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post #4 of Old 09-07-2003
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Keel Lift

Date: Sep. 07 2003 10:05 PM
Author: aflanigan
Subject: Re:Keel Lift
Nonsense. Your question was an excellent one; it forced me to think. And it illustrates what a complex physical system a sailboat is.

Consider the fact that a sailboat moves forward because the force acting on the sail (lift plus drag) points at an angle of less than 180 degrees to the angle of the force acting on the keel:


Generally, for airfoils like a sail or a keel, the angle of the force (lift plus drag) acting on the airfoil is roughly perpendicular to the angle of attack. Thus, as we pull the sail closer to the wind, and cross the sail over the center of the boat and push it to windward, we reach a point where the angle of attack of the sail to the wind and the angle of attack of the keel to the water are more or less parallel, and the angle between the forces acting on the sail and the keel is exactly 180 degrees. The boat stops moving forward. This is why you can''t keep pointing closer to the wind by simply pushing the boom of the mainsail to a higher angle relative to the boat''s centerline.

Of course, if we "backwind" the sail by pulling it even farther up to windward, the angle between these forces becomes greater than 180 degrees, and the boat starts sailing backward:


Dinghy sailers use this technique to back away from a starting line before a race, for example.

Allen Flanigan

Amateur physicist & sailor
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