The secret to a wing is that its shape accelerates the flow of wind over the low pressure side. It actually creates its own wind.....

Lets say the true wind speed is 10 knots, then the windspeed on the leeward side could easily be 12 knots, while the wind speed on the upwind side stays very close to 10 knots.

Using this principle very fast boats (low resistance) can sail directly downwind much faster than the true wind.  Ice boats do it all the time.  Interestingly the old square riggers can do the same thing, but the airflow is over the top and down the front of the sails creating lift in front of the sails as they sail downwind, airflow has to be maintained under the square sails too.

Sails with non rotating masts can only take advantage of the wing shape at specific angles and are often only about 1/3 -1/2 as efficient as a properly oriented wing.

In general, a properly oriented wing with half the surface area of a sail will produce twice the driving force in a much wider range of wind directions.  If the boat has high resistance though, the wing won't work going downwind. Can anyone say motor sail?  Heresy I know, but it works.....  All the engine has to do is overcome a little resistance and the boat will sail very well down wind, faster than the wind.

Here is the point, a wing with half the surface area of a sail will generate more drive than the sail (except at about 100˚ apparent wind) and always have less forces on the mast, so it can be lighter and weaker.  That isn't precisely true because the wings Center of Effort is higher so it has a higher moment, but it is close.

Talador

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