In. | Ft. | No. | Kts. | n.p. | Kts. | | Common Screw | 11 | 0 | 21 | 4 | 82 | 17-283 | 325 | 11-822-26 | 11-585 | | | 11 | 0 | 21 | 4 | 79 | 16-737 | 321 | 11-586 | 11-586 | | | 11 | 0 | 20 | 0 | 75 | 16-082 | 263 | 11-667-76 | 11-736 | | Griffith's Screw with feathering blades | 13 | 1 | 19 | 0 | 74 | 14-704 | 21 | 11-60 | 11-603 | | | 13 | 1 | 17 | 0 | 77 | 13-406 | 155 | 11-98-55 | 11-284 | | | 13 | 1 | 16 | 0 | 77 | 13-00 | 105 | 12-27-00 | 11-640 | | | 13 | 1 | 15 | 0 | 83 | 12-197 | 42 | 12-65-00 | 11-508 | | | 13 | 1 | 18 | 0 | 75 | 13-411 | 145 | 12-20-20 | 11-461 | | | 13 | 1 | 17 | 0 | 77 | 13-00 | 111 | 12-65-80 | 11-552 |
A very interesting series of experiments has been recently made with the screw-frigate Doris, to determine the most suitable form of screw-propeller for our steam-vessels of war. The following is a résumé of the principal results arrived at, the first five trials having been made with the common or admiralty screw, and the last three with Griffith's propeller. The engines of the Doris (by Messrs J. Penn and Son) are of 800 nominal horse-power. The draught of water while under trial was kept constant at about 19 ft. 6 in. forward and 21 ft. 9 in. aft—exact mean, 20 ft. 6 in.; giving an immersed midship-section of 7421 square feet. Pressure of steam in boilers, 20 lbs.; indicated horse-power, about 3000. The first trial with the admiralty screw was with a diameter of 18 feet, the vessel's speed being 11-823 knots. On the second trial, with the diameter increased to 20 feet, the speed realized was 11-826 knots, with a great increase of vibration; steering imperfect. On Steam Na-
the third trial, the "leading" corner of each blade was cut off, and in this form the common screw attained its greatest speed, giving a result of 12-032 knots, with 50 revolutions of engines per minute, and 2884 indicated horse-power; vibration reduced, and steering good. On the fourth trial, both the corners of each blade were cut off, so as to assimilate the blades to Griffith's form, when, with a greater number of revolutions, the speed fell off to 12-012 knots. In the fifth trial, with the "following" corner of each blade cut off, but the screw restored to its perfect form in every other respect, a result of 11-815 knots was obtained. The common screw was then removed, and the next trial was made with Griffith's propeller, 20 feet diameter and 32 feet pitch; this gave a result of 11-981 knots, there being scarcely any vibration, and the ship steering well. The second trial, with the same Griffith's propeller, having the blades set at 26 ft. 6 in. pitch, gave a speed of 12-269 knots, being the highest of the series; steering perfect, and no vibration perceptible; indicated horse-power, 3091. The third trial of Griffith's 20-feet screw, with the blades set at a medium pitch of 30 feet, gave 12-158 knots.
Several important points connected with the screw-propeller seem to have been proved by these trials—1st, That Doris's leading edge of the screw is the part that mostly affects the steering of the ship, and also causes the greater part of the vibration; 2d, That increased diameter of the screw is better than increased pitch for reducing the speed of the engines, but it considerably increases the vibration with the common screw; whereas with Griffith's it did not produce that effect, in consequence of its chief propelling surface being towards the centre. The common screw, when its blades are cut to the form of Griffith's, is not so effective as when the centre sphere is applied to them. The power required to obtain the same speed was very much the same for both screws.
The annexed sketch is interesting from the peculiar markings shown upon the surface of the screw, which is that of the steamer Croesus. This vessel had on one occasion got under way while the paint on her screw was still wet, and on being docked soon afterwards the paint was found streaked by the water, as here shown.
Official explanation of the Table of "Results of Trials made in her Majesty's Screw-ships."
"The numbers in the last two figure-columns of the table show approximately the relative excellence, in respect of speed, of the forms of the various vessels, conjoinedly with the relative efficiency of the propeller, as adapted to each of them.
"The formula by which the calculations are made are founded on the assumption that the resistance of the vessel varies as the square of the velocity; therefore, that the power required to produce that velocity varies as the cube, and that the usual effect of the engine—that is, the effect which remains after deducting the power absorbed in overcoming friction, working air-pumps, &c.—bears a constant ratio to the power developed in the cylinder, known by the term 'Indicated horse-power.' The resistance is, in the first of these columns, assumed to vary, ceteris paribus, as the area of the midship-section, and in the last column as the square of the cube-root of the displacement." None of these assumptions, however, more especially the last two, are absolutely correct, but probably they are not so far from the truth as to render useless and uninteresting a comparison of which they are the basis, made between the performances of any two screw-vessels; while between two vessels which do not materially differ in engines and displacement, or in the area of their midship-sections, such a comparison is not only highly interesting, but it may prove of great value in pointing out the forms of vessels and proportions of propellers which ought to be adopted. In some cases, however, it becomes necessary to make any other comparison than that of speed. For example, as may be seen in the table printed in 1850, the Texier after her form had been improved, went above a knot an hour faster with 40-horse engines than she had previously gone with engines of 100 horse-power. Again, these engines of 100 horse, when transferred to the Rideau—a vessel approaching to double the tonnage—drove her, after her form had been altered, as fast as she was previously driven by engines of double the power, and nearly two knots faster than the same engines drove the smaller vessel before the alteration of her after-body.—Admiralty, August 1856.
The only other mode of steam-propulsion which has been attended with any considerable success is that known as Ruthven's water-jet system, in which the propelling power is derived from the reaction, or recoil, of two jets of water projected, at a high velocity, from nozzles at the ship's side. The first experimental vessel on this principle was built by Messrs. Ruthven, of Edinburgh, in 1843, and was tried on the Firth of Forth, when it attained a speed of from 6½ to 7 miles an hour. This was an iron-boat, 40 feet long.
More recently, in 1853, the Enterprise was constructed on Ruthven's principle, for deep-sea fishing, a preference being given to the jet propeller in this case, from its being less likely to interfere with the fishing-nets than the screw or the paddles. The dimensions of the Enterprise are as follow:—length of deck, 95 feet; length on the water-line, 87 feet; breadth of beam, 16 feet; depth, 8 feet; draught at load-line, 4 feet; burthen, 100 tons. The propelling power is derived from two pairs of horizontal oscillating cylinders, each 12 inches in diameter, and 24 inches stroke (condensing), working a vertical shaft. There is one cylindrical boiler, 6 feet in diameter, and 5 feet long, with two fire-tubes running through it, each 22 inches diameter, and 105 return flue-tubes, each 5 feet long, and 2 inches internal diameter. The propeller consists of a fan-wheel, or centrifugal pump, 7 feet in diameter, with curved blades, keyed on the lower end of the vertical crank-shaft; this revolves horizontally in a water-tight casing into which the water from the sea flows (along a covered passage), through crescent-shaped openings in the bottom of the hull. The water is expelled laterally, from the fan-wheel, in two continuous streams, through curved pipes with nozzles, 10 inches in diameter, protruding from the sides of the hull. The nozzles turn in collars fixed to the ship's side, so that they can be pointed a-stern or a-head, as required, for forward or backward motion, or downwards, when the vessel is to remain at rest. These changes can be made rapidly and easily from the deck, since the nozzles alone require to be operated upon, while the engine continues to work at full speed. Again, by setting the nozzles in opposite directions, one pointing a-head and the other a-stern, the vessel can be turned on the spot, swinging on her beam without the aid of the rudder; and she could thus be steered by the nozzles in case of the rudder being lost or disabled, the manoeuvring of the vessel being entirely in the hands of the officers on deck. The vessel progressed very smoothly, without tremulous motion.
