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How a sailboat moves

Airflow over sails separates on to the windward (high pressure) and leeward (low pressure) sides. The turbulence is created at the leading trailing edges. The difference between the positive and negative pressure creates the aerodynamic force that pushes the sail forwards and moves the boat along. This force is is sufficient to overcome the resistance of water, hull, sails and rigging.
The sails must always be adjusted to make maximum use of the speed and direction of apparent wind.
True wind is the real wind experienced by a static object
Apparent wind is the wind experienced by a moving object

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windward side apparent wind true wind induced wind turbulence air flow separation negative low preasure positive high preasure leeward side