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Can a sailing boat go faster than the wind?

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Title:
Can a sailing boat go faster than the wind?
Topic:
Sport
Question: 

Can a sailing boat go faster than the wind?

Answer: 

Yes, a boat can sail faster than speed of wind. It is effected by "apparent wind", which is the sum of two vectors: atmospheric wind (when the boat is at rest) and wind which is created by movement of the boat itself. Direction and speed of apparent wind depend on the atmospheric wind speed, the direction and velocity of the boat and the angle of its sail relative to the direction of atmospheric wind.

 Speed depends largely on the area of the sail. Newtons Second Law controls the movement. Two forces disturb the system: friction and the force of wind (which is proportional to the sail area). Note that friction force is nonlinear to boat speed and will increase when the speed of sailboat increases.

 To help sail even faster, we must decrease friction between the water and the submerged part of the boat. This can be done by making the boat very light and adding small wings (like an airplane) under water. These wings have a controllable angle of attack. Hydrodynamic forces from the wings lift the boat above the water leaving only the wings submerged. In this condition boat friction in the water can decrease by 10-20 times, which allows acceleration to speeds much higher than the wind itself. Very similar conditions can be achieved in ice boats which use ice blades to move on top of the ice. They can reach speeds of 30-40 mph (50 – 65 km/h) in a 15-20 knot (38-37 km/h) wind, but it is a very cold sport!

Solo racing sailor Olivier Desport writes:
A fast boat such as a racing multihull can certainly go faster than the wind. One of the problems is friction with the water. Floaters on a 60-ft racing trimaran are designed to minimize the wetted area while giving enough buoyancy to avoid capsizing the boat. These boats can reach 1.5 times the wind speed in a 15 to 20 knot wind. Another thing to consider is that faster you go, the more apparent wind you get, and this apparent wind will be coming more and more from the front as speed increases. So you need to have very efficient sails.

 Typically the fastest speeds are reached with a wind coming at 130º to 145º from your course. This is the true wind, but the apparent wind is coming from 45-50 degrees most of the time. This is why you rarely see a mutlihull with the sails open and also never with a spinnaker but only a gennaker as they are always sailing relatively close to the apparent wind. For the same reason, they always need to use a highly efficient dagger board or foils to resist the leeway force generated by the sails. This is of course only true for the top racing multihulls. A cruising multihull still uses a spinnaker as it never goes faster than the wind and still receives wind from the back when sailing down-wind.

To know how boats can go faster than the wind, visit The physics of sailingfrom the University of New South Wales and Sailing: From Work To Funon the SEED web site.


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