Flingwing,
A couple of guesses;
~ The rear propeller came about before the global warming. Perhaps early IceBreakers were destroying too many bow-mounted propellers.
~ Submariners weren't too happy about timing torpedo launches with the rotation of the blades.
How about if we compromise and put the prop in the middle.
Seriously;
~ Perhaps a tractor prop on a ship would spend a lot of its time thrashing air and waves.
~ Again perhaps, the slower speed of the submarine and the density of the water results in a clean flow entering the pusher propeller. Just a guess.
Mart,
I did not mean to imply that the prop should be operational during hover and transition. One of the advantages of a pusher prop is that it can be a folding one, thereby minimizing drag and contact with foreign objects at slow speeds.
Dave