Scuba diving ("scuba" originally being an acronym for Self Contained Underwater Breathing Apparatus, although now widely considered a word in its own right) is a form of underwater diving in which a diver uses a scuba set to breathe underwater for recreation, commercial or industrial reasons.

Unlike early diving, which relied exclusively on air pumped from the surface, scuba divers carry their own source of breathing gas (usually compressed air), allowing them greater freedom than with an air line. Both surface supplied and scuba diving allow divers to stay underwater significantly longer than with breath-holding techniques as used in snorkelling and free-diving.

According to the purpose of the dive, a diver usually moves underwater by swimfins attached to his feet, but external propulsion can come from an underwater vehicle, or a sled pulled from the surface.

History

The first commercially successful scuba sets were the Aqualung open-circuit units developed by Emile Gagnan and Jacques-Yves Cousteau, in which compressed gas (usually air) is inhaled from a tank and then exhaled into the water, and the descendants of these systems are still the most popular units today.

The open circuit systems were developed after Cousteau had a number of incidents of oxygen toxicity using a rebreather system, in which exhaled air is reprocessed to remove carbon dioxide. Modern versions of rebreather systems (both semi-closed circuit and closed circuit) are still available today, and form the second main type of scuba unit, most commonly used for technical diving, such as deep diving.

Etymology

The term SCUBA (an acronym for Self-Contained Underwater Breathing Apparatus) arose during World War II, and originally referred to United States combat frogmen's oxygen rebreathers, developed by Dr. Christian Lambertsen for underwater warfare.

The word SCUBA began as an acronym, but it is now usually thought of as a regular word—scuba. It has become acceptable to refer to "scuba equipment" or "scuba apparatus"—examples of the linguistic RAS syndrome.

Types of diving

Scuba diving may be performed for a number of reasons, both personal and professional. Most people begin though recreational diving, which is performed purely for enjoyment and has a number of distinct technical disciplines to increase interest underwater, such as cave diving, wreck diving, ice diving and deep diving.

Divers may be employed professionally to perform tasks underwater. Most of these commercial divers are employed to perform tasks related to the running of a business involving deep water, including civil engineering tasks such as in oil exploration, underwater welding or offshore construction. Commercial divers may also be employed to perform tasks specifically related to marine activities, such as naval diving, including the repair and inspection of boats and ships, salvage of wrecks or underwater fishing, like spear fishing.

Other specialist areas of diving include military diving, with a long history of military frogmen in various roles. They can perform roles including direct combat, infiltration behind enemy lines, placing mines or using a manned torpedo, bomb disposal or engineering operations. In civilian operations, many police forces operate police diving teams to perform search and recovery or search and rescue operations and to assist with the detection of crime which may involve bodies of water. In some cases diver rescue teams may also be part of a fire department or lifeguard unit.

Lastly, there are professional divers involved with the water itself, such as underwater photography or underwater filming divers, who set out to document the underwater world, or scientific diving, including marine biology and underwater archaeology.

Breathing underwater

Water normally contains dissolved oxygen from which fish and other aquatic animals extract all their required oxygen as the water flows past their gills. Humans lack gills and do not otherwise have the capacity to breathe underwater unaided by external devices. Some experiments indicate the possibility of filling and ventilating artificially the lungs with a dedicated liquid (Liquid breathing) — this currently has only medical applications.

Early diving experimenters quickly discovered it is not enough simply to supply air in order to breathe comfortably underwater. As one descends, in addition to the normal atmospheric pressure, water exerts increasing pressure on the chest and lungs—approximately 1 bar or 14.7 psi for every 33 feet or 10 meters of depth—so the pressure of the inhaled breath must almost exactly counter the surrounding or ambient pressure to inflate the lungs. It generally becomes difficult to breathe through a tube past three feet under the water.

By always providing the breathing gas at ambient pressure, modern demand valve regulators ensure the diver can inhale and exhale naturally and virtually effortlessly, regardless of depth.

