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.
RigsSailing 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, 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.
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.
HistoryArchaeological studies of the Cucuteni-Trypillian culture ceramics show use of sailing boats from the sixth millennium BCE onwards. Excavations of the Ubaid period in Mesopotamia provides direct evidence of sailing boats.
Square rigsSails 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 rigsThe 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.
The proto-Austronesian words for sail, lay, 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, 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 and double-hulled 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.
Lateen rigsIn 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 forcesAerodynamic 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 is called the "point of sail". The speed of the craft at a given point of sail contributes to the apparent wind —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 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 —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. 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
TypesEach rig is configured in a sail plan, appropriate to the size of the sailing craft. A sail plan is a set of drawings, usually prepared by a naval architect which shows the various combinations of sail proposed for a sailing ship. Sail plans may vary for different wind conditions—light to heavy. Both square-rigged and fore-and-aft rigged vessels have been built with a wide range of configurations for single and multiple masts.
Types of sail that can be part of a sail plan can be broadly classed by how they are attached to the sailing craft:
- To a stay – Sails attached to stays, include jibs, which are attached to forestays and staysails, which are mounted on other stays that support other masts from the bow aft.
- To a mast – Fore-and-aft sails directly attached to the mast at the luff include gaff-rigged quadrilateral and Bermuda triangular sails.
- To a spar – Sails attached to a spar include both square sails and such fore-and-aft quadrilateral sails as lug rigs, junk and spritsails and such triangular sails as the lateen, and the crab claw.
- To a halyard – Spinnakers are the predominant sail supported solely by a halyard.
ShapeThe shape of a sail is defined by its edges and corners in the plane of the sail, laid out on a flat surface. The edges may be curved, either to extend the sail's shape as an airfoil or to define its shape in use. In use, the sail becomes a curved shape, adding the dimension of depth or draft.
- Edges – The top of all sails is called the head, the leading edge is called the luff on fore-and-aft sails and on windward leech symmetrical sails, the trailing edge is the leech, and the bottom edge is the foot. The head is attached at the throat and peak to a gaff, yard, or sprit. For a triangular sail the head refers to the topmost corner.
- Corners – The names of corners of sails vary, depending on shape and symmetry. In a triangular sail, the corner where the luff and the leech connect is called the head. On a square sail, the top corners are head cringles, where there are grommets, called cringles. On a quadrilateral sail, the peak is the upper aft corner of the sail, at the top end of a gaff or other spar. The throat is the upper forward corner of the sail, at the bottom end of a gaff or other spar. Gaff-rigged sails, and certain similar rigs, employ two halyards to raise the sails: the throat halyard raises the forward, throat end of the gaff, while the peak halyard raises the aft, peak end.
- Draft – Those triangular sails that are attached to both a mast along the luff and a boom along the foot have depth, called draft, which results from the luff and foot being curved, rather than straight as they are attached to those spars. Draft creates a more efficient airfoil shape for the sail. Draft can also be induced in triangular staysails by adjustment of the sheets and the angle from which they reach the sails.
Traditionally, sails were made from flax or cotton canvas. Materials used in sails, as of the 21st century, include nylon for spinnakers—where light weight and elastic resistance to shock load are valued—and a range of fibers, used for triangular sails, that includes Dacron, aramid fibers—including Kevlar, and other liquid crystal polymer fibers—including Vectran. Woven materials, like Dacron, may specified as either high or low tenacity, as indicated, in part by their denier count.
ConstructionConventional sails comprise panels, which are most often stitched together, other times adhered. There are two basic configurations, cross-cut and radial.
Cross-cut sails have the panels sewn parallel to one another, often parallel to the foot of the sail, and are the least expensive of the two sail constructions. Triangular cross-cut sail panels are designed to meet the mast and stay at an angle from either the warp or the weft to allow stretching along the luff, but minimize strutting on the luff and foot, where the fibers are aligned with the edges of the sail.
