Re: 70' Hylas grounded, dismasted in Penobscot Bay...
I think you're on to something with the compression theory. I used to design and build composite airplanes. Carbon fibre parts are designed for specific load paths and while they do a superb job loads they do not yield. A carbon fibre part will continue to deflect until you reach its breaking point and then it typically cracks/shatters. Since the mast was designed to handle lateral loads, not compressive loads (other than those necessary to keep the mast upright), if the grounding did force the keel up, it's quite possible that could have been enough compressive force to cause the mast to break.