Because the diver's nose and eyes are covered by a diving mask; the diver cannot breathe in through the nose, except when wearing a full face diving mask. However, inhaling from a regulator's mouthpiece becomes second nature very quickly.

Open-circuit

The most commonly used scuba set today is the "single-hose" open circuit 2-stage diving regulator, coupled to a single pressurized gas cylinder, with the first stage on the cylinder and the second stage at the mouthpiece. This arrangement differs from Emile Gagnan's and Jacques Cousteau's original 1942 "twin-hose" design, known as the Aqua-lung, in which the cylinder's pressure was reduced to ambient pressure in one or two or three stages which were all on the cylinder. The "single-hose" system has significant advantages over the original system.

In the "single-hose" two-stage design, the first stage regulator reduces the cylinder pressure of about 200 bar (3000 psi) to an intermediate level of about 10 bar (145 psi) The second stage demand valve regulator, connected via a low pressure hose to the first stage, delivers the breathing gas at the correct ambient pressure to the diver's mouth and lungs. The diver's exhaled gases are exhausted directly to the environment as waste. The first stage typically has at least one outlet delivering breathing gas at unreduced tank pressure. This is connected to the diver's pressure gauge or computer, in order to show how much breathing gas remains.

Rebreather

Less common are closed and semi-closed rebreathers, which unlike open-circuit sets, which vent off all exhaled gases, reprocess each exhaled breath for re-use by removing the carbon dioxide buildup and replacing the oxygen used by the diver.

Rebreathers release few or no gas bubbles into the water, and use much less oxygen per hour because exhaled oxygen is recovered; this has advantages for research, military, photography, and other applications. The first modern rebreather was the MK-19 that was developed at S-Tron by Ralph Osterhout that was the first electronic system. Rebreathers are more complex and more expensive than sport open-circuit scuba, and need special training and maintenance to be safely used.

Gas mixtures

For some diving, gas mixtures other than normal atmospheric air (21% oxygen, 78% nitrogen, 1% trace gases) can be used, so long as the diver is properly trained in their use. The most commonly used mixture is Enriched Air Nitrox, which is air with extra oxygen, often with 32% or 36% oxygen, and thus less nitrogen, reducing the likelihood of decompression sickness. The reduced nitrogen may also allow for no or less decompression stop times and a shorter surface interval between dives. A common misconception is that nitrox can reduce narcosis, but research has shown that oxygen is also narcotic.

Several other common gas mixtures are in use, and all need specialized training. The increased oxygen levels in nitrox help fend off decompression sickness, however below the maximum operating depth of the mixture, the increased partial pressure of oxygen can lead to oxygen toxicity. To displace nitrogen without the increased oxygen concentration, other diluents can be used, often helium, when the resultant mixture is called trimix.

In cases of technical dives, some of the cylinders may contain different gas mixture for each phase of the dive, typically designated as Travel, Bottom, and Decompression. These different gas mixtures may be used to extend bottom time, reduce inert gas narcotic effects, and reduce decompression times.

Because compressed air is 78% nitrogen, much of the weight of air in conventional open-circuit scuba is nitrogen. Rebreathers do not have to carry nitrogen in addition to the oxygen, so they can be lighter while using the same amount of oxygen. Because the nitrogen in the system is kept to a minimum, decompressing is much less complicated than traditional SCUBA systems divers can stay down longer. Because rebreathers produce very few bubbles, they do not disturb marine life or make a diver’s presence known. This is very good for underwater photography.

Windsurfing is a surface water sport using a windsurf board usually two to four meters long and powered by the effect of the wind on a sail. The rig is connected to the board by a free-rotating universal joint and comprises a mast, wishbone boom and sail. The sail area ranges from less than 3.0m² to more than 12m² depending on the conditions, the skill of the sailor and the type of windsurfing being undertaken.

At one time referred to as "surfing's ginger haired cousin" by the sport's legendary champion, Robby Naish, windsurfing has long struggled to present a coherent image of the sport to outsiders. Indeed, until the 1990s participants would regularly use different names to describe the sport, including sailboarding and board sailing. Despite the term "Windsurfing" becoming the accepted name for the sport, participants are still called "sailors" and not "surfers".