In a trial trip with the Enterprise on the 16th of January 1854, from Granton to Kirkcaldy, in the Firth of Forth, and back, a distance of 10½ miles each way, the speeds obtained were 9½ statute miles per hour going, and 9 miles per hour returning, giving an average of 9¾ miles per hour—the engine making 50 revolutions per minute. On another occasion, she is stated to have made a considerably higher speed, the engine making 65 revolutions per minute. Steam Na-
The draught of the vessel, during the trial, was 3 feet 2 inches; and the immersed midship-section, 40½ square feet. The indicated horse-power of the engine was not known, no indicator-diagrams having been taken. In such an arrangement, much power is necessarily lost in communicating to the water which enters the propeller a velocity equal to that of the ship, besides a considerable loss from friction, eddies, &c.; but upon the whole, the power of the engine seems to be applied to considerable advantage. Even allowing that the speed attained does not equal that Advan-
from paddles or screw, the jet-propeller possesses other un-
tages of the doubted advantages which recommend it for special cases, water-jet
as, for instance, in the Government floating-batteries and propeller,
steam-rams, where the screw and the rudder are particu-
larly liable to be fouled by wreck and cordage. It would also be preferable to the screw in cases of river-steamers
of very light draught, where the paddle might not be ap-
plicable. Several of the large floating fire-engines on the
Thames have been fitted with this propeller, the water
being ejected by the powerful steam-pumps with which
these vessels are fitted. The speed, however, has in these
cases not proved satisfactory. A steamer, called the
Albert, propelled on this principle, was placed on the
Rhine, as a passenger-boat, a few years since, but did not
attain a speed proportional to her power or consumption
of fuel.
It is frequently asked, Whether is the paddle-wheel or Paddle
the screw the most efficient propeller? This question may and screw
now be safely answered, by asserting, that when both are compared,
in their best trim, and both are equally well proportioned
to the engines and vessel, they are, as nearly as possible,
equally efficient. It follows, therefore, that the preference
for one or the other, in any particular case, depends entirely
upon the class of vessel and the nature of her service.
The objections to the paddle, as compared with the screw,
may be thus briefly stated, namely, the unequal immersion
of the wheels, according as the vessel swims light or deep;
the obstruction to the sailing of the ship caused by the
resistance of the paddle-boxes to the wind; and the dragging
of the paddle-boards through the water when the engine
power is not used, and the wheels are not disconnected;
and, in the case of steam-vessels of war, the exposure of the
wheels and machinery to an enemy's shot. The advantages
of the paddles, on the other hand, are, that they are not so
much affected by the pitching motion of the ship, when
steaming head to wind, as the screw is; that they do not
require such a speed of engine (or else gearing); and that,
from the disposition of the weights in respect to the centre
of gravity of displacement, the movements of the vessel are
easier than those of a screw-steamer, a matter of consider-
able interest to the passengers at least. With regard to
the screw, its efficiency is but little impaired by variations
of trim in the ship, but it is most injuriously affected by the
pitching motion. Its advantages in facilitating the sailing
of the ship are self-evident, and have been already alluded
to. In fine, the superiority of the screw for sea-going
steamers appears to amount to this, that it retains its
efficacy as a propelling agent under a greater variety of
conditions of sea, weather, and trim, than the paddles, and
that it admits of more use being made of the sails and a
greater display of seamanship in the navigation of the vessel.
Under proper management, therefore, it appears to be more
economical of steam-power than the paddle-wheels; and
this, it may be remarked, is the actual experience of the
Peninsular and Oriental Company, whose steam fleet is
composed of vessels on both principles.
Having thus briefly considered, firstly, the engine-power
of the steamship, and secondly, the immediate propelling
agents employed to produce locomotion, it will now be
necessary to view her as a completed whole, and to ex- Steam Navigation.
amine some of the general properties and qualifications inherent in, or demanded by, this complicated structure, as well as the relations they severally bear to each other. The construction of the steamer's hull will be found amply detailed in the article Ship-Building, so this need not be here adverted to.
When a steamer is once set in motion, the motion is, of course, continued by her momentum, and she would then evidently continue to advance at a uniform speed, without any more force being applied to her, were it not for the opposition of external causes. These external forces which she encounters, and which are constantly at work to destroy her momentum, and bring her to a state of rest, are the resistance of the water to her hull, and the resistance of the air to her upper works and rigging; the impetus of the waves and the winds being exerted sometimes in her favour, and at other times against her. The power of the steam-machinery is, therefore, applied to counteract these retarding forces, and to maintain a certain amount of progressive motion in the ship, depending upon the resistance on the one hand, and the power of the engines on the other.
The resistance offered by the water to the passage of the hull must be divided into two parts; firstly, that due to the dividing and displacing of the water, to make room for the hull of the ship to pass through (which is analogous to scooping out a long trough or canal, of the full breadth of the ship); and, secondly, the resistance arising from the friction of the water upon the sides and bottom of the vessel. Of these resistances, the first is by much the more serious, although the second must not be overlooked. The resistance offered to the passage of the hull depends mainly upon the area of the immersed midship-section of the ship, (or its greatest cross-section), but also very materially upon the form of the vessel's lines under the water. There is considerable discrepancy of opinion as to the relative value of these two functions of the ship; one naval constructor relying for speed upon a small immersed midship-section, while another holds that fine lines for dividing and closing the water are still more essential. The lines of a ship undoubtedly exert a great influence upon her speed, as has been shown experimentally in numerous instances. In the case of the Government dispatch-boat, Flying Fish, (already referred to at page 660), this vessel, of 1050 tons displacement, attained a speed of only 11·73 knots, with 1166 indicated horse-power. That performance did not equal the expectations of the Admiralty authorities; and without making any variation in the other parts of the vessel, they added, not a new bow, but an elongated bow, 18 feet in length, in advance of the original one, to divide the water more freely. The result was, that with the same draught of water, the velocity of the vessel increased from 11·73 knots to 12·55 knots an hour.
Mr J. Scott Russell, in the course of a discussion on this subject at the Institution of Civil Engineers,1 has given some very interesting results of experiments, all tending to show that the shape of the vessel has a very decided influence upon her speed, irrespective of her engine-power. He relates that he had, on one occasion, the control of four timber ships of the same dimensions, the same displacement, and the same horse-power; but each had different lines, being constructed by different ship-builders. The engines were all alike, being made by the same firm. The result was that, upon a run of 16 miles, their several speeds were 12½, 12, under 11, and between 10 and 11 miles an hour. In another instance, a steamer, constructed to go both ways, but built with one end finer than the other as an experiment, went fully a knot faster one way than the other, although the midship-section and the horse-power were necessarily identical at all times. A third case was the following:—Two vessels were built of the respective lengths of 190 feet and 186 feet, their breadths being equal. The engines were the same in each, the cylinders being 48 inches diameter and 4 feet 6 inches in stroke, making 39 revolutions per minute. The speed attained by the first vessel, however, was 15·08 knots, while that of the second was but 11·32 knots. The difference in the two vessels consisted mainly in the shape, the other and minor elements being much in favour of the slower vessel. For instance, the faster vessel had 124 feet of midship-section, whilst the slower vessel had but 71 feet of midship-section to drag through the water. The faster vessel drew 6 feet 8 inches of water, whilst the slower vessel drew only 2 feet 10 inches. The difference in length was only 4 feet, yet a radical difference in shape thus reduced the velocity, with equal power, from 15 knots to 11 knots per hour.
Colonel Beaufoy's experiments determined the resistance of the water to a ship with a square head only, and it has since been found that a semicircular or round head offers two-thirds of the resistance derived from his formula
\[ R = \frac{av^2}{2g} \]
and an elliptical head considerably less.2 By making the bow still finer, the resistance had been gradually reduced to one-sixth, and one-eighth, of that given by the formula; and Mr Scott Russell believes that the engine-power required to drive a large vessel through the water has now, in some cases, been reduced as low as one-twelfth. We learn from the same authority (the highest, indeed, which it is possible to adduce on this subject), that with a vessel of proper form, measuring about 1500 tons, the resistance of a ship can be reduced to 50 lb. per square foot of immersed midship-section, while steaming at the rate of 10 knots an hour. This is the direct resistance of the water upon the hull, and Mr Scott Russell asserts that he has thus been enabled to calculate confidently, to within a quarter of a knot, the amount of steam-power necessary to propel a given ship at a given speed, basing the calculation upon his own peculiar form of "wave-line," there being necessarily a shape for every speed. For instance, when a speed of 10 knots an hour was desired, he provided engine-power for 50 lb. per square foot of immersed midship-section (exclusive of the resistance of the machinery, which brought it up to 65 lb. per square foot), for a vessel of about 1500 tons, built on the "wave-line" construction. These figures, 50 lb. and 65 lb., are gross resistances, and include friction of skin.
