First of all this happened many years ago and everyone is fine but we've never really been certain of the cause. Cruising across the lake at about 40-45mph in light chop going straight with five people on board. Without warning the boat rolls hard onto the right chine, shoots 90 degrees left and rolls over. Motor was running fine at the time it happened and didn't seize. First guess was that we'd wrapped something around the prop shaft. Nope. Second was that we'd peeled some glass off one side of the bottom. The boat is glass over wood. Nope. Third guess is that the v-drive shifted. We didn't have bracing on the gear box mount and noticed cracking of the stringers after the crash. Steering wheel never moved in my hand. I tried to correct but the boat had its mind made up. Steering was all intact after crash. We had drive rebuilt after crash but shafts were spinning freely. It is a Menkens gear box. Any guesses? The boat ran great as designed for about 40 years prior to this happening. We are in the planning phase of building another one so if there's something we've overlooked we want to correct it on this one. As many of you know, flipping a boat just isn't that much fun.