Something like that happened on my old Scout boat...so I now get to subject you all to story time!
"Shaft Alley" (the part of the bilge area where the propeller shaft runs before it exits the boat) was under the galley, and there was a piece of plywood above the shaft, to allow people to go down there safely while the boat was underway. The galley guys were in the habit of storing ice chests on top of this board.
Well, one fine October evening, we were cruising along in the bay, and I was standing watch in the engine room. All of a sudden, the EOT (5 Brownie Points for anyone who knows what that means) rings "Engine Stop". I brought the engine to idle, pulled the gearbox into neutral, and sat waiting for further orders. After about 5 minutes, someone called me up out of the pit, and I got to find out what happened.
One of the galley guys had put two ice coolers into shaft alley, and used some line to tie them off. Problem is, he left the slack hanging...and the line got caught on the propeller shaft and got wrapped around countless times. When the shaft finally wrapped all of the slack line around itself, it pulled the line taut enough that the line cut halfway through the board (about 1 1/2 feet) and demolished the two ice coolers...and, fortunately, at that point the line broke before any more damage could be done.
Being the only engineer on the boat for that cruise, I got the unpleasant task of pulling the line off of the shaft, which IIRC was so tightly wound up that it took nearly an hour to do so.
Really, we were just lucky that no one got hurt or that no mechanical damage was done...and a valuable lesson was learned that evening.