I got this from the multihull boatbuilder mailing list, seems to make a lot of sense.

Brian,
I presume you are using what is known as a "Dynamite Payson" joint. I fiddled with these on a rowboat about two years ago. They are stiff!. And if you try to make them less stiff by using less glass they will fail (with a loud pop!) under load.

The problem seems to be not only the higher modulus of the glass as opposed to wood but the fact that the glass is applied on top of the surface. This effectively takes the compression and tension loads farther away from the neutral axis - sort of an I-beam effect.

One solution is to hollow out the joint before laminating so the glass lays closer to the neutral axis. Be careful not to grind too much of the outer ply away or the joint may then fail at the edge of the glass. It's still going to be a guessing game. I suppose fiber orientation is a factor as well.

Doug