Small outboards use a "sear" pin (sorry I couldn't resist) that breaks when you hit something, larger ones use a rubber hub designed to slip if you hit something. Not sure where Yamaha draws the line between the two but am guessing 15 HP will be a rubber hub. If so then the garden you are dragging around will put quite a strain on the rubber hub and it could be slipping, like a worn clutch
in a manual shift car trying to go up a steep hill.
Some props are two part and the hub can be replaced. Others are built together and have to be replaced or sent out to a prop shop.
Either way replacing the prop is not to tricky. If stuck on the hook I would give it a go, unless you relish two weeks of isolation.