If run into something and you can't spin your prop, it really wouldn't help. Say you lost you rudder, like the whale collision. You don't want to be driving around at 7 or so knots in tight circles. Or say you get holed and the problem is forward. Or say you hit a ledge and you can't move forward.

The other concern I have is that if there was water up to the shaft, you would be driving aroun with a lot of water in the boat. Your ability to make way would be dramatically different. I think. This would work better on a shallower hull-engine compartment vessel that was moving all the time --say a motor vessel that is fishing like a trawler.