The sail alone is not sufficient to drive the boat in any desired direction, as a sail by itself would only push a boat in the same direction as the wind. Sailboats overcome this by having another physical object below the water line. This may include, a keel, centerboard, or some other form of underwater foil or even the hull itself (as in catamarans without centreboard or in a traditional proa) web development. Thus, the physical portion of the boat which is below water can be regarded as functioning as a "second sail". Having two surfaces against the wind and water enables the sailor to travel in almost any direction and to generate an additional source of lift from the water. The flow of water over the underwater hull portions creates a hydrodynamic force internet marketing. The combination of the aerodynamic force from the sails and the hydrodynamic force from the underwater hull section allows motion in almost any direction, except straight into the wind. This can be likened, in simple terms, to squeezing a wet bar of soap with two hands which causes it to shoot out in a direction perpendicular to both opposing forces. Depending on the efficiency of the rig, the angle of travel relative to the true wind can be as little as 35° to over 80°. This angle is called the tacking angle. With a 35° tacking angle on either side of the wind, it is possible for a sailboat to sail directly over 290° of the compass (360° − 2 × 35° = 290°).

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