Ebb,

Grab yourself a copy of "Chapman Piloting" and go to Chapter 10, "Seamanship Under Sail" and read the section there about why boats sail. Chapman explains in text and graphic form how a rudder, and how a keel for that matter, function as foils. I just happened to turn to that section this morning for quite a different reason before I saw your post. Anyway, pretty pictures there to behold: pictures that are worth a thousand words.