In fact windsurfing can be said to straddle both the laid-back culture of surf sports and the more rules-based environment of sailing. Although it might be considered a minimalistic version of a sailboat, a windsurfer offers experiences that are outside the scope of any other sailing craft design. Windsurfers can perform jumps, inverted loops, spinning maneuvers, and other "freestyle" moves that cannot be matched by any sailboat. When compared to surfing, Windsurfers were the first to ride the world's largest waves, such as Jaws on the island of Maui, and, with very few exceptions, it was not until the advent of tow-in surfing that waves of that size became accessible to traditional surfers. Extreme waves aside, many expert windsurfers will ride the same waves as surfers do (wind permitting) and are themselves usually very accomplished without a rig on a conventional surfboard.

The sport has a potentially shallower (longer) learning curve when compared to other so-called "extreme" sports, like snowboarding, freeride Mountain Biking or kitesurfing. The average beginner starting off on a 3.8 m long board with a tiny triangular sail in less than 5 knots of wind on a shallow lake often struggles to see the similarity between what they are doing and the images they see in magazines of a more advanced sailor using a 2.25 m board to ride waves in 20-30 knots of wind.

Key to this is the difference between displacement sailing and hydroplaning (referred to as "planing"). The former takes place in light winds (up to 10 knots) and involves the hull moving through the water using (typically) a centreboard and fin or skeg for stability and lateral resistance. Directional control is achieved via the rig and weighting one or other side the board, or sinking the tail.

When the wind gets above 8-10 knots (typically 15 knots+ for recreational equipment) the board ceases to move through the water and instead planes on top of the water, skimming over the surface at much higher speeds. To make the most of planing conditions, the board needs to be smaller and can dispense with the centreboard as sufficient lift and lateral resistance are provided by the fin (or combination of fins). When planing, changing direction is achieved via rotating the rig and engaging one of the rails (edges) of the board which is referred to as carving. Though windsurfing is possible in winds from near 0 to 50 knots, the ideal planing conditions for most recreational sailors is 15-25 knots.

Beginners must develop their balance and core stability, acquire an understanding of sailing theory, and learn a range of techniques before they can progress to planing windsurfing.

Initial lessons can be taken with a Windsurfing School, which exist in reasonable numbers in most countries. With coaching and favorable conditions, the basic skills of sailing, steering, and turning can be learned within a few hours. Competence in the sport and mastery of more advanced maneuvers such as planing, carve gybing (turning downwind at speed), water starting, jumping, and more advanced moves can require lengthy practice. Training DVDs exist which are useful in a sport where it is difficult for a coach to be close to a pupil particularly when learning the more advanced maneuvers.

Nevertheless, windsurfing is a sport which, once mastered, can be enjoyed, even at an advanced level, well into retirement and then at a more sedate level for considerably longer still. This is partly down to the fact that windsurfing crashes tend to cause less injury than those sports which take place on harder surfaces (although being reckless whilst windsurfing in advanced conditions can still cause serious injury due to the speeds and altitudes involved).

Windsurfing is predominately undertaken on a non-competitive basis. Organised competition does take place at all levels across the world and typical formats for competitive windsurfing include speed sailing, slalom, course racing, wave sailing, superX, and freestyle.

The boom of the 1980s led windsurfing to be recognized as an Olympic sport in 1984. However, windsurfing's popularity saw a sharp decline in the mid-1990s, as equipment became more specialized, requiring more expertise to sail. Now the sport is experiencing a modest revival, as new beginner-friendly designs are becoming available.

History

Windsurfing, as a sport and recreational activity, did not emerge until the later half of the 20th century. But before this, there have been sailing boats of various designs that have used wind as the driving force for millennia, and Polynesians have been riding waves for many of them, undertaking day trips over oceans standing upright on a solid board with a vertical sail. Therefore, crediting a single person with the invention of windsurfing would be presumptuous.

