LEEWARD-SHIP

LEEWARD

LEE-WAY

This movement is produced by the mutual effort of the wind and lea upon her side, forcing her to leeward of the line upon which She appears to sail; and in this situation her course is necessarily a compound of the two motions by which she is impelled, of which the one presses forward, according to the line of her keel, from H to K, fig. 5. plate V. whilst the other, acting in the line B A, pushes her to leeward of the course from B. towards A, with a motion which is usually in proportion to the force of the wind, and the rate of her velocity, as appears by the following theory.

Plate V

When a ship is close-hauled, and the head-sails are in perfect equilibrio with those abaft, the resistance of the water from A to B. fig. 5. plate V. is equal to the impulse of the sails, whether it is impressed upon the center of gravity H of the ship, or any other point of her length before or abaft it. In this situation, the ship will as readily bear away as come nearer to the wind, with regard to the resistance of the water upon her bottom on one side, and the impulsion of the wind upon the sails on the other. But it must be observed, that the united effort of the sails acts upon the Ship according to a direction B A, perpendicular to their surfaces, and commencing it's action in some point H, being the mean d between the different effects CG, of the sails afore and abaft, which should exactly correspond with the resistance of the water from A towards B; so that the vessel is pushed to leeward of the course I K, which she steers in the direction B A of the effort of the sails. But the resistance of the water, acting upon the lee-side of her bottom, counterbalances this effort, and becomes stronger, in proportion to the greater facility with which She divides the fluid with her stem; so that the will really advance in the course N R, which lies nearer the line of her keel than B A. Thus the angle K H R of the lee-way is proportional to the greater or less resistance the ship meets with from the fluid upon her lee-fide, respectively with her greater or less facility of dividing it with her fore-part; so that the lee-way is very inconsiderable, except when the ship is close-hauled, and is accordingly disregarded whenever the wind is large.

This demonstration might be pushed further by a fact founded on daily experience, which proves that not only the lee-way depends on the form of the vessel, but also the degree of velocity with which she advances; and perhaps never, intirely, upon the greater or less obliquity of the sails with the keel, as some authors have pretended. For when a swift-sailing ship is close-hauled, with all her sails out, in a very light wind, and scarcely having steerage-way, the lee-way is considerable even in smooth water. This is occasioned by the tardy motion of the vessel, which being feebly pushed forward cannot impress the water with a forcible effect, and by consequence feels no resistance from it, but is accordingly carried with facility by her sails, in the direction of their effort B A: and if we consider the situation of the ship's side, which presents a great surface of sail above the water, it appears that the lee-way will become yet more perpendicular to the keel. But when the wind makes a forcible impression, the velocity of moving forward is considerably augmented; the ship strikes the fluid with a force, expressed by the square of two or three leagues of swiftness, from B towards A. in the space of an hour, whilst the water repels her effort in a contrary direction. The resistance of the water is then in the ratio of this square to the square of her first velocity, or head-way; and in this state will not readily yield to it's effort. The lee-way immediately decreases, and will be still further diminished, if the ship's course is accelerated. If then at the moment when the ship advances with great rapidity, she bears away 12 or 15 degrees, or even two points, without altering the general arrangement of her sails, their obliquity remains the same; the ship therefore ought to have the same lee-way, according to the opinion of those who have written on the theory of sailing. The velocity is augmented, because the sails then receive the wind by a greater fine of incidence, and thereby acquire a more powerful effort, whilst the ship's bead is always struck by the water in the same parts, and by the same sine of incidence; so that the lee-way is also diminished, because the water resists more, in consequence of the accelerated swiftness; and because the resistance is more exerted on the ship's side than on her head, which is less opposed to it's impulsion. Hence we may conclude, that the lee-way of a ship does not entirely depend on the disposition of her sails; that it is different in different vessels, because they are neither formed alike, nor are their sails equally trimmed in the same oblique courses: and finally, because they have always a different velocity, at the same time, and under the same sail. Thus it is evident, that the lee-way is always composed of the ship's comparative velocity; of her form, which gives more or less proportional resistance upon the side than on the fore-part; and of the disposition of her sails, as forming a greater or smaller obliquity with the keel. See also CLOSE-HAULED, DRIFT, and SAILING.

LEECHES

The leeches of all sails, whose tops and bottoms are parallel to the deck, or at right angles with the mall, are denominated from the ship's side, and the sail to which they belong; as the starboard leech of the main-sail, the lee leech of the fore-top-sail, &c. but the sails which are fixed obliquely upon the masts, have their leeches named from their situation with respect to the Ship's length; as the fore-leech of the mizen, the after-leech of the jib, or fore-stay-sail, &c.

Reference Works