Hi,
I have been trying to read alot about rudders and foils and lift and such, trying to understand what shape an asymmetrical rudder should be, when I realized that I am missing something fundamental.
I don't see how a rudder (or a keel) generates lift. I understand how a sail does it, but the bernoulli equation only applies to a gas (if I remember my college physics; it has been 30+ years). The idea of the flow around the outside of a sail reducing the pressure implies that the density of the gas decreases. Since water is incompressible, I don't see how that applies, as the density remains constant.
It seems to me that keels and rudders act by introducing resistance to the leeway producing forces generated by the sails, resulting in an effective force vector in the direction of motion by cancelling out leeway forces, not by adding lift in the opposite direction.
If this is right, then the shape of the keel/rudder should not be critical for lift, but only to prevent seperation of flow from the rudders surface so that you don't get turbulence or a gap that can suck air down and reinforce the separation.
I am somewhat encouraged in this view by this article I found from the school of physics in NSW, Australia that mentions lift from the sail, but does not mention it in relation to the keel.
Am I right about this? If not, can someone point me to a good explanation of how a keel/rudder generates lift?
Thanks,
- Gardner
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