However, because of the financial stakes in the manufacture and sale of windsurfing equipment, and the way in which a sport invented by core activists was eventually commercialised, there has been considerable dispute and litigation between parties claiming the rights to the invention. As with many modern commercial disputes, the origin and ownership of the final design was resolved in court, but it is accepted that the earliest modern design originated in the United Kingdom.

Universal joint, moveable sail, steering

In 1964, over a discussion on water sports over a brandy at his home in Southern California, RAND Corporation aeronautical engineer Jim Drake and his former Rockwell boss and now good friend Fred Payne, who worked at The Pentagon, discussed options for creating a wind-powered water-ski which would allow Payne to travel on the Potomac River.[4] That night they developed the idea of a kite powered surfboard. On later reflection, Drake didn't like the integrity of the idea and dismissed it. There were already a number of sailboard designs available, and Drake also was concerned about the integrity of a design needing taut wire close to a human body to keep the sail upright.

Still developing the idea, Drake's wife met the pregnant Diana Schweitzer, and the two families became good friends through their children. Drake mentioned the idea to surfer Hoyle Schweitzer who wanted to develop it, but Drake was still unsure of how to control and steer what he envisaged in a design concept as a surfboard with upright sail design, whereby the sailor stood upright on the board holding the sail.

The technical problem was that most boats steer by varying the angle of attack in the water between the centre board and the rudder, and Drake's question came down to simple operation of how a standing person could control both the power of the sail as well as the direction of the craft.

In 1967, while driving between his home and a contract at the Norton Air Force Base in San Bernadino, Drake had time to reflect on early 1600s based sail ship control. Rudders then were weak and ineffective, mostly used for trimming course. Hence with multi-masted boats, the sailors would trim the upper sails on the forward and rearwards masts to steer the ship.

Dismissing the idea of a design with two upright sails, Drake decided to move the sail by rotation, as moving it linearly would require a mechanical system. Experimenting with a rotational design which became the concept for the universal joint, whereby the angle of attack of the sail to the board could be varied to allow control of both power and craft direction. Drake finished the design by using an earlier but for them failed invention of East Coast racing sail, and added a wishbone boom.

Windsurfing International

In 1968, Drake and Hoyle together as individuals filed the very first windsurfing patent, which was granted by the USPTO in 1970. There is no evidence that they had knowledge of any prior inventions similar to theirs, but Drake accepts in retrospect that although he can be credited with invention, he was "probably no better than third," behind Englishman Peter Chilvers and mid-west based Newman Darby.

The early windsurfing boards were made of foam in the garages of Drake and Hoyle, with the booms, tees and daggerboards hand crafted in teak. Hoyle sub-contracted the manufacture of the teak items to boat builder Ennals Ives in Taiwan, but the quality and costs of transportation brought other issues. One of the early customers was Bert Salisbury, and the first international shipment of a container of boards went to Sweden. Early customers also included Lufthansa pilots who had read about the board, who simply included one as personal luggage on their return journey form Los Angeles International Airport.

To ensure the quality of the product and handle marketing, in 1968 Hoyle and Diana Schweitzer founded the company Windsurfing International in Southern California to manufacture, promote and license a windsurfer design. The jointly owned patent was wholly licensed by Drake and Hoyle to Windsurfing International. Working in a factory unit in Torrance, California, Hoyle, who had previously built personal surf boards in his garage, was unhappy with the durability of the early "Baja Board." He therefore developed a new mould, based on an old Malibu surfboard design that Matt Kivlin had developed, which the company sub-contracted for mass manufacture to Elmer Good.

The company registered the term "windsurfer" as a trademark at the United States Patent and Trademark Office in 1973, launching the craft as a one-design class. Going one-design was influenced by the success of the Laser and Hobie Cat classes. Each Windsurfer had an identical computer-cut sail, a technology new at that time and pioneered by Ian Bruce and the Laser class.

