How thick is your hull

[Bajansailor]

Re boats bouncing over reefs, here is a link to some photos of a J 44 that came ashore here a few years ago - http://www.ybw.com/forums/showthread.php?t=111396
OK, these conditions were a bit extreme, but she broke up pretty rapidly. Her hull was balsa cored even below the waterline, with about 3 mm of fibreglass on the outside, and maybe half that on the inside (although it would have been single skin and much thicker in way of the keel).
If she had been built like a Westsail (massively heavy fibreglass Colin Archer types, aka 'Wet snails'), then she would probably have coped a bit better, but she wouldn't be a J anymore.

Re hull thicknesses, our Challenger 35 is about 20 mm (I think) of solid laminate in way of the log transducer, and this is not too far away from the keel.

 

[Stimp]

Thick enough to not want to have to drill through it... on a rising tide!!

 

[TradewindSailor]

The keel has gone. Presumably it was the pile driving action of the keel that caused the hull to break up. With a cast keel there would be no elasticity in the structure as there would be in either a wooden long keel structure or a glassfibre integral keel. i.e. all the shock loads would be concentrated around the perimeter of the keel joint with the hull, creating tremendous shear loads in the hull ..... rather than being distributed by time due to elasticity in the other forms of structure.

This would suggest that the structure of boats built like the J would need significantly more structural material in the keel support matrix than that of a more traditionally built boat to stand up to this extreme punishment.