A Review of Modern Sail Theory (1981) [pdf]
It's interesting to me that even in this theory keel is not mentioned at all, though it is essential for sailing, together with the sails.
Few things:<p>- sailboat without the keel, will not sail at all. It will just move sideways and back<p>- sailboat driven by a motor in a lull, with sails up and perfectly trimmed, will go slower than without the sails<p>- plane and air gliders comparison is always wrong, because plane is driven, and both operate in the same medium - air<p>Sails alone cannot drive boat forward. 
Only with interaction from the keel on the boundary of two medium, water and air with different density.