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patrickbryant
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Sailboat maximum airspeed

Colleagues:

After completing a particularly challenging voyage on the S.F. Bay last Sunday, it occurred to me that there must be a reason why my boat progressively slowed down while I was on a close reach into an increasing wind (topping out at about 40 knots). Any comments on my explanation below will be greatly appreciated.

There appears to be a maximum air speed (wind speed) at which the sails of a sailboat on a reaching point of sail can no longer overcome the induced drag produced by the sails themselves plus the parasitic drag of the wind-exposed area of the boat (the combined exposure of the freeboard and all of the other wind-exposed areas). Above that speed, "the curves cross" and the sails can no longer produce enough lift (forward propulsion) to overcome the aerodynamic drag of the boat -- so an equilibrium is reached: the boat will go no faster. If the air speed increases, the boat will slow down.

Lift (propulsion) of an airfoil (wing or sail) increases as the square of the velocity. But the power needed to overcome drag increases as the cube of velocity, so at some velocity, drag overcomes lift (propulsion). The same effect is seen with airplanes: if an airplane that cruises at 100 knots requires 85 horsepower to maintain level flight (lift-to-drag equilibrium), the amount of power needed to make the same airplane cruise level at 200 knots (assuming you could add a bigger engine without increasing the airplane’s weight) would be 85 X (2 cubed) , or 85 X 8 = 680 horsepower. In the case of a sailboat penetrating forward (reaching) in the airmass, when the wind speed doubles, the drag increases by a factor of 8, while the lift (propulsion) produced by the sails only increases by only a factor of 4. Equilibrium is reached at some critical speed, and the boat will go no faster. Increase the wind speed any more, and the boat slows down.

The challenge of reaching into a high wind can be mitigated somewhat by reducing the sail area (reefing), which reduces its induced and parasitic drag more than the reduction in lift, but the parasitic drag produced by the freeboard and other exposed areas (cabin top, rigging, etc.) cannot be reduced. So, regardless of what one does, any given boat has a maximum wind speed into which it can reach before it begins to slow down. The more freeboard and other wind exposed surfaces, the lower is that maximum speed.

Last edited by patrickbryant; 01-17-2012 at 04:36 PM.
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