# Stuck understanding Bernoulli's Principle, would love some help!

by avidstr
Tags: bernoulli, love, principle, stuck
 P: 1 Hi all - I was having a debate with some students in my flipped class today and I can't wrap my head around Bernoulli's Principle as it relates to the sea. Can anyone explain how Bernoulli's principle works with a sail boat? I don't know how the force vectors resolve to push a sail boat faster into the wind than with the wind. Thanks so much for any help!
 HW Helper P: 6,900 A sailboat can't sail directly upwind. A very low drag sailboat may be able to tach downwind such that the net downwind speed is greater than the wind speed. A better example would be an ice boat, where there is very little drag, and these can acheive downwind speed component much greater than the wind speed. There are also directly down wind faster than the wind (DDWFTW) vehicles that use the wheels to drive a propeller, and these can also go faster than the wind, with a full scale model achieving close to 3x wind speed with a wind around 10 mph. This works because the wind speed versus the ice / ground / water speed remains the same regardless of how fast the craft moves. Bernoulli equation approximates the amount of pressure versus speed of the air as the air is deflected by a sail or wing, but it's not the key reason for the high speed that these craft can achieve, other than for these vehicles to work the sail or drive train and propeller need to be efficient. Wind powered vehicles get their energy by slowing down the wind. It doesn't matter if the wind is slowed down directly by the sail or propeller, or slowed down due to an upwind component of the wash of air diverted by a sail or accelerated by a propeller. Other threads with links to even more threads: http://www.physicsforums.com/showthread.php?t=562993 http://www.physicsforums.com/showthread.php?t=421733

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