With regard to the absorption of power by the friction of prismatic skin, it is seen by every day's experience how much a vessel will fall off in speed by the fouling of her bottom. This often amounts to a loss of one-fifth of her original speed, the engine-power exerted remaining the same, so that, under these circumstances, double the power would be required to attain the same speed as before. The total immersed surface of the Rattler's hull has been calculated at 7000 square feet, and according to Beaufoy's experiments on the friction of immersed surfaces, the resistance thus arising would be eight-tenths of a pound per square foot for a speed of 10 knots an hour, and nearly 1 lb. per square foot at 11 knots. At the speed and friction first named, the power absorbed would be equivalent to nearly 170 r.h.p., the total r.h.p. of the engines amounting to 428 r.h.p. only. In the case of the Himalaya, an immersed surface of about 18,000 square feet is exposed, the friction from which, at a velocity of 13 knots an hour,
---
1 See Transactions of Institute of Civil Engineers, Session 1856–57. 2 In the formula in the above sentence \( a \) represents the midship-section in square feet, \( v \) the velocity of the vessel in linear feet per second, \( g \) the accelerating force of gravity \( = 32 \frac{1}{2} \) and \( w \) the weight of a cubic foot of sea water at 61\(\frac{1}{2}\) lb. avoirdupois. would absorb about 650 l.h.p., supposing the bottom to be perfectly clean.
The consideration of frictional resistance, of course, places a limit to increase of length in a steamer, although many instances have occurred in which the vessel has gone as fast, or very nearly as fast, with the same engines, and on the same draught of water, after some 30 or 40 feet have been added in midships. The Candia is a remarkable instance of this, as will be seen by the following comparison of her speed when originally built, and after she was lengthened in midships by 33½ feet, her load displacement being thereby increased about 470 tons:
| Date of trial | Draught of water | Length | Displacement | Tons | Tons | Number of boilers | Speed of steam | Speed of water | |---------------|-----------------|--------|--------------|------|------|------------------|---------------|---------------| | May 31, 1854 | 18½ ft | 281 | 2,620 | 50 | 364 | 2 | 12½ knots | 12½ knots | | Aug. 12, 1857 | 19½ ft | 314½ | 3,600 | 100 | 33 | 2 | 12½ knots | 12½ knots |
Although in this instance, from some unexplained cause (owing possibly to improved trim, or circumstances of wind and sea), it would appear from the trial trips that a considerable increase of length has been obtained without any corresponding absorption of power, there must necessarily be a limit where further extension of length is more than neutralized by increased frictional resistance.
It is universally admitted that the gross resistances (direct and frictional) to which a vessel is subject increase as the square of the velocity, and therefore, as a necessary consequence, the power expended in producing this velocity varies as the cube of the velocity. For instance, if the resistance to one square foot of midship-section propelled through the water at 5 miles an hour be 5 lb., then the resistance at 10 miles an hour would be four times 5, or 20 lb. But the latter resistance has acted over double the space, so that the result must be again doubled for the measure of the power expended; and hence the power developed in one hour must necessarily and unavoidably be as the cube of the velocity. This rule cannot be expected to hold strictly good in all steamers alike, looking to the great diversity of form and displacement which exists, but in the great majority of cases it is fully borne out in practice. Thus, in H.M. screw-steamer Desperate, the following relation between power and speed was found to obtain:
| Indicated Horse-Power | Knots | Coal per l.h.p. p.hour | |-----------------------|-------|-----------------------| | With 4 boilers and 4 cylinders | 805-89 | 9½ | 4½ | | With 3 boilers and 4 cylinders, working expansively | 579-32 | 8½ | 5½ | | With 2 boilers and 4 cylinders, do. | 363-87 | 7½ | 5½ | | With 1 boiler and 2 cylinders, do. | 169-32 | 5½ | 5½ |
The Retribution, paddle-wheel steamer, had a speed of 10¾ knots with 1092 l.h.p., and a speed of 6½ knots with 226 l.h.p. The Onyx, with 2 boilers and 533 l.h.p., realized a speed of 13½ knots, whilst with one boiler and 158 l.h.p. the speed was 8½ knots. The Minx, with 234 l.h.p., made 9½ knots, and with 31½ l.h.p. 4½ knots. These, and many other instances, are all in accordance with the rule, that the power and consumption of fuel vary as the cube of the velocity.
The practical value of this rule will be made apparent by the following examples:
1. If it be wished to find the speed corresponding to a diminished consumption of fuel for any particular steam-vessel, the calculation will be effected thus:—The vessel, we will suppose, has engines which propel her at the rate of 12 knots, with a consumption of 33 tons of coal per diem, and we wish to find the speed corresponding to a consumption of 25 tons per diem; then—
\[ \frac{35}{25} : : \frac{12^3}{V^3} \] (cube of required velocity).
When reduced, \( 7 : 5 :: 1728 : V^3 \)
As an equation, \( 5 \times 1728 = 7 V^3 \);
or, \( \frac{8640}{7} = V^3 \).
And \( \sqrt[3]{1234} = V^3 \) 10-726 knots = V, the required velocity.
It is thus seen, that by reducing the consumption of fuel by 10 tons per diem, we lose in this instance about 1½ knot per hour.
2. If it be wished to increase the speed of the vessel, on the other hand, from 9 to 11 knots, and we desire to know the increased consumption attending the increase of speed, this will be in the proportion of \( 9^3 : 11^3 \), or as the numbers 729 : 1331, or as 1 : 1.825.
All we have to do, therefore, is to multiply the present consumption by this latter number.
3. If a certain steamer consumes, say 220 tons of coal, during a run of 1600 miles, performed at the average speed of 11 knots per hour, and we wish to find her probable consumption of coal for a longer voyage of 2400 miles, at a reduced speed of 9 miles, the calculation will then be as follows:
220 tons coal : C (regular consumption) :: 11½ knots × 1600 :
\( \frac{220}{C} \) knots × 2400 miles,
Then \( C \times 121 \times 1600 = 220 \times 81 \times 2400 \);
or, \( C \times 193,600 = 42,768,000 \).
Reduced to \( C = \frac{427,680}{1936} = 220-9 \) tons, required consumption.
It is thus seen that the consumption of fuel is almost exactly equal in these two cases, showing that the same vessel would steam 1600 miles at 11 knots, or 2400 miles at 9 knots, with the same quantity of coals.
4. Supposing that we have a steamer with stowage-room for only 460 tons of coal, which she has nearly expended during a trip of 1800 miles, while steaming at the speed of 11½ knots an hour, and we wish to place her upon another station, where she must run 2500 miles without coaling, it is required to find at what reduced speed she must steam so as not to run short of coals?
| tons | knots | knots | tons | knots | |------|-------|-------|------|-------| | 460 | 11½ | 1800 | 460 | 2500 | | | | | | \( V^2 \) required velocity;
or, \( 460 \times 132\frac{1}{2} \times 1800 = 460 \times 2500 \times V^2 \);
reduced to \( 109,503 = 1150 V^2 \);
or, \( V^2 = \frac{109,503}{1150} = 95-04 \).
Therefore, \( V = \sqrt{95-04} = 9-75 \) knots, required velocity.
We thus find that the same vessel which ran 1800 miles at a speed of 11½ knots, and with a consumption of 460 tons of coal, must reduce her speed to 9½ knots, to enable her to run 2500 miles with the same consumption.
