TACK, is also applied, by analogy, to that part of any sail to which the tack is usually fastened.
A ship is said to be on the starboard or larboard tack, when she is close-hauled, with the wind upon the starboard or larboard side: and in this sense the distance which she sails in that position is considered as the length of the tack; although this is more frequently called board. See that article.
To TACK, to change the course from one board to another, or turn the ship about from the starboard to the larboard tack, in a contrary wind. Thus the ship A, being close-hauled on the larboard tack, and turning her prow suddenly to windward, receives the impression of the wind on her head-sails a, by which she falls off upon the line of the starboard-tack a. Tacking is also used in a more enlarged sense, to imply that manœuvre in navigation by which a ship makes an oblique progression to the windward, in a zigzag direction. This, however, is more usually called beating, or turning to windward.
Thus, suppose the ship A bound to a port B lying to windward, with the wind northerly, as expressed by the arrow. The sails a, b, c, being braced obliquely with the keel, the wind also falls upon their surfaces in an oblique direction, by which the ship is pushed to leeward, as explained in the article LEE-WAY. Hence, although she apparently sails W. N. W. upon the larboard-tack, as expressed in the dotted line Ad, and E. N. E. upon the other df, yet if the lee-way is only one point, (and indeed it is seldom less in the smoothest water), the course will accordingly be W. by N. upon one tack, and E. by N. upon the other, as represented by the lines Ae, and eg.
If the port A were directly to windward of the ship, it is evident that both tacks ought to be of equal length; or, in other words, that she ought to run the same distance upon each tack: but as the place of her destination lies obliquely to windward, she must run a greater distance upon one tack than the other; because the extremities of both boards should be equally distant from the line of her true course BA; so the larboard tack Ae, crossing the course more obliquely than the other eg, will necessarily be much longer.
As the true course, or the direct distance from B to A is only 12 leagues, it is evident that with a favourable wind she could reach it in a few hours. On the contrary, her distance is considerably increased by the length of her boards, in a contrary wind; which, by its obliquity with her sails, operates also to retard her velocity. Thus her first board Ae, on a W. by N. course, is equal to 5.7 leagues. The second tack eg is 9.2 leagues E by N. the third tack, parallel to Ae, is 11.5: the fourth, parallel to eg,
is 9.2: and the fifth, parallel to the first, 11.7 leagues. Finally, the sixth board is 4.8 leagues parallel to the second, which brings her to the port B. By this scheme it appears that she has run more than four times the extent of the line AB, her primitive distance; and this is the most favourable circumstances of a contrary wind, viz. when the sea is smooth, and when she may carry her full topsails. For if the wind blows stronger, to render it necessary to reef the topsails, she will soon make two points of lee-way, and accordingly run east on one board and west on the other. In this situation she will neither approach nor recede from the place of her destination: but if the wind increases, the sea will also be enlarged; a circumstance that still farther augments the lee-way. Hence the vessel will gradually fall off from the port, in proportion to the augmentation of the wind and sea, which occasions a proportional increase of lee-way.
In order to explain the theory of tacking a ship, it may be necessary to premise a known axiom in natural philosophy, That every body will persevere in a state of rest, or of moving uniformly in a right line, unless it be compelled to change its state by forces impressed; and that the change of motion is proportional to the moving force impressed, and is made according to the right line in which that force is exerted.
By this principle it is easy to conceive how a ship is compelled to turn into any direction, by the force of the wind acting upon her sails in horizontal lines. For the sails may be so arranged as to receive the current of air either directly or more or less obliquely. Hence the motion communicated to the sails must of necessity conspire with that of the wind upon their surfaces. To make the ship tack or turn round with her head to the windward, it is therefore necessary, after she has received the first impression from the helm, that the head-sails should be so disposed as to diminish the effort of the wind in the first instant of her motion; and that the whole force of the wind should be exerted on the after-sails; which, operating on the ship's stern, carries it round like a weathercock. But since the action of the after-sails to turn the ship will unavoidably cease when her head points to the windward, it then becomes necessary to use the head-sails, to prevent her from falling off and returning to her former situation. These are accordingly laid aback on the lee-side, to push the vessel's fore part towards the opposite side, till she has fallen into the line of her course thereon, and fixed her sails to conform with that situation.
It has been observed above, that the first effort to turn the ship in tacking is communicated by the helm, which is then put to the lee-side. This circumstance being announced by the pilot or commanding officer, who then calls out Helm's a lee! the head-sails are immediately made to shiver in the wind, by casting loose their sheets or bowlines. The pilot then calls, Up tacks and sheets! which is executed by loosening all the ropes which confine the corners of the lower sails, in order that they may be more readily shifted to the other side. When the ship has turned her head directly to windward, as in d, the pilot gives the order to turn about the sails on the main and mizzen masts, by the exclamation, Haul main-sail, haul! the bowlines.
bowlines and braces are then instantly cast off on one side, and as expeditiously drawn in on the other side, so as to wheel the yards about their masts. The lower corner of the main-sail is, by means of its tack, pulled down to its station at the cheetree; and all the after-sails are at the same time adjusted to stand upon the other board. Finally, when the ship has fallen off five or six points, the pilot cries, Haul off all! or, Let go, and haul! then the sails on the fore-mast are wheeled about by their braces; and as the ship has then a tendency to fall off, she is checked by the effort of the helm; which for that purpose is put hard a-lee. The fore-tack, or the lower corner of the fore-sail, being fixed in its place, the bowlines are hauled; and the other sails, which have been neglected in the hurry of tacking, are properly arranged to the wind; which exercise is called trimming the sails. See LEE-WAY, and SAILING.