On my Eska , Which is the same motor in many respects has a shear pin right behind the prop. And yes that can shear and needs replaced after suffering some type of trauma. Like jerking the gears around or hitting something with the prop while under way and such. NOW, There is another pin that drives the prop gear inside the gear case ( the very bottom of your lower unit with the fin ) 4 bolts and your in there. Theres a little pin that drives the gear that is right behind the prop. That drives the prop shaft. That pin can rot out somehow or get damaged and viola , You have no drive to the prop. Now you know.