I agree with Alan, a keel produces lift (in this case the lift is sideways)
by changing the direction of the water, just in the same way your sail is
working.
It needs an angle of attack to do so, and this angle of attack generates the
leeward way you are making.
compressibility of the fluid (air or water) does not matter for the
principle.
on www.sailtheory.com/sail.html this is explained in more detail.

Alan Baker schreef in berichtnieuws
...
| In article ,
| "Mike" wrote:
|
| In books about sailing, I have read that the keel, going through water,
| produces lift in a similar way to a sail. Given that water is a
| non-compressible medium, I wonder how this can be so. Has it ever been
| documented?
|
| What makes you think that lift depends on compressibility?
|
| As has been pointed out before, lift really depends on changing the
| momentum of the fluid through which the foil is moving, and you can
| certainly do that regardless of whether or not the fluid is compressible.
|
| --
| Alan Baker
| Vancouver, British Columbia
| "If you raise the ceiling 4 feet, move the fireplace from that wall
| to that wall, you'll still only get the full stereophonic effect
| if you sit in the bottom of that cupboard."


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