To Shawn's explanation, I'd like to add a word of caution. I paddle a Nordkapp HM, which has and integral fixed skeg. While the boat tracks like a super tanker, I find that it can be a real handful in seas off the stern quarter and in surf. The skeg gives the water a lot of leverage on the stern of the boat and makes if difficult to avoid broaching. Constant leans and sweep strokes on the windward side are the order of the day under these conditions.

One other caveat is that an excessively large skeg will cause the boat to leecock (turn downwind), which is also difficult to deal with. Given the choice, a boat that weathercocks slightly is easier to control than one that leecocks.