The sail of a yacht is like the wing of an aircraft except that it is oriented vertically rather than horizontally, so an air flow across it produces thrust instead of lift. A yacht uses wind as the source of its motive power. As it gains speed its motion through the air increases the airflow across its sail and it is able to reach speeds in excess of the wind speed. Land yachts which are just yachts on wheels can reach speeds of 3 to 4 times the wind speed.