They are exactly the same. Both are sub-disciplines of Fluid Dynamics. Even in air, you have different flow speeds and medium densities that have to be accounted for. You only have to look at the wide range of airplane wings to see that in action.
This.
Guys. Look again at my diagram. I'm not making this stuff up! ;^)
Of course, this happens all the time on modern yacht design. This is done to balance the rig to the keel/righting moment. Using modeling programs called VPPs (Velocity Prediction Program), designers can model all the physical attributes of a yachts, and then calculate expected sailing performance (polar charts) to a VERY high degree of accuracy.
Any higher performance keel you are likely to see in a boatyard has a rounded leading edge tapering to a trailing edge of maybe 1/8" - same with rudders. A foil with a sharp leading edge will stall out very easily with small changes in angle of attack through the water by suddenly losing the laminar flow around the windward side, which produces the keel's lift. Do this in an aircraft at low altitude and you suddenly drop out of control, crash and burn!
Indeed, ALL performance keels will have a NACA foil shape, designed to maximize lift and laminar flow for an intended speed. Other design considerations can factor, like how wide the AOA can very before stalling etc, but they will have this form. Key to the creation of laminar flow is the rounded bulb nose. If you do not have this your keel will suck.
I agree that Jackdaw is not making this stuff up! Lift is generated perpendicular to the flow over it and in the direction of the low pressure side. With the sail acting like an air foil, lift is generated to leeward, and when sailing upwind, should have a slight forward component. Due to the inherent side slip of a sailboat, the flow over the keel is asymmetric and therefore generates lift to windward. That is the basically the lateral resistance force to try to keep the boat from side slipping (note that this is not the same as drag -- the drag force is parallel to the flow). When you arrange the force vectors correctly, as Jackdaw has done in an earlier post, you'll see that the net force is forward.
If I do the math correctly for a chord length of 4' (48"), it would seem that the paint thickness applied to the keel could make a difference between a keel for normal use and a high performance keel ... wow!Indeed, ALL performance keels will have a NACA foil shape, designed to maximize lift and laminar flow for an intended speed. Other design considerations can factor, like how wide the AOA can very before stalling etc, but they will have this form. Key to the creation of laminar flow is the rounded bulb nose. If you do not have this your keel will suck.
The point tip trailing edge is ideal but for keels but VERY hard to manufacture. That is why it is usually squared off.
Scotty,If I do the math correctly for a chord length of 4' (48"), it would seem that the paint thickness applied to the keel could make a difference between a keel for normal use and a high performance keel ... wow!
Except, of course, that the seed analogy is completely wrong. Nothing is being squeezed. The exact opposite is happening, two forces that are almost in opposite directions work together by the slight positive sum of their vectors.
OK, I'm missing something here. If two forces are acting in almost opposite directions, would there not be a pinching affect? I realize the forces are in two different planes, but the nature of the affect would still be the same. What am I picturing that is not accurate?
I'm surprised that the drag would be that dramatic. Are you using speed through the water or over ground? I'm guessing that if you're using SOG a large part of your 6kts with the keel up is leeway, and by lowering the keel you reduce the leeway, lowering your SOG, but helping your VMG.
