Archive for February 8th, 2015

February 8, 2015

Downwind Faster than the Wind

blackbird

Sailing faster than the wind is the technique by which vehicles that are powered by sails (such as sailboats, iceboats and sand yachts) advance over the surface on which they travel faster than the wind that powers them. Typically, such devices cannot travel faster than the wind when sailing dead downwind using simple square sails that are set perpendicular to the wind. They require sails set at an angle to the wind, which utilizes the lateral resistance of the surface on which they sail (for example the water or the ice) to maintain a course at some other angle to the wind.

For those craft it is impossible to sail dead downwind faster than the wind because the apparent wind will be zero if the speed of the vehicle equals the speed of the wind. However, certain sailing craft (such as ice boats and high performance catamarans) can achieve overall downwind speeds faster than the wind by tacking back and forth across the wind: they do this by using the surface on which they sail to capture the energy of the wind. Similarly, it is possible to sail dead downwind faster than the wind if a mechanical device is used to transfer energy from the surface on which the machine is moving in order to capture the energy of the wind and use it (not through a sail). By using a propeller instead of a conventional sail, and coupling the propeller to its wheels, a land yacht can proceed dead downwind faster than the wind.

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