igorn,
The only thing I worry about with an outboard on the back of a catamaran is that if you have small children on board, you have to make sure that when the outboard is running, you do not let the children sit on the bows or hang over the front beam, because if, God forbid, somebody fell in the water between the hulls, they might get ground up by the propeller.

My golden retriever is a little clutzy, and I had him out on my Wave one day, and he fell off over the front beam between the hulls, and, as I passed over him, I grabbed his neck and pulled him back up onto the boat. It might have been a bad ending if I had been running an outboard motor and not been able to shut it off quickly enough. <img src="http://www.catsailor.com/forums/images/graemlins/frown.gif" alt="" />

This applies to adults, too, for that matter.