The forward end of our Jimmy Skiff's skeg is about 1/2", the aft end is about 5 3/4".

CAPT-K,

The blank that I glued up to cut the skeg from was 7 1/2" at the aft end.  I based this on the idea that I wanted the bottom of the skeg to be at about the same depth as the center of the hull when she is sitting level on her lines.  This worked out quite well as the bottom of the transom will be out of the water when the trim is correct.

See this page on my building blog... http://jimmyskiff.blogspot.com/2007/02/two-seats-skeg-and-mast-spar.html

I just had a chance to measure the finished depth of my skeg, and it is about 5 1/2".