A sail is a surface aimed at generating thrust when the wind acts on it. The principle is identical to a vertical wing generating lift through aerodynamic forces induced by the wind acting on it. The force exercised by the wind is called sail thrust and is always at right angles to the sail's profile whatever its angle to the wind. This is due to a fluid dynamics principle called the Bernoulli principle and the sail thrust is the result of the different wind pressures on the sail's windward and downwind sides. The sail has a concave shape to minimize the sail's initial angle of impact with the wind.
The sail thrust can break down in two forces in two separate directions: one in the direction of the boat's movement and the other at right angles to it. The first is called propelling thrust and is the useful force that pushes the boat forward. The second is called the leeway and causes lateral displacement of the boat (that is to say the leeway) and also makes it list.
In sail boats, a keel or a centreboard are used to counteract the lateral push so as to use only the propelling thrust to move the boat forward. In this way, the boat can reach a speed of 2-3 times that of the wind.
The sail's shape influences the thrust; its shape can be altered by working on the boats rigging identified as fat and thin for sails giving more or less power.