The preceding examples all show that an increase of speed Efficiency is obtained only by the expenditure of a very great increase of power. Hence, to draw even the most superficial comparison between the efficiency of different steam-vessels, their speeds must first be reduced to a common standard, and the relation must then be found between the consumption of fuel at the standard speed, and the size or tonnage of the vessel, the maximum speed of each being treated as a separate question. The value of the term efficiency also varies so much for different classes of vessels, that steamers of the same class only can be justly compared together. The number of tons displacement that 100 gross or indicated horse-power will propel, at the rate of 10 knots an hour, has been proposed as a standard of comparison between different steamers.
A vessel, for instance, is known to have a speed of 12 Modes of knots an hour, the engines exerting 1620 indicated horse-power, at a displacement of 2240 tons.
Then, as 12 knots : 10 knots :: \( \frac{1620}{2240} \) h.p. : \( \frac{1620}{2240} \times 937\frac{1}{2} \);
or, \( 1728 : 1000 :: 1620 : 937\frac{1}{2} \);
and \( 937\frac{1}{2} : 2240 :: 100 : 2389 \) = tons displacement propelled by 100 l.h.p., at 10 knots an hour.
By making similar calculations for other vessels, their relative efficiency may be, to a certain extent, compared one with the other. It is found, in practice, however, that the form of the vessel influences the ratio existing, theoretically, between the power exerted and the resulting speed. Steam Na. Thus, in the Flying Fish, before she was altered, an increase of only 1-68 knots in the speed of the ship was obtained by doubling the indicated horse-power; but after a fine bow was fitted, she gained 2-452 knots by doubling the power, the latter increase of speed being just proportional to the cube of the extra power exerted.
A formula frequently employed in comparing the relative merits of vessels is \( V^3 = \frac{D^3}{H.P.} \), which is thus expressed in words:—The cube of the speed in knots, multiplied by the square of the cube-root of the displacement, and divided by the indicated horse-power. The resultant number is called the co-efficient of dynamic duty for that particular steamer, and forms a criterion of the cost at which she performs her work, the higher the co-efficient the greater being the economy. In a steamer of good average performance, the co-efficient, as calculated by this rule, should lie between 250 and 320, or thereabouts.
As the preceding formula does not take note of the area of immersed midship-section, the following is also useful as a means of comparison:
\[ V^3 \times \text{midship-section} = \frac{H.P. \times G.}{D^2} \]
The two formulae next to be given are used indiscriminately for estimating the probable speed of a steamer, viz.—
No. 1. \( V^3 = \frac{\sqrt{H.P. \times G.}}{D^2} \); or, when expressed in words, the cube of the velocity equals the square-root of the nominal horse-power, multiplied by the diameter of the cylinder in inches, divided by the square of the cube-root of the displacement.
No. 2. \( V^2 = \frac{2}{3} \times \frac{H.P. \times 100}{\text{mid. sect.}} \)
The speed of the Candia, when measured by the first of these rules, is 11-38 knots, and by the second, 11-26 knots; while her actual speed, under the same conditions of displacement, &c., is 11-93 knots. In the same way the speed of the Pera, by the first rule, is 11-22; by the second, 11-25; and actual, 12-55 knots. The actual speed, in both of these cases, is therefore in excess of that calculated by the formulae.
In proportioning the horse-power of a steamer, the fact must be borne in mind that the effective power of the engines increases in a higher ratio than simply as the tonnage, since the resistance varies as the square of the cube root of the tonnage. Thus, if a vessel of 1200 tons and 400 horse-power have a speed of 12 knots, a similarly constructed vessel of 1650 tons and 550 horse-power (with the same proportion of power to tonnage), ought to have a considerably higher speed, since the square of the cube root of 1200 being 112-78, and the square of the cube root of 1650 being 139-47, the proportion will then be 112-78 : 400 :: 139-47 : 494-6 horse-power, instead of 550 horse-power.
The proportion of horse-power to tonnage recommended for different classes of ocean steamships may be stated as follows:—For full-powered passenger paddle-steamers of from 500 to 1200 tons (builders' o.m.), 1 horse-power to 3 tons; for ditto, of from 1200 to 3000 tons, 1 horse-power to 4 tons; for full-powered passenger screw-steamers of from 500 to 1200 tons, 1 horse-power to 4 tons; for ditto, of from 1200 to 3000 tons, 1 horse-power to 5 tons; for auxiliary screw-steamers, 1 horse-power to 6, 7, or 8 tons, according to size.
It must not be supposed that a steamer can, or ever will be constructed by the sole aid of "formulae," which are themselves, for the most part, empirical. They serve, however, to assist in estimating the value and tendency of the several proportions and attributes of the structure, a wide margin being left for the exercise of practical sagacity and experience on the part of the constructor. To sum up, in a few words, the main elements upon which economy in steam navigation seems to depend, these are—a fine form, a moderate speed, considerable magnitude, a clean bottom, and a high ratio of length to breadth; to which may be added, effective engines, and a properly proportioned propeller. Again, after the naval constructor and engineer have each done their best, much still remains for the skill of the commander and officers of the ship.
With regard to the relative proportions of the hull, our fastest and most successful ocean merchant-steamers of the present day have their length and breadth as 7, 7-5, or even 8 to 1 for iron screw-steamers, wooden hulls being proportionally confined to 6-5 and 7 times the beam. The proportions of depth vary very much, but this should never exceed eight-tenths of the breadth, and is better limited to six-tenths. A very deep steamer is always unweatherly and unmanageable, and often dangerous. The proportions in actual use will be seen by inspecting the tables of steamers in the royal navy, and the table of merchant-steamers, at pages 669, 670, 671.
With reference to the management of steamers at sea, a full-powered passenger-steamer is generally so tied to time of sea that she cannot afford to disuse any portion of her steam-power, contenting herself with expanding more or less in the cylinders, according as the wind and sea are propitious or otherwise. Every opportunity of a fair wind, however, should be eagerly seized for hoisting sail, which, in the case of a screw-steamer especially, affords a great addition to the power of the engines. Steam-vessels of war, on the other hand, are expressly designed to sail well, in addition to their steaming powers; and in estimating the performance of a Government steamer, we should rather look to the direct distance run by the combined action of steam and sails, at a moderate but uninterrupted speed, and with a small consumption of fuel, than to the attainment of a high velocity, which is seldom wanted in war-steamers. In steaming against a strong head wind, with a paddle-wheel steamer of moderate steam-power, it is found preferable to keep the vessel in a direct course as long as possible; but, so soon as her head begins to fall off for want of good steerage way, the fore and aft sails should be set, and the vessel tacked, the engines being kept working. In the screw-steamer, on the other hand, there is no economy in keeping a direct course against a head wind and sea, after her speed is reduced to three or four knots an hour, since the engines keep up their usual number of revolutions, and the steam is mostly wasted in slip, or during the "racing" of the machinery. In such a case, therefore, the ship's course should be altered, and the sails set to assist the screw, so soon as the speed falls to this amount. In paddle-wheel engines, the waste of steam is not so great while going head to wind, since the revolutions decrease with the speed of the vessel. When a vessel is steaming against an opposing stream or tide, it is found that her engine-power is most economically applied when she goes half as fast again as the velocity of the stream. Notwithstanding the many undoubted improvements which have been recently introduced in the application of the screw propeller, and the extended experience we have now had of its operation in different classes of ships, and under every variety of trial, the position which it holds in the merchant service, either as an antagonist to the paddle-wheel in full-powered steamers, or as an auxiliary to the sails in sailing ships, is still far from being well defined. Before bringing this article to a close, it is proposed to give a few examples of steam-vessels which have either proved unusually successful, and may, therefore, stand as types of their class, or have some peculiarity of structure which seems to point them out for special notice.
1. Duke of Wellington, 131 guns, steamship of the line, is 240 feet 6 inches long between perpendiculars, and 60 feet broad. Tonnage, 3825; builders' o.m., 780; indicated n.p., 2500. Machinery by Robert Napier & Sons, of Glasgow. Has 4 tubular boilers, each containing 5 furnaces of the following dimensions, viz.—7 feet 4 inches long + 2 feet 9 inches wide; the total space occupied by the machinery being 70 feet in length. The diameter of the shaft next the engines is 12½ inches. The screw has a double thread, 18 feet diameter, 16 feet 3 inches pitch, and 3 feet 4 inches long. The driving-wheel of screw-panning is 10 feet 6 inches diameter, working into a pinion (with wooden cogs), 4 feet 6 inches diameter, and 4 feet 5 inches broad. The speed of the vessel at her trial trip was 10 2/3 knots.
