SAIL, a quantity of canvas attached to the yards or stays of a boat or ship in order to receive the impulse of the wind, and thus give motion to the vessel. The depth of a sail is capable of being diminished at pleasure, according to the force of the wind, by means of the reef points.
The principal sails of largo vessels can be placed at right angles to the direction of the keel of the ship, and this position is given to them when the vessel goes before the wind; in other cases the same sails may, by means of the braces, be placed obliquely to the keeL The sails which are attached to the ship's stays, and the sails of boats or small vessels, are generally in a vertical plane passing through the keel ; a certain degree of obliquity to that plane may, however, be given to them at their lower extremities if necessary. Sails are strengthened by ropes, called bolt-ropes, sewn along their edges in order to prevent them from being easily torn by the action of the wind.
When a vessel is in still water, the pressure of the wind against the sails overcomes its inertia, and motion takes place in some direction. The motion goes on increasing by the accelerative power of the wind ; but at the end of a certain time the resistance in an opposite direction, both of the air against the sails and hull of the ship, and of the water against the latter, becoming equal to the accelerative power of the wind, the ship acquires a terminal or uniform velocity, and in this state (neglecting the resistance of the air) there may be said to be an equilibrium between the pressure of the wind against the sails and of the water against the vessel.
The principal problem connected with the motion of vessel', on the water has for its object the determination of the relation between the velocities of the wind and of the vessel; and its solution consists in finding algebraic expressions for those pressures, and making them equal to one another. But many practical difficulties present them selves in investigating that relation ' • for the pressure of the wind is modified by the form which the sail assumes when acted upon, by the obliquity of the wind's direction to the general plane of the sail, and by the interference of one sail with another, by which interference the wind may be partly intercepted, or currents may be produced in directions different from the general direction of the wind. The resistance of the water is also greatly modified by the form of the ship's hull, and by the direction of its motion with respect to the lino of the keel. These difficulties cannot be removed ; therefore the results of mathematical researches concerning the motion of ships can only be considered as very remote approximations to the rules which should guide the practice of tho seaman. And in order to simplify the problem, it is necessary to suppose that the ship is furnished with only one sail, whose area is such that the action of the wind upon it may be equivalent to the efficient action of the wind upon all the sails.
The centre relique, as it hi called by foreign writers, or the centre of pressure or elfori, insult also be supposed to be at the centre of gravity of the sail. That part of the ship's surface which is resisted by the water must moreover be represented by a plane surface whose area is such that thin resistance shall be equivalent to the efficient action of the water on the ship.
The pre/entre of the air perpendicularly against a plane surface equal to one square foot is nsuallv estimated at 4,1b. avoirdupois, the surface prowled being at rest, and ill() wind moving with a velocity equal to one foot per second, or about 038 mile per hour ; also the resistance of water against a like surface and moving with an equal velocity is estimated at The pressure or resistance, by the laws of hydro dynamics, varies with the square of the velocity; and, from the reso lution of forces, it may be shown (Annonntamtcm] that the effective force with which a fluid strikes a plane surface obliquely, when estimated in a direction perpendicular to the plane, varies with the square of the sine of the inclination to the plane. This, however, is only an approximation for practical purposes, its insufficiency arising from want of allowance for the accumulation of force depending on the shape or curved surface of the sail ; and, from the experiments of Borant, D'Alembert, and Condorcet, it appears to hold good only for inclinations between lie• and 90'. The experiments of Smeaton indicate that the pressures vary nearly with the sine of the inclination, when the latter is between 50' and tiO*; at greater Inclinations the pressure ie some fractional power of the eine, and at very small nations it approaches nearly to the square. lint the following formula of Smcaton gives the effective velocity very nearly as From the experiments of Dr. Dutton it is found that at inclinations between 5(1' and 90' the pressures vary nearly as the sines of the inclination's In determining the pressure of a fluid against the surface, which is in motion, it Town be observed that, by the laws of the collision of bodies, the efficient velocity of impulse is to be expressed not by the absolute, but by the relative velocity of the impelling power. Hence, when the wind and Ali], arc moving in the same direction, the effective velocity is the difference, and when they are moving in opposite directions, it is the sum of their several velocities. It must also be observed that the force of the wind and the reaction of the water are to be considered as taking place in horizontal directions, and that the effective pressure of the wind on a flat sail is in a direction perpen dicular to the plane of the sail, whatever be the position of the latter and the direction of tho wind.