You need to TRAIN anyone on your boat the correct way to tighten those down, or you'll end up with more broken ones, even on the new ones!
The correct way is to use you THUMB to push on the window right next to the clamp, while you're tightening the clamp LIGHTLY with the other hand. Go around all of them twice with this method and you'll never break one and never have a leak (as long as the seals are half-way decent!).
If someone just cranks on the knobs to tighten them they'll start to develop cracks all around and eventually break right off.
