A sail is any type of surface intended to generate thrust by being placed in a wind — in essence a vertically-oriented wing. Sails are used in sailing.

Sail aerodynamics

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Sails propel the boat in one of two ways. When the boat is going in the direction of the wind (i.e. downwind - see Points of sail), the sails merely act to catch the air as it flows by. This method of propulsion is simple enough that it really doesn't need any additional explanation — the wind simply blows the boat downwind and the sails act more like a parachute than like a wing.

The other way sails propel the boat occurs when the boat is travelling across the wind or into the wind. In this situation, the sails propel the boat by redirecting the wind coming in from the side towards the rear. In accordance with the law of conservation of momentum, air is redirected backwards, making the boat go forward. This driving force is called lift.

(Often, Bernoulli's principle is invoked to explain the action of sails and other airfoils, but this explanation is often based on the incorrect assumption that the two parcels of air which separate at the leading edge of a wing must meet again at the trailing edge.)

The lift can be resolved into two components; forward force and sideways force (see vectors for an explanation of this principle). These forces are acting against opposing forces generated by the hull and the keel

On a sailing boat, a keel or centreboard is used to convert this lateral force into forward movement of the boat with some sideways leaning, or 'heel'. The forward motion is created because the shape of the keel has a much smaller cross section in the fore and aft axis and a very large cross section on the athwart axis (across the beam of the boat). The resistance to motion along the smallest cross section is "easy" while motion across the large cross section is "hard". Forces across the boat are resolved by balancing the sideways force by the mass of the ballast in the keel being raised against gravity by the boat heeling, while forward forces are balanced by velocity through the water and friction between the hull, keel and the water. In other words it is easier for the sail to push the boat forward rather than sideways.

Use of sails

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Sails are primarily used at sea, on sailing ships as a propulsion system. For purposes of commerce, sails have been greatly superseded by other forms of propulsion, such as the internal combustion engine. For recreation, however, sailing vessels 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 sails of tall ships are attached to wooden timbers or "spars".

The jib is secured along its leading edge to a forestay (strong wire) strung from the top of the mast to the bowsprit on the bow (nose) of the boat. A genoa is also used on some boats. It is a type of jib that is larger, and cut so that it is fuller than an ordinary jib.

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 and/or vice versa, 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 non-stretch fabric (ref New technology below). The former adds draft, while the latter allows the sail to keep a constant shape as the wind pressure increases. The draft of the sail can be reduced in stronger winds by use of a cunningham and outhaul, and also by increasing the downward pressure of the boom by use of a boom vang.

Other sail powered machines include ice yachts and windmills.

Sail construction is governed by the science of aerodynamics.

Parts of the Sail

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The lower edge of a triangular sail is called the "foot" of the sail, while the upper point is known as the "head". The lower two points of the sail, on either end of the foot, are called the "tack" (forward) and "clew" (aft). The forward edge of the sail is called the "luff" (from which derives the term "luffing", a rippling of the sail when the angle of the wind fails to maintain a good aerodynamic shape near the luff). 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.

An alternative approach to sail design is that used in Junks, originally an oriental design. It uses horizontal sail curving to produce an efficient and easily controlled sail-plan. ^*.

New technology

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Sail producers are competing hard with the latest technology to produce the best, i.e. the most efficient sail: North sails with their 3DL sail, Doyle the 4D and Elvström Sobstad the Genesis sail. These sails are produced with carbon fiber, spectra, pbo, vectran and other low-stretch low weight fibers and are then glued and baked. This is quite different when compared with panelled sails built of woven cloth.

See also

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• Sailing
• Points of sail
• Sail-plan
• Rigging
• Wing
• Rudder
• Fin
• Solar sail
• Sail twist
• Sail Class Markings
• Marine canvas

Types of sails

• Flanker
• Lugsail
• Ringtail
• Royal
• Skysail

External links

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• International Sailing Federation
• The quest for the perfect sailshape

Sailing vessels and rigging | Marine propulsion | Sailboat anatomy

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