M_Boats: Aft skeg for Montgomery 17

After looking at the pics on Sean Mullilgan's site and seeing that horrendous crack in his aft skeg and noticing a hairline crack in mine just below where the skeg fairs into the hull, I would like to know just what is inside the skeg and why, short of slamming into the bottom, a hull would crack like that? Is it at all connected to the expanding ballast type situation in the centerboard housing? Any information about this would be most appreciated. Tom B, Mont 17 #258, 1977