I once repaired a sheared off A-cat daggerboard for a friend. He hit a large underwater rock on his first outing with the boat, and the daggerboard sheared off cleanly below the hull. When removing the pain and some glass for repair, it became evident that the manufacturer had drilled and filled a 10mm hole at the leading edge. The shear started just at the hole, by design I suppose. Much better to break the board than cleaving the hull.
If you look at the bottom picture, you can just make out the hole at the leading edge in front of the light foam.