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The Physics of Sailing


gdaunton

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Sailing is very very old. The first representation of a sailboat was in Kuwait dating between 5000 and 5500 BC. This technology is not only old but also has some cool physics behind it.

The basic principle of sailing is as follows:

-As a boat sails across the wind the sail redirects the wind, causing the wind below the sail to be slower than the wind above the sail. This causes a low pressure system under the sail which causes lift to push the boat sideways and forwards.

-As a boat sails with the wind, the wind simply pushes on the sail causing the boat to accelerate.

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However, in this new age, boats are being made that go faster than the wind that pushes them. How is this possible?

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Well think about this: doesn't it seem that when you roll down the windows on the highway that the wind is a lot faster outside the car than when you are stopped?

The same thing happens with these massive sailboats. As the boat moves faster the relative wind speed increases and the boat continues to accelerate. To find this relative wind speed we use the equation vr = vb + vw where vb = the boats velocity and vw = the winds velocity. According to this equation the boat can continue to accelerate until the drag from the water matches the force of the relative wind pushing the boat forward.

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But in the case of the AC72's racing in the Americas cup, in order to sail faster and faster, they needed to find a way to reduce the drag from the water. To do this the boats rise up on hydrofoils and literally fly on top of the water.

These foils also use a very basic physics principle: Bernoulli's principle. As water moves over the top of the foil it slows down where as the water under the foil stays the same speed. This creates a low pressure zone above the foil which creates lift.

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Air planes use this same principle to create lift to stay in the air.

All these principals come together to make a 72 foot boat go 50 mph in 24 mph winds

My sources:

http://www.phys.unsw.edu.au/~jw/sailing.html

http://www.unc.edu/~thriveni/sailing/lift.html

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