How can sailboats travel faster than the wind

A.� As the boat moves in response to the apparent wind it creates its own additional wind or ‘slipstream’ that adds to the propelling forces. As long as the sailors know how to fix the sail to harness this ‘extra’ wind then the sailboat is able to travel faster than the speed of the apparent wind.

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Q.� So, what are the best conditions for the boat to sail faster than the wind

A.� When the apparent wind hits the sail at right angles then, as the boat picks up speed it also creates its own slipstream. As long as the crew align the sail to harness both the apparent wind and the beneficial effects of the slipstream then the joint propulsive force increases, making the boat go faster than the speed of the apparent wind.

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Q.� How do the dimensions of the boat factor in

A.� When the sailboat is travelling using the joint propulsive forces of the apparent wind and its own slipstream it could theoretically keep going faster and faster until it capsized. However the hull of the boat meets resistance from the water, which creates a drag that slows the boat down.

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Therefore a sailboats top speed is determined by the drag on the bottom of the boat, rather than wind speed.

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Q.� How does the hull affect speed

A.� Because boats create a wave as they move through water that becomes longer the faster the boat moves until the wavelength is equal to the length of the boat. At this point the boat sinks into the trough formed between the two wave peaks, which prevents it picking up any more speed.

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by Lisa Cardy