Not a new lesson, but thought you'd like to see the damage from last weekend.
We did not ground out, but hit the side wall of the slipway (Chew is full and working out where the vertical brick walls are is suprisingly tricky). Sadly the impact was at a few mph and loaded. The unzipping happened when we descided to sail on it anyway.
The plus side of this is that we don't need to break it further to effect the repair, the negative side is that the bit of core in the impact zone has gone missing.