To TACK

Plate XI

Thus, suppose a ship A, fig. 2. plate XI. 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 the apparently sails W. N.W. upon the larboard tack, as expressed in the dotted line A d, and E. N. E. upon the other d f, 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 A e, and e g.

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 A e, crossing the course more obliquely than the other e g, 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 A e, on a W. by N. course, is equal to 5. 7 leagues. The second tack e g, is 9. 2 leagues E by N.: the third tack, parallel to A e, is 11. 5: the fourth, parallel to e g, 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 A B, her primitive distance; and this in 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 leeway, 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 further 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.

Reference Works