Looking at the picture I think that the prop was probably slipping the whole time and the double-nut kept it from coming off. YS prop nuts have to be seriously tight to keep this from happening, much tighter than any other engine I have used.

A trick that I picked up from Dave a while back is to cut a disk from fine emory paper and put it between the drive washer and the spinner backplate with the rough side toward the backplate. This helps a whole lot- but you still have to tighen the x;!k!! out of the nut. Troy do you have a better trick than this?

Walt