Tug, the frame and decks were made from 5-ply aircraft plywood. The lower portions of the hull that I was beginning to fill were made from balsa that I was sanding to shape. I was doing one side of the ship at a time with the block, and I noticed that the keel was taking on a curve from the pressure between the ribs. So, I cut out a little bit of one of the blocks and tried to bend the keel back a bit. Apparently I used WAY too much pressure and the whole thing failed catastrophically. I think I had too much other stress built up in the frame, as I had to make several alterations to get the scale and shape correct. There were also several weak joints from said alterations, so that just compounded the issue.
Anyway, live and learn. I am going to finish up New Zealand, and then I am going to attempt another scratch build. I have a the plans for a few neat ships that do not have glass hulls, so I will probably do one of those.