Sailing is the movement of a boat powered by wind across a body of water. The science of steering a sailboat is the application of a series of physical principles that underlie the fundamental relationship between wind speed and direction, the size and shape of the boat, and the characteristics of moving water.
The wind forces acting upon a sail are often compared to the effect of wind upon the wing of an aircraft. A boat may be thought of as possessing two wings, one above the water (the sail), and one below the water (the keel). As wind strikes the sail, the sail acts as a foil, causing the air current striking the sail to move at different speeds on each side of the sail. The side of the sail exposed to slower moving air is subject to greater air pressure, creating lift. Lift is a force that acts perpendicular to the direction of the force created by the air, as lift causes the boat to move in the direction of the higher speed air, which results in lower air pressure on the sail. This effect above water is counterbalanced by the water forces acting upon the keel, the portion of the boat that extends below the waterline to provide both ballast (counterweight) to the boat and to prevent the boat from moving sideways.
The steering of the boat is performed through the combined action of the positioning of the sail, using the sheet (the rope or ropes that control the sail position), and the rudder, an airfoil shaped board attached by a hinge to the stern (rear) of the boat where it immersed in the water. The rudder is controlled by the helmsman (the sailor responsible for steering the boat) through a tiller extension, a mechanism that permits the turning of the rudder by either a steering wheel or on a smaller sailboat, by direct linkage to the rudder called a tiller.
The steering of the sailboat, as a part of the setting and the maintenance of a particular course across the water involves four fundamental sailing manouvers—tacking, jibing, heading up, and bearing away. The combined effects of the sail and the keel mean that the boat may be steered in any direction except directly into the wind. The fastest and most nimble racing vessels can only sail as close to the wind direction by as much as approximately 35% into the wind.
The rudder and the tiller operate as a large lever. A turn is accomplished by pushing the rudder through the water. To turn the boat to the right, the tiller must be pushed to the right; a left turn requires a push to the left. The foil design of the rudder creates a pressure differential that steers the stern of the boat into the low-pressure zone. The bow swings to the opposite direction.
Tacking (also known as "coming about") is a steering concept that recognizes that the boat cannot sail directly into the wind. Tacking to achieve a change in direction is distinct from the "tack" (the direction that a boat may travel). Tacking is the maneuvering of the boat in an upwind direction (against the wind), creating a zigzag motion across the direction of the wind, without ever proceeding directly into the wind, where the sails would be ineffective. The bow of the boat is at all times facing the direction of the wind when the boat is tacked. As the wind moves in a direction across the boat into the sails that is perpendicular to the boat's direction of travel, the boat is not pushed in the direction of the wind due to the drag force exerted by the keel. The drag force acts to negate the wind effect, and as the keel is aligned with the hull of the boat, the boat is directed forward and not sideways. A sailboat will often perform a number of tacking movements to achieve a desired steering result across a body of water. At each tack, the sails are brought from one side of the boat to the other, through the movement of the boom, the pivoting hardware to which the sails are attached, mounted perpendicular to the mast to redirect the force of the wind; the rudder is correspondingly adjusted. In many tacks, depending upon the wind speed, the size and construction of the boat, or the condition of the water, the boat may heel, the crew of the boat will have to move from one side of the boat to the other to stabilize the boat against the forces of the wind during the tacking process.
Jibing (also spelled in various authorities as gybing and gibing) is a steering technique that is similar to tacking, except that jibing is performed when the boat is being steered in the same direction as the wind, with the stern at all times facing the wind. Jibing is often a more dynamic steering movement than a tack, because the sails remain full of wind throughout the maneuver. A boat moving on a downwind path is able to navigate more directly to an intended destination; variations in course direction a downwind course are achieved through jibing. As with a tack, the crew will usually move to the side of the boat opposite the sails to maintain the balance of the boat (a movement that recalls the old expression "maintaining an even keel").
Heading up the boat means to steer the boat closer to the direction of the wind. To achieve this result, the sails must be brought closer to the center-line of the craft; if boat is too close to a position directly opposed to the wind direction, the sails will flutter, or luff, without power and the boat will not move. Heading up the boat is the preliminary aspect of tacking. Bearing away the boat is to direct the boat to permit the wind to come from the stern; jibing is the steering technique that will follow an effort to bear away.