2. Mersey, 40 guns, screw steam-frigate, is 300 feet long between perpendiculars, and 52 feet beam. Tonnage, 3726; nominal h.p., 1000; indicated n.p., 4000. Makers of the machinery, J. Penn and Son. Pressure of steam, 20 lb.; mean number of revolutions of engines (direct), 554. Screw, diameter, 20 feet; pitch, 29 feet; immersion at trial, 6 inches; revolutions of screw, 554. Draught of water of ship, forward, 20 feet 8 inches; aft, 22 feet 7 inches. Coal on board, 800 tons; consumption of coal at full speed, about 140 tons per day, or 24 hours. Number of furnaces, 6; length of stoke-hole, 68 feet 10 inches; breadth of ditto, 10 feet; temperature of ditto, 100° Fahr. The tops of the boilers are 4 feet under the water-line; fitted with three auxiliary engines, two of which supply the boilers, and the third acts as a steam-fire-engine. Weight of shot fired by one broadside, 1652 lb. Speed at trial, 13 2/3 knots.
3. Rattler, screw-sloop, 179 feet 6 inches long between perpendiculars; 32 feet 8½ inches beam; 888 tons builders' o.m.; 13 feet 6 inches mean draught of water at trial; area of immersed midship-section, 330 square feet; displacement at trial, 1078 tons; nominal h.p., 200; ditto, indicated, 436; diameter of cylinders, 4 feet 10 inches each; length of stroke, 4 feet. Revolutions during trial, 120. Diameter of screw, 11 feet; length, 1 foot 3 inches; multiple of gearing, 4:1; revolutions of engines, vertical geared. Makers of the machinery, Mandalay, Sons, and Field. Maximum speed of the vessel at trial, 10 knots.
4. Growler, screw gun-boat. Length between perpendiculars, 100 feet; breadth, 22 feet; draught of water, mean, 6 feet 10½ inches; immersed midship-section at this draught, 130 square feet; horse-power—nominal, 60; indicated, 200; high pressure engines working direct, with 2 cylinders, each 15½ inches in diameter, and 18 inches stroke. Makers of the machinery, Maudslay, Sons, and Field. Pressure of steam, 50 to 60 lb.; weight of engines, 8 tons 14 cwt. Boilers, 3 in number, are cylindrical, with internal tubes. Length of boiler, 15 feet 4 inches; 4 feet diameter, with four furnaces in each, 2 feet 2 inches broad × 3 feet deep for each. Each boiler contains 82 iron tubes, 2 inches in diameter, and 3 feet long. Total grate-bar surface in the three boilers, 29½ square feet; weight of the three boilers complete, 13 tons 1 cwt.; weight of water in the three boilers, 9 tons. Screw, two-threaded, 6 feet diameter; 8 feet pitch; 16 inches long; weight, 840 lb. Total weight of the machinery with spare gear, 32½ tons. Coals carried in bunkers, 28 tons. Speed at above immersion, 8¾ knots; speed of engines and screw, 154 revolutions per minute; slip, 31 per cent.
5. Warrior, iron-cased steam-frigate, is built of iron; extreme length, 380 feet; breadth, 58 feet; depth, 41 feet 6 inches; tonnage, 6177. Weight of hull, about 5700 tons; fitted with engines of 1200 nominal horse-power, weighing 950 tons. Builders of the ship, the Thames Iron Co.; makers of the machinery, J. Penn & Son. She will carry 950 tons of coal, and the weight of armament, stores, &c., will amount to about 1200 tons. The total weight at sea will thus be about 9000 tons. Sheathed with wrought-iron armour-plates 4½ inches thick from 5 feet below the water-line to the level of the upper deck for 220 feet of the broadside, each plate being 15 feet long by 4 feet broad. Behind the iron armour-plates there is a thickness of 24 inches of teak, protecting all the fighting portion of the vessel. The bow and stern are not thus sheathed, being merely plated with thick iron plates in the usual way, and crossed by several water-tight bulkheads. Armament will consist of Armstrong guns, each capable of throwing a 100 lb. shot a distance of 5 miles. The total cost of each frigate will be about L320,000. Estimated speed, 14 knots an hour.
6. Victoria and Albert, H.M. steam-yacht, is built of tim-Steamb Na- vigation. ber, and has the following dimensions.—Length between the perpendiculars, 200 feet 1 inch; breadth of beam, 33 feet; depth of hold, 23 feet 9 inches; burthen in tons, builders' o.m., 2343; horse-power, nominal, 600; horse-power, indicated, at trial 2800. Propelled by paddle-wheels. Has oscillating engines by J. Penn and Son, with two cylinders, each 38 inches in diameter, and 7 feet stroke, the total weight of her machinery being 4011 tons. Her draught of water when complete for sea, with her reserve fuel on board, is 15 feet forward and 15 feet 9 inches aft. Revolutions of engines at this draught, 22; displacement at medium load-draught, 2120 tons. Has four tubular boilers, containing in all 3024 brass tubes, each 6 feet 5 inches long by 2½ inches external diameter. The boilers have also together, 24 furnaces, each 7 feet long by 3 feet wide, fired from two stoke-holes. The pressure of steam on the safety-valves, 16 lb. The steam is superheated; has two funnels, each 5 feet 6 inches diameter, and 40 feet 3 inches high from top of boiler. The coal-boxes contain 410 tons of coal. The paddle-wheels are feathering, 29 feet extreme diameter, 14 boards to each wheel, each board 11 feet 6 inches by 4 feet 2 inches. The wheels can be disconnected by a friction-disc and break. Speed at trial trip, 17 2/3 knots.
7. Fairy, H.M.'s screw steam-yacht, is built of iron, and has the following dimensions.—Length between the perpendiculars, 144 feet 8 inches; breadth extreme, 21 feet 1½ inches; mean draught of water at trial, 4 feet 10 inches; area of immersed midship-section, 71½ square feet; mean displacement at trial, 168 tons. Tonnage, builder's o.m., 312; nominal horse-power, 128; indicated horse-power, 364. Builders, Mare & Co.; machinery by J. Penn & Son. Diameter of screw, 5 feet 4 inches; pitch, 8 feet; length, 1 foot. Revolutions of screw at trial, 250 per minute; slip of the screw, 34 per cent. Has two vertical, oscillating, geared engines, with cylinders 12 inches diameter, and 3 feet stroke, making 51½ revolutions per minute. Speed of the vessel at trial, 15 3/4 knots.
8. Himalaya, steam troop-ship, propelled by paddle-wheels, is built of iron, and has the following dimensions.—Length between the perpendiculars, 341 feet; breadth, extreme, 46 feet 4 inches; depth of hold, 35 feet; tonnage, 2550; horse-power, nominal, 700. Has horizontal direct engines, with cylinders 8¼ inches diameter, and 3 feet 6 inches stroke, making 59 revolutions per minute. Boilers on Lamb and Summer's sheet-pan principle. Pressure of steam, 14 lbs. She can store 1000 tons of coal; her daily consumption, at full speed, being 702 tons. Fitted with the common 3-bladed screw, 18 feet diameter, by 28 feet pitch, making 59 revolutions per minute. Speed at trial, 13 3/4 knots. This steamer has served as transport, escorting troops made the trip from England to Alexandria, on several occasions, at an average speed of 12 knots an hour; and with a favorable breeze, she has been known to run 16 knots within the hour.
