Sailing is the control of a vessel moving across a body of water that uses wind power for propulsion. The physics of sailing has many subtleties, and the act of successfully sailing a boat is a combination of art, scientific principles, and the experience of the sailor. Those subtleties will be grounded in three distinct aspects of sailing—the function of the sails, the shape and the configuration of the hull, and the function of the keel.
The sails are the most distinctive physical feature of any wind-powered vessel. The sail is designed to act as a foil, with similar physical properties to those of an aircraft wing. As wind moves across the surface of a sail, the air moves at a higher speed where the wind strikes the surface. The faster moving air produces lower pressure upon the sail than does the slower moving air on the opposite side of the sail. It is a rule of physics, referred to as the Bernoulli principle, that the vessel will move into the area of lower air pressure, in the same fashion that an aircraft wing, angled upwards, will lift when air is directed against it.
If the wind striking a sail is coming from directly behind a sailboat, the boat will travel in the same direction as the wind and at the same speed as the wind. The boat cannot travel faster than the wind speed because the sails block the path of the wind, an act that decelerates the wind. When the wind is striking the sails at an angle toward the direction of travel of the boat, the power of the wind is directed into forward motion through the stabilizing effect of the keel. The keel is the device attached to the hull that extends below the water surface and provides both ballast as well as a counterforce to the wind. The wind force that strikes the sails at an angle to the path of the boat does not take the boat in a lateral direction, because the keel counters all such direction through the creation of opposite pressures in the water.
The function of a sail as a foil means that a boat can be steered upwind (into the wind), so long as the boat is not headed directly into the wind. The degree to which a boat can be sailed upwind varies with the construction of the boat and the type of sails. As a general rule, sails will not act as a wind foil if the wind direction is less than 30° from the centerline of the boat, with the bow first.
Applying these principles, the sailboat is sailed upwind using a combination of the rudder and wind direction to traverse a zigzag path across the water, a technique known as tacking.
Sails on boats large and small are generally controlled by one or more pulleys. A pulley is a type of lever, that may both change the direction of a force applied, or when used in combination, the pulleys may multiply the forces applied to it. The rope found in a common pulley is the lever, and the grooved wheel in which the rope rests is the device used to change the forces.
When a single pulley is employed to move a sail, the amount of force required to move the mass of the sail is unchanged. The pulley makes the action of applying the force easier, as the direction from which the force is applied is adjustable to the convenience of the user. Normally, the more pulleys used to move a sail, the less force required.
As a general rule (although different physical considerations apply to vessels such as windsurfers), the larger the sailboat, the faster it will be able to travel. Where boats are designed to displace water, as in the case of a keel boat, the length of the boat is an important component in determining the physics of the maximum boat speed. The maximum boat speed is referred to as hull speed, calculated using the formula 1.34 multiplied by the square root of the length of the waterline (in feet). The waterline is the location of the hull where the boat rests on the surface. The result provides a maximum hull speed in knots, the measure of speed in nautical miles per hour. (A nautical mile is 1.15 miles, the distance described by 1 minute of 1 degree of longitude.)
As an example, the hull speed of a sailboat with a hull that is 25 ft long at the waterline would be calculated as 1.34 × 5, or 6.7 knots.
The boat cannot travel faster than the wave created by the bow of the boat as it moves through the water. Windsurfers are not subject to these considerations because they do not displace water like a keelboat does, but are intended to plane on top of the water. Windsurfers do not push water aside, as does a large keel boat, but skips along the water in the fashion of a stone that is thrown to bounce across the surface of the water.