Radial sails have panels that "radiate" from corners in order to efficiently transmit stress and are typically higher-performance than cross-cut sails. A bi-radial sail has panels radiating from two of three corners; a tri-radial sail has panels radiating from all three corners. Mainsails are more likely to be bi-radial, since there is very little stress at the tack, whereas head sails are more likely to be tri-radial, because they are tensioned at their corners.
Higher-performance sails may be laminated, constructed directly from multiple plies of filaments, fibers, taffetas, and films—instead of woven textiles—and adhered together. Molded sails are laminated sails formed over a curved mold and adhered together into a shape that does not lie flat.
Conventional sail panels are sewn together. Sails are tensile structures, so the role of a seam is to transmit a tensile load from panel to panel. For a sewn textile sail this is done through thread and is limited by the strength of the thread and the strength of the hole in the textile through which it passes. Sail seams are often overlapped between panels and sewn with zig-zag stitches that create many connections per unit of seam length.
Whereas textiles are typically sewn together, other sail materials may be ultrasonically welded—a technique whereby high-frequency ultrasonic acoustic vibrations are locally applied to workpieces being held together under pressure to create a solid-state weld. It is commonly used for plastics, and especially for joining dissimilar materials.
Sails feature reinforcements of fabric layers where lines attach at grommets or cringles. A bolt rope may be sewn onto the edges of a sail to reinforce it, or to fix the sail into a groove in the boom, in the mast, or in the luff foil of a roller-furling jib. They may have stiffening features, called battens, that help shape the sail, when full length, or just the roach, when present. They may have a variety of means of reefing them, including rows of short lines affixed to the sail to wrap up unused sail, as on square and gaff rigs, or simply grommets through which a line or a hook may pass, as on Bermuda mainsails. Fore-and-aft sails may have tell-tales—pieces of yarn, thread or tape that are affixed to sails—to help visualize airflow over their surfaces.
Running riggingThe lines that attach to and control sails are part of the running rigging and differ between square and fore-and-aft rigs. Some rigs shift from one side of the mast to the other, e.g. the dipping lug sail and the lateen. The lines can be categorized as those that support the sail, those that shape it, and those that control its angle to the wind.
Fore-and-aft rigged vesselsFore-and-aft rigged vessels have rigging that supports, shapes, and adjusts the sails to optimize their performance in the wind, which include the following lines:
- Supporting – Halyards raise sails and control luff tension. Topping lifts hold booms and yards aloft. On a gaff sail, brails run from the leech to the spar to facilitate furling.
- Shaping – Barber haulers adjust a spinnaker/jib sheeting angle inboard at right angles to the sheet with a ring or clip on the sheet attached to cordage which is secured and adjusted via fairlead and cam cleat. Kicking straps/boom vangs control a boom-footed sail's leech tension by exerting downward force mid-boom. Cunninghams tighten the luff of a boom-footed sail by pulling downward on a cringle in the luff of a mainsail above the tack. Downhauls lower a sail or a yard and can adjust the tension on the luff of a sail. Outhauls control the foot tension of a boom-footed sail.
- Adjusting angle to the wind – Sheets control angle of attack with respect to the apparent wind, the amount of leech "twist" near the head of the sail, and the foot tension of loose-footed sails. A preventer attaches to the end of the boom from a point near the mast to prevent an accidental gybe. Guys control spinnaker pole angle with respect to the apparent wind.
- Supporting – Halyards raise and lower the yards. Brails run from the leech to the spar to facilitate furling. Buntlines serve to raise the foot up for shortening sail or for furling. Lifts adjust the tilt of a yard, to raise or lower the ends off the horizontal. Leechlines run to the leech of a sail and serve to pull the leech both in and up when furling.
- Shaping – Bowlines run from the leech forward towards the bow to control the weather leech, keeping it taut and thus preventing it from curling back on itself. Clewlines raise the clews to the yard above.
- Adjusting angle to the wind – Braces adjust the fore and aft angle of a yard. Sheets attach to the clew to control the sail's angle to the wind. Tacks haul the clew of a square sail forward.
Sails on high-performance sailing craft.
Sails on craft subject to low forward resistance and high lateral resistance typically have full-length battens.