9. Troop River-steamers for India. These are now being built for Government of the following proportions.—Length on the water-line, 350 feet; length over all, 375 feet; breadth, 46 feet. Built of steel-plates, weighing about 5 lb. per superficial foot, with the exception of the keel-strakes, which are 7 lbs., and the girder-strakes 15 lb. per square foot. They are flat-bottomed, to draw only 2 feet of water, with machinery, fuel, stores, and 800 troops on board. The hulls are estimated to weigh only 370 tons. They are to be propelled by paddle-wheel engines of 200 horse-power. Their speed will be given by two large central steering-blades. The hull is stiffened by two iron girders, rising above the deck; and running for 300 feet of the length, from which vertical and diagonal trusses are carried. They will be sent to India in parts, after being put together and tried in this country. Estimated speed, 12 miles an hour.
10. Great Eastern, of iron. Length between the perpendiculars, 680 feet; length on deck, 691 feet; breadth, extreme, 83 feet; depth of side, 58 feet; draught of water, from 20 to 30 feet. Gross tonnage, 22,500. Nominal horse-power, 2600. Builder, J. Scott Russell. Her screw-engines of 1600 horse-power, by James Watt and Company; and paddle-engines of 1000 horse-power, by J. Scott Russell. The screw-engines are horizontal, direct, with 4 cylinders, each 84 inches diameter, by 4 feet stroke, making 48 strokes per minute; with tubular boilers, making 25 lb. pressure. The screws are of the common construction, with 4 blades, 2 feet diameter, with a pitch of 44 feet. The paddle-wheel engines are oscillating, with 4 cylinders, each 74 inches diameter, by 14 feet stroke. Boilers, tubular, 25 lb. pressure. Paddle-wheels are of the common construction, 50 feet diameter, 13 feet length of floats, and 3 feet depth. Total coals carried, 10,000 tons. Speed about 15 knots. Her ship-draught is shown in the plates illustrating article Ship-Building.
11. Persia, transatlantic mail-steamship, of iron, has the following dimensions.—Length between the perpendiculars, 360 feet; breadth of beam, 45 feet; depth of hold, 29 feet 8 inches. Medium load-draught of water, 21½ feet. Displacement at this draught, Steam Navigation.
16. Tachtalina, river-steamer for shallow navigation, is built of iron, and has the following dimensions: Length, 150 feet; breadth, 20 feet; draught of water, with machinery and passengers on board, 12 inches. Builders of the vessel and makers of the machinery, Messrs J. and A. Blyth. She is propelled by 4 condensing engines, of the collective power of 40 horses, acting through 4 paddle-wheels, each 6 feet in diameter and 6 feet wide; the forward engines making 87 revolutions and the after engines 95 revolutions per minute, and the speed through still water being about 11 miles per hour. The hull is formed of very thin plates, stiffened by frequent transverse bulkheads, and webs of plate surrounding the vessel internally. By employing 2 pairs of paddle-wheels, each pair driven by a distinct pair of engines, not only are the weights of the machinery lighter, and coals diffused over the vessel, but the revolving water is also widely distributed over the stream. The use of 4 paddle-wheels in place of 2 greatly improves the steering of the vessel, always a matter of much difficulty with boats of shallow draught running on swift rivers. Lightness of machinery in this vessel is promoted by the substitution of gun-metal for cast-iron in the condensers, &c., and of cast-steel and wrought-iron for the framing of the engines. The boilers are constructed of Lowmoor plates throughout, without angle-irons, to save weight. Vessels of this class are well adapted for the rivers of India. A representation of the Tachtalina is given at page 664.
17. Screw-steamer for the rivers of India, built of iron. Length, 70 feet; breadth, 7 feet 6 inches; depth, 3 feet 6 inches; draught of water, 2 feet. Boat and machinery constructed by Messrs G. Rennie and Sons. Propelled by two screws, one on each quarter (see annexed wood engraving, fig. 33); diameter of the screws, 2 feet 2 inches; pitch, 4 feet. Driven by a pair of disc engines, acting direct. Speed of the engines and screws, 260 revolutions per minute. Speed of the boat, 10 knots per hour. Weight of the boat, 3 tons 8 cwt.; weight of the machinery, 3 tons. Consumption of coals, 100 lbs. Power of traction, at slow speed, 250 tons. Cost of trackage, 1½ d. per ton per mile.
American Steamboats (by a correspondent of The Engineer).—Going aboard at a late hour in the evening, the scene which presented itself to our eyes was novel in the highest degree. Painted a pure white, as nearly all American river-steamboats are (for the anthracite coal burned under their boilers makes no smoke whatever), the enormous mass of the vessel rose like a giant iceberg above the water. Hurrying over the broad gangway, we found ourselves in a crowd of nearly 700 passengers, more than one-third of whom were ladies. We were upon the main-deck, although under a lofty ceiling, over which was a grand saloon of... Steam Navigation.
palatial proportions and magnificence. Looking aft, a broad entrance, flanked with gilded columns and luxurious drapery, opened to the ladies' saloon—a sanctum sanctorum not to be profaned by the footsteps of a bachelor, although steamboat etiquette was not so strict nor steamboat regulations so inflexible, as to forbid the monstrosity of such things of gentlemen accompanying their wives, or other fair charges, to be brought into the cabin at dinner-time. On either side of this entrance are broad staircases descending to an immense lower cabin, along the sides of which were more than 400 berths. The supper tables were then set out with a degree of splendour for which an English traveller would be altogether unprepared. Nearly amidships, on the main-deck, a grand staircase, sweeping both to the right and left, conducted to the great saloon, or state-room hall, nearly 300 feet in length, several yards in width, and having an upper gallery, with a second story of state-rooms—a lofty arched ceiling, glazed with ground and coloured glass, and supported by richly-carved columns, covering the whole. In its conception this steamboat (the New World) is totally unlike anything ever seen in Britain before. It was originally 275 feet long, originally 376 feet long, it was afterwards lengthened to 468 feet over all. With a breadth of beam of 50 feet, the main-deck is extended by means of platforms, or "guards," projecting over the water to the full width across the paddle-boxes, 85 feet, being thus wider than the main-deck of the Great Eastern. Yet the vessel, which is flat-bottomed, with bilges nearly or quite square, draws only 6½ feet of water, the whole displacement being about 2500 tons, and the immersed mid-section 275 square feet. All American boats have wooden hulls, and how to stiffen such a vast and shallow craft, flat-bottomed as Noah's Ark? There are no tabular cells, no "double skin," nor is there a hundredweight of boiler-plate, excepting in the boilers themselves, in the whole structure. As to the locomotive strain, the boilers, weighing 100 tons each, are placed upon the "guards" outside the hull, and are connected several feet above the load-line. To make the whole, as right as a tubular girder, two enormous arched trusses, placed one over each side of the hull, extend over nearly 350 feet of the length of the boat. These great bows, like the arches of a bow-string bridge, are connected to king-posts and queen-posts, and strapped and fastened, so that the whole is as stiff as a man-of-war. Then there are four or five large king-posts, or masts, stepped upon the keel, and carrying the weight of the projecting "guards" by long diagonal tension rods. These masts carry no spars, booms, or rigging of any kind, all of which would be so much top hamper, worse than useless, at a speed of 20 miles an hour. These posts, like nearly all the rest of the wood-work, are painted a dazzling white, and surmounted by gilded balls. The lines of the hull are very sharp, and at twenty-two statute miles an hour, a speed not unfrequently attained, there is only a thin sport of water breaking into spray to mark the keen entrance of the cutwater."
We subjoin a list of those parts which are considered most necessary to be carried as spare gear for sea-going PADDLE-WHEEL ENGINES of a large class:
- 100 bolts and nuts for paddle-wheels; 50 bolts and nuts for paddle-flats; 6 paddle-flats; 2 sets of gearing for paddle-flats (feathering-wheels); 1 connecting-rod for ditto, ditto; 1 driving-arm for ditto, ditto; 4 large pins and 2 small for brackets, ditto; 2 radius boss pins for ditto, ditto; 2 bushes for ditto, ditto; 8 brass washers for gearing, ditto; 4 bolts and nuts for radius-boss, ditto; 4 segments of paddle-centres; 4 arms for paddle-wheels; 18 iron washer-plates; 2 brass linings for outer-bearings; 120 brushes for boiler-tubes; 36 stoking-irons; 60 scrapers, circular and forked; 1 set of stocks, taps, and dies from ¼ in 1½ in.; 1 air-pump rod; 1 cylinder-cover, bush, and gland; 1 piston and rod; 1 piston-pan cap, complete; 2 complete sets of all India-rubber valves; 1 set of fire-bars; 1 set of bearing-bars for one furnace; boiler-plate, about 60 lbs. per boiler-tube; 100 ferrules for boiler-tubes; 8 handles for boiler-tube brushes; 24 drags (short and long) for tubing; 12 mandrills for ditto; 1 crank-pin for engine; 1 eccentric-band, complete; 1 feed-pump rod, complete; 1 bilge-equip rod; 1 gross iron washers of various sizes; 120 bolts and nuts; 8 glass gauge-tubes for boilers; 2 glass tubes for barometers.
