Sails translates the movement of wind into thrust that propels the boat. Sailing has a place in human civilization that likely extends as far back in time as man has possessed the ability to make crafts that could float on water. Sailing has a shorter history as a competitive sport, but it has been one of the most popular and intensely contested of sporting activities throughout the world. People have engaged in the organized racing of sailboats, crafts that range in size from dinghies (small open boats with one sail), to very large, multi-masted ocean going vessels for over 200 years.

Modern competitive sailing is governed by the International Sailing Federation (ISF), whose predecessor organization was founded in 1907. The chief purpose in the formation of the ISF was to bring standardization to the rules of international sailing. Today sailing is a vibrant Olympic sport; there are 11 different Olympic sailing events. In the international arena, the ISF sanctions racing in 81 different categories of sailboats, the largest of which are the multi these boats may be over 60 ft (18 m) in length. Sailing is one international sport where the racing activity on the club and regional levels throughout the world attracts greater interest than Olympic competition.

The best known of the international sailing races, the America's Cup, has been contested since 1851. The racing boats are classed as America's Cup class yachts and adhere to a complicated mathematical formula that considers boat length, beam (width), sail area, hull depth, keel size and dimensions, and other physical characteristics of the boat. Sailors, the sailor's home club, and syndicates that are formed in support of a particular boat, commence an America's Cup competition through the issuance of a formal challenge to race the Cup holder, known as the defender. The technology involved in the design and construction of these crafts is so sophisticated that challenges are mounted and races staged for the Cup in several year intervals.

The beauty and the enduring appeal of the sport of sailing springs from the fact that the principles of sailing are universal, applicable from the smallest of open sail boats, to the most sophisticated America's Cup challenger. The propulsion of a sail boat is achieved through the effect of the wind and combined function of the sails, the hull, and the keel (or centerboard). The techniques employed by a sailor to sail the boat in a particular direction are determined by the force and the direction of the wind.

The function of the sail in powering a boat is simple when the wind is directly behind the sailboat and the intended direction of travel is the same as the wind direction. The sail is positioned to permit the wind to strike it at angle perpendicular to the line of the sail. In this position the wind pushes the sail, and consequently the boat. It is for this reason that when the boat and the wind are moving in the same direction, the boat can never travel faster than the wind. Enlarged billowing sails called spinnakers, provide maximum surface area to catch the following winds.

Key West Race Week.

When the wind is moving in a direction that is angled to the direction of the boat, the sail no longer functions simply as a surface against which the force of the wind acts. The sail on a boat, with a curvature similar to an aircraft wing, now has the same relationship to the wind as does an aircraft wing to the air flowing both over and under it. As the wind strikes the leading edge of the surface of a sail at an angle, the airflow separates. The faster moving air naturally creates a pocket of lower air pressure in contrast to the higher air pressure formed on the opposite side of the sail that is in contact with the slower airstream. The sail, and, consequently, the boat will move into the region of lower air pressure.

Sails are airfoils in accord with Bernoulli's equation, which states that static pressure plus dynamic pressure equals total pressure. The sails are designed to maximize lift and minimize drag. The upper surface of a typical airfoil has a curvature greater than that of the lower surface. This extra curvature is known as camber. The straight line, joining the front tip or the leading edge of the airfoil to the rear tip or the trailing edge, is known as the chord line. The angle of attack is the angle that the chord line forms with the direction of the air stream.

The stagnation point is the point at which the stream of air moving toward the wing divides into two streams, one flowing above and the other flowing below the wing. Air flows faster above a wing with greater camber since the same amount of air has to flow through a narrower space. According to Bernoulli's principle, the faster flowing air exerts less pressure on the top surface, so that the pressure on the lower surface is higher, and there is a net upward force on the wing, creating lift. A sail is a wing oriented along the axis defined by the mast of the boat and so lift becomes a horizontal driving force, a form of thrust, to propel a boat through the water. The camber of a sail is varied by devices that slightly alter the shape of the sail.

A sailboat can be sailed into the wind (upwind) to within approximately 30° of a direct upwind position. So long as the wind direction permits the creation of the two pressure regions on the sail, the wind will power forward movement. Any attempt to sail closer to the direction of the wind causes turbulence and a loss of air contact with the sail surface. Lift, and hence the driving force of the sail, is lost.

The hull of the boat is the physical outer structure that is in contact with the water. The front or nose of the hull is the bow; the rear is the stern. Attached to the hull in the approximate center of the boat is the keel, a larger structure that extends from the hull into the water. The general function of the keel is to provide stability to the boat. The specific purpose of the keel in relation to the function of the sails is to resist any lateral forces created by the wind as it pushes against the sails. If the sailboat did not have a keel, when the wind struck the sails from an angle the boat would be pushed both forward and laterally. The keel operates to convert the lateral forces of the wind into forward motion.

Tacking is the most common steering technique used in sailing. Tacking is the maneuver employed when the destination of the sailboat is in an upwind direction. The sailboat is steered in a series of angular courses forming a zigzag pattern across the water into the wind, to maximize the wind power available. Tacking requires the sailor to use the rudder to change direction, while making a simultaneous shift in the position of the sail to capture the maximum available wind. The boat moves into the wind by taking turns to the left and right of the wind to maintain a proper angle of attack for the sail. The tacking results in net forward motion into the wind with the movement to the left and right of the desired direction of travel offset by the opposing tacks.

The hull of the sailboat and its characteristics are also subject to intense scientific testing as the basis for the hull designs used in racing craft. The performance of a hull in water is a branch of the science known as hydrodynamics. Computer technology permits a significant level of design testing to be done by way of computer simulation, where an infinite manner of variables, including different sail configurations, weight, wind, lake or ocean current, and material design may be examined and tested at a relatively low cost.

SEE ALSO Computer simulations as a training tool; Sailing and steering a sailboat; Windsurfing.