sail

A sail is a panel of woven fabric, historically linen or canvas and now often synthetic material like dacron or aramid, rigged to a mast or spar in such a way that wind pressure on one side creates a force vector with a forward component. The sail does not simply catch the wind and push: the curve of the sail (its draft) and its angle to the wind (the angle of attack) generate lift perpendicular to the airflow, much like an aircraft wing. This lift is resolved into forward driving force and sideways heeling force, the latter resisted by the hull, keel, or centerboard.

The fundamental sail types reflect the points of sail at which they work best. A square sail, hung from a horizontal spar called a yard, develops power across a broad range of wind angles and was the workhorse of ocean-going cargo vessels for centuries. A fore-and-aft sail, set along the line of the mast and controlled by sheets and boom, points more nearly into the wind and dominates modern yachts and small craft. Working sails include the mainsail and jib; storm sails are smaller, heavily built versions cut from heavier cloth, deployed in high wind to reduce canvas area and preserve the rig. Specialized sails like the spinnaker, cut with enormous fullness, catch wind on downwind points where square sails excel.

Construction and attachment

A sail is sewn from panels of cloth joined at the seams, with the edges (the leech, foot, and luff) finished with a rope called the bolt rope stitched into a reinforced hem. Modern racing sails use narrow panels aligned to the direction of maximum stress, while cruising sails use wider panels. The luff of a fore-and-aft sail attaches to the mast by slides or a groove; the foot attaches to the boom by slides or a continuous seam; the clew (corner) and tack (corner) are reinforced with patches and fittings so that control lines, or sheets, can be attached and tensions taken. Batten pockets sewn into the leech hold horizontal stiffeners (battens) that maintain the sail's aerodynamic shape and prevent flutter and distortion.

Wear and failure come from UV degradation of the fabric, fatigue at the seams and corners, and distortion of the weave after years of stretching. Salt spray accelerates rot in natural fibers and corrosion of metal fittings. A sail that has lost its shape, develops soft spots, or tears along the seams is often beyond economic repair. Chafe at the spreaders, the leech rubbing the shrouds, and the foot dragging across the boom all cause rapid local damage. Proper furling, stowing out of sunlight, and periodic inspection of seams and fittings extend working life significantly.

The word sail comes from Old English segl and Old Norse segl, themselves of obscure origin; the Germanic root may relate to cutting or division, perhaps from the way a sail divides the wind. In maritime trade, sail denotes not just the physical object but the collective canvas on a vessel: a ship under full sail carries all working canvas deployed; to shorten sail is to reduce canvas area in rising wind. In modern usage, a sail is measured by area in square meters or feet and by aspect ratio (the ratio of luff length to foot length), with high aspect ratios favored in racing and moderate ratios in cruising work.