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."