A sail derives power from the wind to provide motive power for sailing craft, including sailing ships, sailboats, windsurfers, ice boats, and sail-powered land vehicles. Sails mobilize lift and drag properties as air passes along the surface, functioning similarly to a wing in a vertical orientation. In most cases sails are supported directly by a mast rigidly attached to the sailing craft or on a wire stay attached to the mast, however some craft employ a flexible mount for a mast. Sails also employ spars and battens to help determine their shape, aligned with airflow. As a result, sails come in a variety of shapes that include both triangular and quadrilateral configurations, usually with curved edges that promote three-dimensional curvature of the sail.
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).
Archaeological studies of the Cucuteni-Trypillian culture ceramics show use of sailing boats from the sixth millennium onwards. Excavations of the Ubaid period (c. 6000 -4300 BC) in Mesopotamia provides direct evidence of sailing boats.
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. The proto-Austronesian words for sail, lay(r), and other rigging parts date to about 3000 BCE when this group began their Pacific expansion. Greeks and Phoenicians began trading by ship by around 1,200 BCE.
Triangular fore-and-aft rigs were invented in the Mediterranean as single-yarded lateen sails and independently in the Pacific as the more efficient bi-sparred crab claw sail, 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 mainsail, improving the upwind sailing ability of European vessels.
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.
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
Each 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 (typically wire cable) 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.
High-performance yachts, including the International C-Class Catamaran, have used or use rigid wing sails, which perform better than traditional soft sails but are more difficult to manage. A rigid wing sail was used by Stars and Stripes, the defender which won the 1988 America's Cup, and by USA-17, the challenger which won the 2010 America's Cup. USA 17's performance during the 2010 America's Cup races demonstrated a velocity made good upwind of over twice the wind speed and downwind of over 2.5 times the wind speed and the ability to sail as close as 20 degrees off the apparent wind.
The 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.
- A fore-and-aft triangular mainsail achieves a better approximation of a wing form by extending the leech aft, beyond the line between the head and clew on an arc called the roach, rather than having a triangular shape. This added area would flutter in the wind and not contribute to the efficient airfoil shape of the sail without the presence of battens. Offshore cruising mainsails sometimes have a hollow leech (the inverse of a roach) to obviate the need for battens and their ensuing likelihood of chafing the sail. The roach on a square sail design is the arc of a circle above a straight line from clew to clew at the foot of a square sail, which allows the foot of the sail to clear stays coming up the mast, as the sails are rotated from side to side.
- 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.
- The corner where the leech and foot connect is called the clew on a fore-and-aft sail. On a jib, the sheet is connected to the clew; on a mainsail, the sheet is connected to the boom (if present) near the clew. Clews are the lower two corners of a square sail. Square sails have sheets attached to their clews like triangular sails, but the sheets are used to pull the sail down to the yard below rather than to adjust the angle it makes with the wind. The corner where the leech and the foot connect is called the clew. The corner on a fore-and-aft sail where the luff and foot connect is called the tack and, on a mainsail, is located where the boom and mast connect.
- In the case of a symmetrical spinnaker, each of the lower corners of the sail is a clew. However, under sail on a given tack, the corner to which the spinnaker sheet is attached is called the clew, and the corner attached to the spinnaker pole is referred to as the tack.  On a square sail underway, the tack is the windward clew and also the line holding down that corner.
- 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.
Sail characteristics derive, in part, from the design, construction and the attributes of the fibers, which are woven together to make the sail cloth. There are several key factors in evaluating a fiber for suitability in weaving a sail-cloth: initial modulus, breaking strength (tenacity), creep, and flex strength. Both the initial cost and its durability of the material define its cost-effectiveness over time.
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 (a unit of measure for the linear mass density of fibers).
Conventional 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 (on the bias) 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 (spinnakers and jibs) 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 (reducing sail area), 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.
The 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 vessels
Fore-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 – Guys control spar angle with respect to the apparent wind. A preventer attaches to the end of the boom from a point near the mast to prevent an accidental gybe. Sheets control foot tension of loose-footed sails, angle of attack with respect to the apparent wind and/or the amount of leech "twist" near the head of the sail.
Square-rigged vessels require more controlling lines than fore-and-aft rigged ones, including the following.
- 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 (outer vertical edges) 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 (i.e. to rotate the yard laterally, fore and aft, around the mast). 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.
Types of sails
- Genoa jib
- Anti-UV covering
- Head foil attachment
- Leech control
- Foot control
- Furling marks
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