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A sail is a surface intended to generate thrust by being placed in a wind. Depending on the angle between the wind and the surface, one side of the sail will have a higher air pressure than the other one. This difference will cause the sail to pull towards the area of lower pressure.

Sails are primarily used at sea, on sailing ships as a locomotive system, but have been rendered commercially obsolete by other forms of propulsion, such as the steam engine. For recreation, however, sailing vehicles remain popular.

The most familiar type of sailboat, a small pleasure yacht, usually has a sail-plan called a sloop. This has two fore-and-aft sails: the mainsail[?] and the jib.

The mainsail extends aftward and is secured the whole length of its edges to the mast and to a boom also hung from the mast.

The jib is secured along its hypotenuse (or luff[?]) to a forestay[?] (strong wire) strung from the top of the mast to the bowsprit[?] on the bow (nose) of the boat.

Fore-and-aft sails can be switched from one side of the boat to the other, in order to alter the boat's course. When the boat's stern crosses the wind, this is called jibing[?]; when the bow crosses the wind, it is called tacking. Tacking repeatedly from port to starboard, called "beating", is done in order to allow the boat to follow a course into the wind.

A primary feature of a properly designed sail is an amount of "draft", caused by curvature of the surface of the sail. When the sail is oriented into the wind, this curvature induces lift, much like the wing of an airplane. Modern sails are manufactured with a combination of broadseaming[?] and stretchable fabric. The former adds draft, while the latter makes it possible to adjust the draft for different levels of wind.

Other sail powered machines include ice yachts and windmills

Sail construction is governed by the science of aerodynamics.

Parts of the Sail

The lower edge of a triangular sail is called the "foot" of the sail, while the upper point is known as the "head". The halyard, a line which raises the sail, is attached to the head. The lower two points of the sail, on either end of the foot, are called the "tack" (forward) and "clew[?]" (aft). A line called a "cunnigham[?]" is sometimes attached to the tack, and an "outhaul[?]" is sometimes attached to the clew of a sail.

The forward edge of the sail is called the "luff", which inspires the term "luffing", a condition where the sail ripples because wind is crossing over the front and back side simultaneously. The aft edge of a sail is called the "leech".

Modern sails are designed such that the warp[?] and the weft[?] of the sailcloth are oriented parallel to the luff and foot of the sail. This places the most stretchable axis of the cloth along the diagonal axis (parallel to the leech), and makes it possible for sailors to reduce the draft of the sail by tensioning the sail, mast and boom in various ways.

See also: Sail-plan, Wing, Rudder, Fin

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