Sail facts for kids
A sail is a tensile structure—made from fabric or other membrane materials—that uses wind power to propel sailing craft, including sailing ships, sailboats, windsurfers, ice boats, and even sail-powered land vehicles. Sails may be made from a combination of woven materials—including canvas or polyester cloth, laminated membranes or bonded filaments—usually in a three- or four-sided shape.
A sail provides propulsive force via a combination of lift and drag, depending on its angle of attack—its angle with respect to the apparent wind. Apparent wind is the air velocity experienced on the moving craft and is the combined effect of the true wind velocity with the velocity of the sailing craft. Angle of attack is often constrained by the sailing craft's orientation to the wind or point of sail. On points of sail where it is possible to align the leading edge of the sail with the apparent wind, the sail may act as an airfoil, generating propulsive force as air passes along its surface—just as an airplane wing generates lift—which predominates over aerodynamic drag retarding forward motion. The more that the angle of attack diverges from the apparent wind as a sailing craft turns downwind, the more drag increases and lift decreases as propulsive forces, until a sail going downwind is predominated by drag forces. Sails are unable to generate propulsive force if they are aligned too closely to the wind.
Sails may be attached to a mast, boom or other spar or may be attached to a wire that is suspended by a mast. They are typically raised by a line, called a halyard, and their angle with respect to the wind is usually controlled by a line, called a sheet. In use, they may be designed to be curved in both directions along their surface, often as a result of their curved edges. Battens may be used to extend the trailing edge of a sail beyond the line of its attachment points.
Other non-rotating airfoils that power sailing craft include wingsails, which are rigid wing-like structures, and kites that power kite-rigged vessels, but do not employ a mast to support the airfoil and are beyond the scope of this article.
Contents
Rigs
Sailing craft employ two types of rig, the square rig and the fore-and-aft rig.
The square rig carries the primary driving sails are carried on horizontal spars, which are perpendicular or square, to the keel of the vessel and to the masts. These spars are called yards and their tips, beyond the last stay, are called the yardarms. A ship mainly so rigged is called a square-rigger. The square rig is aerodynamically most efficient when running (sailing downwind).
A fore-and-aft rig consists of sails that are set along the line of the keel rather than perpendicular to it. Vessels so rigged are described as fore-and-aft rigged.
History
Archaeological studies of the Cucuteni-Trypillian culture ceramics show use of sailing boats from the sixth millennium BCE onwards. Excavations of the Ubaid period (c. 6000–4300 BCE) in Mesopotamia provides direct evidence of sailing boats.
Square rigs
Sails from ancient Egypt are depicted around 3200 BCE, where reed boats sailed upstream against the River Nile's current. Ancient Sumerians used square rigged sailing boats at about the same time, and it is believed they established sea trading routes as far away as the Indus valley. Greeks and Phoenicians began trading by ship by around 1200 BCE.
Crab claw rigs
The first sea-going sailing ships were developed by the Austronesian peoples from what is now Southern China and Taiwan. Their invention of catamarans, outriggers, and the highly-efficient bi-sparred triangular crab claw sails enabled their ships to sail for vast distances in open ocean. It led to the Austronesian Expansion at around 3000 to 1500 BC. From Taiwan, they rapidly colonized the islands of Maritime Southeast Asia, then sailed further onwards to Micronesia, Island Melanesia, Polynesia, and Madagascar, eventually colonizing a territory spanning half the globe.
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The proto-Austronesian words for sail, lay(r), and other rigging parts date to about 3000 BCE when this group began their Pacific expansion. Austronesian rigs were distinctive in that they had spars supporting both the upper and lower edges of the sails (and sometimes in between), in contrast to western rigs which only had a spar on the upper edge. The sails were also made from salt-resistant woven leaves, usually from pandan plants.