The following is a list of SPARE GEAR for SCREW-ENGINES of a large class:
- 1 Cylinder-cover, complete; 1 connecting-rod; 1 centre-bonnet, for cylinder; 1 air-pump rod; 1 piston and rod, complete; 1 feed-pump rod; 1 bilge-pump rod; 1 guide-block, complete; 1 eccentric-strain-completer; 1 splinter-ring; for escape-valve; 1 cross-head; 1 guide-block and brass (if needed), complete; 1 cap and thrust-block, fitted with white metal; 2 screws for thrust-block; 2 soft metal bearings, various; 2 complete sets of India-rubber valves; 1 wrench for piston-rod nuts; 1 set of taps and dies, complete; 50 bolts, assorted (iron and metal); 90 bolts and nuts (assorted); 40 spanners, various; ½ set of fire-bars; 60 fire-irons, assorted; 110 scrapers (50 circular and 60 forked); 3 bearing-bars; 100 boiler-tubes; 300 ferrules for boiler-tubes; 40 drifts and mandrills for boiler-tubes; 200 tube-brushes, and 4 handles; 140 washers for boiler-tubes; 3 boiler-plates; 16 glass gauge-tubes, and 60 India-rubber rings.
Actual Weights of Steam Machinery in the Royal Navy.
| Name of the Vessel | Nominal Horse-power | Engines complete | Boilers and apparatus | Propeller and gear | Coal-boxes | Sundries | Spare gear | Total weight of machinery | Water in boilers | Grand Total | |--------------------|---------------------|-----------------|----------------------|-------------------|-----------|---------|-------------|--------------------------|---------------|------------| | Mercury | 1000 | 2337 | 659 | 173 | 15 | 258 | 437 | 629 | 136 | 765 | | Shannon | 600 | 84 | 1286 | 615 | 81 | 15 | 218 | 368 | ... | ... | | Melrose | 600 | 752 | 132 | 489 | 92 | 208 | 137 | 310 | ... | ... | | Algiers | 450 | 1284 | 1629 | 469 | 127 | 135 | 96 | 147 | 975 | 30 | | Cornwallis | 200 | 183 | 454 | 143 | 47 | 96 | 37 | 264 | 592 | 112 | | Marlborough | 800 | 2092 | 2118 | 359 | 148 | 37 | 264 | 592 | 112 | 638 | | Royal Sovereign | 800 | 2107 | 2107 | 212 | 116 | 207 | 248 | 505 | 112 | 617 | | Victoria and Albert| 600 | 1832 | 1704 | 973 | 150 | 317 | 260 | 499 | ... | ... |
*Paddles.*
DESCRIPTION OF THE PLATES.
Plate XXIX. represents the usual type of the side-lever marine-engine. The principal parts are—A the cylinder, B the valve-chest, C the condenser, D the hot well, E the air-pump, F the feed and bilge pumps, GG the great lever, G its main gudgeon, H the cylinder side-rods, I the cross-head, K the piston-rod, LL the parallel motion, M the air-pump cross-head, N the air-pump side-rods, O the air-pump piston-rod, P the connecting-rod cross-tail links, Q the cross-tail, R the connecting-rod, S the crank, U the eccentric palley or cam, uu the eccentric-rod, V the valve-shaft, WW the valve-lever and counterbalance lever.
The apparatus for working the valves expansively is distinctly shown. On the crank-axle, T, is placed a series of cams ttt, which act upon the roller of the expansion-valve tumbler. Yyyyy are the expansion-valve connecting-rods and levers. Z is the valve-chest, and the valve is of the kind called equilibrium-valves, or crown-valves.
Plates XXX., XXXI., and XXXII., represent a pair of direct screw-engines of 500 horse-power (nominal), as constructed by Messrs. Ravenhill, Salkeld, and Company, for various ships in the royal navy. The following vessels have been fitted with machinery on this plan, viz.—the Waterloo and Nelson, 98 guns, 500 h.p.; the Undaunted, Glasgow, and Newcastle, 50-gun frigates, and 600 h.p.; the Narcissus, 50 guns, and the Jason, 21 guns, each of 400 h.p. These engines have given much satisfaction, being at once compact, and at the same time easy of access to all the working parts.
The following are the principal dimensions of the 500-horse screw-engines:— Steam Navigation.
Diameter of cylinders (two) ........................................... 71 inches. Length of stroke .................................................................. 3 feet. Revolutions of engines and screw-shaft per min. ...................... 50. Pressure of steam in the boilers per sq. inch ......................... 20 lb. Diameter of the screw ....................................................... 18 feet. Pitch of do. mean ............................................................. 20 feet. Description of screw ......................................................... Griffith's. Mean draught of the ship (H.M.S. Nelson) ......................... 24 ft. 94 in. Speed at the measured mile ............................................... 10 9 knots. Indicated horse-power ...................................................... 2160 horses.
The various parts of the engines will be recognised by reference to the following letters:—AA are the cylinders; B the piston-rods, of which there are two to each cylinder; C the connecting-rods, working between the guiding surfaces DD, and giving motion to the main cranks EE; F is the screw-shaft; G, the thrust-block, on which the thrust of the screw is taken; H the coupling for disconnecting the shaft; I a worm-wheel for turning the engines by hand; K the steam-pipe from the boilers; L the throttle-valve; M the expansion-valves; N the cylinder slide-valves; O the exhaust-passages; P the condenser; Q the air-pump-rods, which work direct from the piston, passing steam-tight through the cylinder covers like small piston-rods. The air-pumps themselves cannot be seen, being concealed by the condenser. R the discharge-pipes; S the feed and bilge plunger pumps; T eccentrics and gear for working the slide-valves.
Plates XXXIII. and XXXIV. represent the engines and boilers of the screw-steamship Thunder, which are possessed of several interesting peculiarities and appliances for economizing fuel and steam. The vessel (built by Messrs. Lungley of London) is of iron, 240 feet long, 30 feet beam, 22 feet 6 inches deep, and 1062 tons n.o.m. Her draught of water is 13 feet 8 inches aft, and 10 feet 8 inches forward. The engines are constructed by Messrs. Dudgeon, of Millwall, London, and have the following dimensions:
| Diameter of cylinders (two) | 55 inches | |-----------------------------|-----------| | Length of stroke | 36 " | | Revolutions of engines and screw per min. | 58 | | Diameter of screw | 15 feet | | Pitch of screw | 294 " | | Nominal horse-power | 210 horses| | Indicated horse-power with full steam | 850 to 1000 | | Do., with expansion when cutting off 1st 1/4th | 698 " | | Speed, with expanded middle-section of 342 square feet and displacement of 1175 tons, the engines making 54 revolutions per minute, and cutting off steam after 1/4th of the stroke | 14 knots | | Maximum speed at trial with full power | 15 " |
The cylinders of these engines are inverted, and are fixed directly over the crank-shaft. They have separate expansion-slides, and double-pan steam-slides. The exhaust is carried round the cylinders by broad belts (see O, Plate XXXIII.), into the condenser P, the belts thus acting as steam-jackets to the cylinders to preserve their temperature. The condensers themselves form part of the framing on which the cylinders stand. The shaft is forged with solid cranks, and the thrust of the screw is taken by the long collared bearing at C, which is supported independently of the engine framing. The pressure of steam in the boilers is 19 lb., the steam being cut off in the cylinders (when working most expansively) after one-fourth of the stroke.
