Hey Bill, folding prop seems to be working properly. Could overtightening the stuffing box cut through a shaft in just 15 hours of running? something sounds unusual here.
Well, anything's possible given the vagaries of alignment, corrosion, and initial build quality as others have mentioned. But, I have to say, it does sound weird to me. I've had two boats with three v-drives over the past 27 years, and have worked on lots of others. It IS hard to align them, but even when alignment isn't perfect I've never seen a broken shaft result.
Overtightening the packing nut could cause excess heat, but....
Guess when you pull the shaft you'll have a better chance at diagnosing the problem.