Crab claw sails used with single-outrigger ships in Micronesia, Island Melanesia, Polynesia, and Madagascar were intrinsically unstable when tacking leeward. To deal with this, Austronesians in these regions developed the shunting technique in sailing, in conjunction with uniquely reversible single-outriggers. In the rest of Austronesia, crab claw sails were mainly for double-outrigger (trimarans) and double-hulled (catamarans) boats, which remained stable even leeward.
In western Island Southeast Asia, later square sails also evolved from the crab claw sail, the tanja and the junk rig, both of which retained the Austronesian characteristic of having more than one spar supporting the sail.
Junk rigs were later adopted by the Chinese Song Dynasty in the 10th century CE from contact with Srivijayan trading ships (known as "Kunlun ships" in Chinese records).
Lateen rigs
In the Mediterranean, single-yarded lateen sails emerged by around the 2nd century CE. Though its origin remains contentious, it is believed that it developed from early contact with Southeast Asian Austronesian trading ships in the Indian Ocean with crab claw sails.
The fore-and-aft rig began as a convention of southern Europe and the Mediterranean Sea: the generally gentle climate made its use practical, and in Italy a few centuries before the Renaissance it began to replace the square rig which had dominated all of Europe since the dawn of sea travel. Northern Europeans were resistant to adopting the fore-and-aft rig, despite having seen its use in the course of trade and during the Crusades. The Renaissance changed this: beginning in 1475, their use increased and within a hundred years the fore-and-aft rig was in common use on rivers and in estuaries in Britain, northern France, and the Low Countries, though the square rig remained standard for the harsher conditions of the open North Sea as well as for trans-Atlantic sailing. The lateen sail proved to have better upwind performance for smaller vessels.
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 mainsail, improving the upwind sailing ability of European vessels.
Aerodynamic forces
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Aerodynamic forces on sails depend on wind speed and direction and the speed and direction of the craft. The direction that the craft is traveling with respect to the true wind (the wind direction and speed over the surface) is called the "point of sail". The speed of the craft at a given point of sail contributes to the apparent wind ( VA)—the wind speed and direction as measured on the moving craft. The apparent wind on the sail creates a total aerodynamic force, which may be resolved into drag—the force component in the direction of the apparent wind—and lift—the force component normal (90°) to the apparent wind. Depending on the alignment of the sail with the apparent wind, lift or drag may be the predominant propulsive component. Total aerodynamic force also resolves into a forward, propulsive, driving force—resisted by the medium through or over which the craft is passing (e.g. through water, air, or over ice, sand)—and a lateral force, resisted by the underwater foils, ice runners, or wheels of the sailing craft.
For apparent wind angles aligned with the entry point of the sail, the sail acts as an airfoil and lift is the predominant component of propulsion. For apparent wind angles behind the sail, lift diminishes and drag increases as the predominant component of propulsion. For a given true wind velocity over the surface, a sail can propel a craft to a higher speed, on points of sail when the entry point of the sail is aligned with the apparent wind, than it can with the entry point not aligned, because of a combination of the diminished force from airflow around the sail and the diminished apparent wind from the velocity of the craft. Because of limitations on speed through the water, displacement sailboats generally derive power from sails generating lift on points of sail that include close-hauled through broad reach (approximately 40° to 135° off the wind). Because of low friction over the surface and high speeds over the ice that create high apparent wind speeds for most points of sail, iceboats can derive power from lift further off the wind than displacement boats.
- Downwind sailing with a spinnaker
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Spinnaker set for a broad reach, mobilizing both lift and drag.
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Spinnaker cross-section trimmed for a broad reach showing air flow.
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Spinnaker downwind, primarily mobilizing drag.
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Spinnaker cross-section with following apparent wind, showing air flow.
Images for kids
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Fijian voyaging outrigger boat with a crab claw sail
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A traditional Maldivian Baghlah with a fore-and-aft rig lateen rig
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Laminated sail with Kevlar and carbon fibers.
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Square sail edges and corners (top). Running rigging (bottom).
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International Moth on foils.
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Formula 16 catamaran.
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DN class ice boat.
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Land sailing craft.
See also
In Spanish: Vela (náutica) para niños