In 1968, Hoyle's computer business collapsed, and he and Diane moved to Newport Beach; at the same time Drake accepted a two year secondment to The Pentagon, and moved to Washington DC. Immediately, Hoyle offered Drake to buy out his half of the patent, and it was only when Hoyle pointed out ownership of the company that the relationship between the pair began to fall apart. Having returned to California, in 1973 Drake sold his half of the patent to Windsurfing International for the sum of $36,000.

Patent disputes

Through the seventies, Schweitzer aggressively promoted and licensed the Windsurfing International design and licensed the patent to manufacturers worldwide, mainly through competition and the publication of a magazine. Resultantly, the sport underwent very rapid growth, particularly in Europe after the sale of a sub-license sold to Ten Cate in Holland.

At the same time, Schweitzer also sought to defend his patent rights vigorously against unauthorized manufacturers. This led to a host of pre-dating windsurfer-like devices being presented to courts around the world by companies disputing Windsurfing International's rights to the invention.

In 1979, Schweitzer licensed Brittany, France-based company Dufour Wing, which was later merged with Tabur Marine - the precursor of Bic Sport. Europe was now the largest growing market for windsurfers, and the sub-licensed companies - Tabur, F2, Mistral - wanted to find a way to remove or reduce their royalty payments to Windsurfing International.

Tabur lawyers found prior art, in a local English newspaper which had published a single story with a picture about Peter Chilvers, who as a young boy on Hayling Island on the south coast of England, assembled his first board combined with a sail, in 1958. They also found stories published about the 1964 invention of the Darby Sailboard by Newman Darby in Wilkes-Barre, Pennsylvania. This used a universal joint, one of the key parts of the Windsurfing International patent.

In Windsurfing International Inc. v Tabur Marine (GB) Ltd. 1985 RPC 59, with Tabur backed financially by French sailing fanatic Baron Marcel Bich, British courts recognized the prior art of Peter Chilvers. Intended to be steered by a rudder, it did not incorporate the curved wishbone booms of the modern windsurfer, but rather a "straight split boom". The courts found that the Schweitzer windsurfer boom was "merely an obvious extension". It is worthy of note that this court case set a significant precedent for patent law in the United Kingdom, in terms of inventive step and non-obviousness; the court upheld the defendant's claim that the Schweitzer patent was invalid, based on film footage of Chilvers. Schweitzer then sued the company in Canada, where the opposition team again financially backed by Bic included Chilvers and Jim Drake, and Schweitzer lost again. After the cases, no longer obliged to pay Windsurfing International any royalty payments, the now renamed Bic Sport became the world’s largest producer of windsurfing equipment, with an annual production of 15,000 boards.

In 1983, Schweitzer sued Swiss board manufacturer Mistral, which is today still a major sailboard manufacturer, and lost. Mistral's defense hinged on the work of US inventor Newman Darby, who in the mid-sixties conceived the "sailboard": a hand-held square rigged "kite" sail on a floating platform for recreational use. Darby's published version did not show any connection between the rig and the board (the mast simply rested in a depression on the board) but it did refer to a "more complex swivel step for advanced riders not shown". He published his "sailboard" design in August 1965 Popular Science magazine. Darby organized Darby Industries Inc in 1964 to build these sailboards. However, the sailboard never gained popularity, and Darby's company ceased operations by the end of the 1960s.

Eventually US courts recognized the Schweitzer windsurfer as an obvious step from Darby's prior art. Schweitzer had to reapply for a patent under severely limited terms, and finally it expired in 1987. Shortly thereafter, having lost its license royalty income, Windsurfing International ceased operations.

In 1983, Australian courts reported a patent case in "Intellectual Property Reports" 3 IPR 449, attributed the first legally accepted use to an Australian boy, Richard Eastaugh. Between the ages of ten and thirteen, from 1946 to 1949, aided by his younger brothers, he built around 20 galvanized iron canoes and hill trolleys which he equipped with sails with split bamboo booms. He sailed these near his home on the Swan River in Perth. There is no evidence that any of the later "inventors" ever sighted the Eastaugh craft of a decade earlier on the other side of the world.

