Sail
From Wikipedia, the free encyclopedia
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.
Contents |
[edit] History of sails
- Further information: Ship
The ships built at around 10,000 BC were just crude log rafts or dug-out canoes. The earliest known ships were papyrus reed boats built by the Egyptians around 4,000 BC. The Greeks and Phoenicians had begun trading by ship around 1,200 BC. The Arabs had invented the sail in about 2,000 BC in an attempt to establish trading routes around the Persian Gulf. The Chinese had designed sails around 3,000 BC, and can still be seen on traditional wooden ships sailing off the coast of Vietnam in Ha Long Bay. Square sails mounted on yardarms perpendicular to the boat's hull are very good for downwind sailing, and dominated in the ancient Mediterranean and spread to Northern Europe, while being independantly invented in China and Ecuador. Although fore-and-aft rigs have become more popular on modern yachts, square sails continued to power full rigged ships through the Age of Sail and to the present day. Triangualar fore-and-aft lateen sails were invented independantly in the Mediterranean (possibly from the square sail through the lugsail), the Indian Ocean, the Pacific (from V-shaped sails) and Peru, and continue to be used throughout the world. During the 16th-19th centuries other fore-and-aft sails were developed in Europe, such as the spritsail, gaff rig, jib/genoa/staysail, and Bermuda rig, which give better upwind sailing ability.
In an interesting recent development, an elderly trawler, TS Pelican, was fitted with what are thought to have been the unorthodox riggings used by the Barbary pirates in the 16th century. The resultant performance has been remarkable, with the Pelican sailing, at speed, over 20 degrees nearer the wind than any square rigger.[1]
[edit] Use of sails
Sails are primarily used on the water by sailing ships and sail boats 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 For purposes of commerce, sails have been greatly superseded by other forms of propulsion, such as the internal combustion engine.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 sails in a fore-and-aft arrangement: 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 large enough to overlap the mainsail, 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 provide propulsion as the sailboat changes direction relative to the wind. When the boat's stern crosses the wind, this is called gybing; 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 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 leading edge of a sail is oriented into the wind, the correct curvature helps maximise lift while minimising turbulence and drag, much like the carefully designed curves of aircraft wings. 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 bending the mast and increasing the downward pressure of the boom by use of a boom vang.
Other sail powered machines include ice yachts, windmills, kites, signs, hang gliders, electric generators, windsurfers , and land sailing vehicles.
Sail construction is governed by the science of aerodynamics.
[edit] Types of rigging
- Further information: Rigging
Generally speaking, 2 main types of rigging can be found to mount the main sails. These include: