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Sails convert the energy in the wind into forward movement of the boat. They do this in the same way as wings of an aeroplane provide lift. The wind causes the sail to take an airfoil shape. The wind passes around the sail and because the distance is greater on the leeward side of the sail, the wind must travel faster. This means the pressure on the leeward side is lower than the pressure on the windward side. This pressure difference results in a force acting perpendicular to the boom in the downwind direction. This force can be resolved as a force on the boat pushing it forward and a force on the boat pushing it sideways. The sideways force results in drift but is (for the most part) avoided by using a centerboard/daggerboard.
A sail has a particular angle to the wind at which it is most efficient. This is usually 45 degrees, which means that boats can travel at 45 degrees to the wind.
When moving in a downwind direction the sail no longer acts like a wing, but more like a parachute, catching the wind and moving the boat along with the wind. It is downwind that boats travel fastest.
Last Updated October 12th, © 2007 QUB Sailing club