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How does a sail work - Sail Pressure

Go To: Sailing - Learn To Sail

Posted on Friday, November 30, 2007 6:23 PM

How does a sail work? You don't just get blown along by the wind, you get pulled along by it too!

You may think a boat is blown, or pushed along by the wind, and you would be partly correct - when a boat is sailing downwind, the boat is blown along. However, when a boat is sailing upwind (beating), it is actually pulled along.

How unseen forces work on a sail

Diagram 1: How unseen forces work on a sail

A boat moves in a windward direction (towards the wind) by forces which are created on both sides of the sail. The forces create a pushing force on the windward side (the positive force) and a pulling force on the leeward side (the negative force), both combined work in the same direction. The negative, pulling is the more stronger of the two forces.

'In 1738 the scientist Daniel Bernoulli discovered that an increase in air flow velocity in relation to the surrounding free air stream causes a decrease in pressure where the faster flow occurs. This is what happens on the leeward side of the sail - the air speeds up and creates a low-pressure area behind the sail.'

If you lightly hold a spoon by a water flow of a tap you expect it to be pushed out the main flow, this is wrong the spoon will actually be pulling/sucked in to the flow. This is the same aspect as a sail.

How air flows around a sail

Diagram 2: How air flows around a sail

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