The letters of reference, previously given, indicate the same portions of the machinery for this and all the remaining plates.
The boilers are tubular, two in number, with four furnaces, and are fired from each end. Each boiler has 360 tubes, 3½ inches external diameter, and 7 feet long. The boilers are fitted with superheating apparatus (A), on Mr. Bearemore's plan; each consists of two steam-chambers, placed one on each side of the chimney, connected by 172 tubes in each, each tube being 2 inches in diameter. The lower end of the chimney is expanded so as to encase the tubes, through which all the steam from the boilers passes on its way to the cylinders. These boilers generate steam with much facility. The advantage of having two funnels in this case is, that the draught thus becomes more direct, and therefore sharper than it would be with one large funnel. The temperature of the superheated steam is about 320°.
These engines have exhibited a very remarkable economy of fuel, the consumption, under favourable circumstances, not exceeding 1½ lb. per l.h.p. per hour; and when the vessel was deeply laden, this did not exceed from 2 to 2½ lb. during a ten days' voyage at sea. The Thunder ran from Plymouth to St Vincent in 9 days, 14 hours, the chief engineer writing thus from the latter place:—"We have run 285 miles during the last 24 hours; and our average speed has been throughout the voyage 11 knots per hour, on a consumption of 15 tons of coals per 24 hours. Pressure of steam 10 lb.; 44 revolutions per minute. Temperature of steam in superheaters, 310°." This is equal to a consumption of about 12 cwt. of coal per hour while steaming at the rate of 11 knots, which, for a displacement of 2000 tons, is an extraordinary result. Whilst on her trial trip her displacement was only half the above, when, under the most favourable circumstances, she went at the rate of 14 knots on a consumption of 8 cwt. per hour.
Plates XXXV. and XXXVI. represent a pair of "combined-cylinder" paddle-wheel engines of 320 horse-power collectively (nominal), as constructed by Messrs. Randolph, Elder, and Company, of Glasgow, in the steamships Callao, Lima, and Bogota.
These vessels are 245 feet long, 36 feet broad, and 23 feet deep, and are designed with lines favourable for speed. Their tonnage is 1650 tons; draught of water, 11 feet forward, 12 feet aft; fitted with feathering wheels 25 feet 2 inches diameter.
The cylinders are four in number, viz., two of 52 inches diameter, and two of 90 inches diameter, and 5 feet stroke. It will be observed that they lie diagonally to each other. During the trial trips the engines made from 23 to 26 revolutions per minute, and indicated from 1000 to 1300 horse-power, the pressure of steam being 26 lb., and the speed of the ships from 12½ to 13 knots per hour. The boilers are tubular, and superheat the steam in the steam-chests by contact with the up-takes only, these being purposely divided and prolonged with this view. The cylinders are further provided with "jackets" kept well supplied with hot steam, to guard against condensation within the cylinders.
These engines have also been attended with a remarkable economy of fuel. The Bogota lately ran from Glasgow to St Vincent, a distance of 2470 nautical miles, in 9 days 21 hours, on a consumption of 232 tons of coal, thus giving an average speed of 10·42 knots, on a consumption of 19 cwt. per hour. The average l.h.p. being 950, this gives an average of 2½ lb. of coal per l.h.p. per hour.
Plate XXXVII. represents a pair of combined-cylinder engines of the same description as shown in the preceding plates, and by the same makers, but designed for driving the screw-propeller. These, it will be observed, are geared engines, driving the screw-shaft by means of internal gearing.
The nature and presumed advantages of "combined-cylinder" engines have been already explained. It may be here repeated, however, that the steam is first admitted into the small cylinder for about one-third of the stroke; and after expanding during the remainder of the stroke in the small cylinder, it enters the large one, and completes its work there by further expanding to the end of its stroke.
Plate XXXVIII. is a section of inboard works of the paddle-steamer Delta, carrying the Indian mails from Southampton to Alexandria. | Labelling | Ship Name | Speed | Date of Trial | Builder | Length between the Masts | Breadth | Depth | Where fitted | Madagascar Section | Engine Type | Horse-Power | |-----------|--------------|---------|---------------|-----------|--------------------------|----------------------------------|-------|--------------------------|--------------------|-------------|-------------| | 1uc spinning | Abekenmon | 1618 | | 3rd | 2 Oct. | | | thronePlayers Bay | | Horizontal | Drinks | | Ditto A | Abubb | | | | | | | thronePlayers Bay | | Stein | | | Devanton | Acagdiness | | | | | | | thronePlayers Bay | | Horizontal | Hapane | | Eagle | Abetà | | | | | | | thronePlayers Bay | | Klein | | | Ironhorse | Akain | | | | | | | thronePlayers Bay | | Mulltter | | | Bonhomme | Alneutral | | | | | | | thronePlayers Bay | | Kenneth | | | Neptune | Awús | | | | | | | thronePlayers Bay | | Mushanlon | | | Divina.s | Anodolve | | | | | | | thronePlayers Bay | | Tu Optus | |
Table of screw-steamers in the royal navy
| Cylinder Weekly (Guaranteed Cylinder) | Name of Manufacturer | |------------------------------------------------------------|--------------------------------------------| | Spare | John Penn and Son | | Cedric | Alexander Smith | | Isla | Proc. Lord Coleraine | | Eliana | Wm. Hack | | Wesley steady | Henry However | | E CAN | Thomas Kenley | | CORAL Vector | Captain Evans | | CYRUS Vector | Carter | | ANDREW Distance | Dryasmar | | CAMINO | Robert Napier | | DOMED | Joshua Longworth | | COLLING | Mullanay Sons and Field | | CONYNGE | Michael Privett | | PORTER | Kilberry Yorkshire | | JED machine | Ramsden Crossbucock | | PEAG ac | Scott Russell & Co. | | KNIGHT SOUND | Maclean Budgen | | KIRKcardi | Cruick | | PARKHORVO | Elancy; viscose yarn markary | | POLLEN | Designed by Hardware Enterprise Industry | | NIendance | Murray, Knowledge Tribal | | ELLIE | Lennox West waqueunn | | PRINDETTE | Luchanan | | ROSEANE | Kelham Ramsden Result | | THOIAH | Murray B/T Universal Ind. | | SEMTREY | Montague Godwin | | SOLORES | Berwick Brothers | | ST CANTY | Harley; ross by Gresham | | CHAMBERLWím | CouDate Batley | | COMANCHE | Lewis Lennon | | DUBLE | Chuddehampey Sot FWH. MRI.mark | | NEW jednoc | Hawken Sproully pompousy mintries | | LIKEY | Ward Romanian yard | | PHRUSSCO | Battersea Stibb | | VOLDHOF uważ owil(wh)nesday age Ing | Tushen writer | | Hospitality Edward | Thor-eae Porsche half | | TAMPA Thomas Cruise | | WHACIDTS | Parker Ecoc 选项 | | TRESTLE | Shingle Cool in martyr | | MAZURE | Dumantote Web | | KEPT | Butter celebrating | | DEPTH INCESSIONS
Table of screwsตารางการนำทางของ蒸汽轮船在皇家海军中 Table of screw-steamers in the royal navy ### Table: Steam Navigation
| Dimensions | Propeller | Ratio of Power | Remarks | |------------|-----------|----------------|---------| | Diameter | Pitch | Length | Dialys | | **18.5** | 4.256 | 59.37 | | | **18.5** | 4.256 | 63 | | | **18.5** | 4.256 | 66.37 | | | **18.5** | 4.256 | 70 | | | **18.5** | 4.256 | 74.16 | | | **18.5** | 4.256 | 78.37 | | | **18.5** | 4.256 | 82.5 | |
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**Notes:**
- The "Dialys" column seems to be a reference to specific trials or measurements. - The remarks section indicates various conditions such as weather and sea state during trials.
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**Wind Conditions:** - **Wind No. 1:** Calm. - **Wind No. 2:** Gusty. - **Wind No. 3:** Moderate wind. - **Wind No. 4:** Strong wind. - **Wind No. 5:** Very strong wind.