It is acknowledged that the separate Chilvers, Darby and Eastaugh inventions all pre-dated the Drake and Schweitzer patent.

Boards and gear

In the 1970s and 1980s, windsurfers were classified as either shortboards or longboards. Longboards were usually longer than 3 meters, with a retractable daggerboard, and were optimized for lighter winds or course racing. Shortboards were less than 3 meters long and were designed for planing conditions. However, this classification by length has become obsolete, as new techniques, designs, and materials have taken the sport in new directions.
A windsurfer using a slalom board to perform a small jump.

Most modern windsurfers (1990s and later) are derived from the shortboard design, and are intended to be used primarily in planing mode, where the board is mostly skipping over the surface of the water, rather than cutting through, and displacing the water. Planing is faster and gives more maneuverability, but requires a different technique from the displacement mode (which is also referred to as slogging or schlogging). Generally, smaller (i.e., lower volume, shorter length, narrower width) boards and smaller area sails are used as the wind increases.

While windsurfing is possible under a wide range of wind conditions, most recreational windsurfers prefer to sail in conditions that allow for consistent planing with multi-purpose, not overly specialized, free-ride equipment. Larger (100 to 140 liters) free-ride boards are capable of planing at wind speeds as low as 12 knots if rigged with an adequate, well-tuned sail in the six to eight square meter range. The pursuit of planing in lower and lower winds has driven the development and spread of wider and shorter boards, with which planing is possible in wind speeds as low as 8 knots, if sails in the 10 to 12 square meter range are used.

Modern windsurfing boards can be classified into these categories:

• Freeride: Boards meant for comfortable recreational cruising (mostly straight-line sailing and occasional turning) at planing speed (aka blasting), mainly in flat waters or in light to moderate swell. They typically fall into the volume range of 90 to 170 liters. The so-called freeride sailing movement diverged from course racing as more recreational sailors chose to sail freely without being constrained to sailing on courses around buoys.
• Formula Windsurfing Class: Shorter boards up to one meter in width, for use in Formula Windsurfing races. See below for a more detailed description.
• Wave boards: Smaller, lighter, more maneuverable boards for use in breaking waves. Characteristically, sailors on wave boards perform high jumps while sailing against waves, and they ride the face of a wave performing narrow linked turns (bottom turns, cutbacks, and top-turns) in a similar way to surfing. Wave boards usually have a volume between 65 and 90 liters, with a length between 230 and 260 centimeters, and 50 to 60 centimeters in width. A general rule is for a sailor to use a wave board whose volume in liters is about the same as the sailor's weight in kilograms - more volume providing additional flotation for sailing in light winds, and less for high winds, where less volume is needed to achieve planing. In recent years, the average width of wave boards has increased slightly, as the length has shrunk, while the range of volume has been maintained the same more or less - according to board designers this makes wave boards easier to use under a wider range of conditions by sailors of differing abilities. The most common sizes of sails used with wave boards are in the range of 4.0 to 6.0 square meters, depending on the wind speed and the weight of the sailor.
• Freestyle boards: Related to wave boards in terms of maneuverability, these are wider, higher volume boards geared specifically at performing acrobatic tricks (jumps, rotations, slides, flips and loops) on flat water. Usually 80 to 110 liters in volume, and about 240 to 250 centimeters in length, with widths frequently in excess of 60 centimeters. Freestyle boards began to diverge more noticeably in design from wave boards in the early part of the 2000 decade, as aerial tricks (the Vulcan, Spock, Grubby, Flaka, and related New School maneuvers, almost all involving a jump-and-spin component) became the predominant part of the freestyle repertoire, superseding Old School moves, in which the board did not leave contact with the water.
• Slalom boards: Shortboards aimed at top speed, rather than maneuverability or ease of use.
• Beginner boards: (sometimes called funboards) these often have a daggerboard, are almost as wide as Formula boards, and have plenty of volume, hence stability.
• Racing longboards: Mistral One Design, or the Olympic RS:X class race boards.

There are many attempts to bridge a gap between two of these categories, such as freerace, freestyle-wave, freeformula, and so on.

The original Windsurfer board had a body made out of polyethylene filled with PVC foam. Later, hollow glass-reinforced epoxy designs were used. Most boards produced today have an expanded polystyrene foam core reinforced with a composite sandwich shell, that can include carbon fiber, kevlar, or fiberglass in a matrix of epoxy and sometimes plywood and thermoplastics. Racing and wave boards are usually very light (5 to 7 kg), and are made out of carbon sandwich. Such boards are very brittle, and veneer is sometimes used to make them more shock-resistant. Boards aimed at the beginners are heavier (8 to 15 kg) and more robust, contain more fiberglass, or even have an indestructible molded plastic shell.

Sails

A windsurfing sail is made of monofilm (clear polyester film), dacron (woven polyester) and mylar. Sensitive parts are reinforced with kevlar mesh.

Two designs of a sail are predominant: camber induced and rotational. Cambered sails have 1-5 camber inducers, plastic devices at the ends of battens which cup against the mast. They help to hold a rigid aerofoil shape in the sail, better for speed and stability, but at the cost of maneuverability and generally how light and easy to use the sail feels. The trend is that racier sails have camber inducers while wave sails and most recreational sails do not. The rigidity of the sail is also determined by a number of battens.

Beginners' sails often do not have battens, so they are lighter and easier to use in light winds. However, as the sailor improves, a battened sail will provide greater stability in stronger winds.
The windsurfer in the foreground is using a camber induced sail and is fully planing using the footstraps, while the other is using a rotational sail and is not planing.

Rotational sails have battens which protrude beyond the back aspect of the mast. They have to flip to the other side of the mast when tacking or jibing, hence the rotation in the name. Rotational sails have aerofoil shape on the leeward side only when filled with wind. They can be absolutely flat and depowered when sheeted out.

In comparison with cambered sails, rotational designs offer less power and stability when sailing straight, but are easier to handle when maneuvering. Also, rotational sails are much easier to rig.

The leading edge of a sail is called the luff. The mast is in the luff tube. The rear edge is called the leech. The front bottom corner of the sail, where the mast foot protrudes, is called the tack, and the rear corner, to which the boom is attached, is called the clew. The bottom edge, between the clew and the tack, is called the foot.

A windsurfing sail is tensioned at two points: at the tack (by downhaul), and at the clew (by outhaul). There is a set of pulleys for downhauling at the tack and there's a grommet at the clew. Most shape is given to the sail by a very strong downhaul, bending the mast in the luff tube. The outhaul tension is relatively weak, mostly to provide leverage for controlling the sail's angle of attack.

The sail is tuned by adjusting the downhaul and the outhaul. Generally, the sail has to be trimmed more for stronger winds. More downhaul tension loosens the upper part of the leech, "spilling" the wind at the gusts and shifting the center of effort of the sail down. Releasing the downhaul tension shifts the center of effort up. More outhaul lowers the camber/draft, making the sail flatter and easier to control, but less powerful, and less outhaul brings more overall depth to the sail, more low-end power, shifts the center of effort upward and to the front, and may limit speed by increasing aerodynamic resistance.

The disciplines of windsurfing (wave, freestyle, freeride) require different sails. Wave sails are reinforced to survive the surf, and are flat when depowered to allow riding the waves like surfers do. Freestyle sails are also flat when depowered, and have high low-end power to allow quick acceleration. Freeride sails are all-rounders that are comfortable to use and are meant for recreational windsurfing. Racing sails provide speed at the expense of qualities like comfort or maneuverability.

The size of the sail is measured in square meters and can be from 3 m2 to 5.5 m2 for wave sails and from 6 m2 to 15 m2 for racing sails, with ranges for freestyle and freeride sails spanning somewhere between these extremes. Learning sails for children can be as small as 0.7 m2 and racing sails being up to 15 m2 large.