[Sans Peur]

If you are new to Diesels I would heavily suggest getting on [Only registered and activated users can see links Click here to register] and getting a subscription. It's pretty cheap and the people on there know friggin everything about these engines. I have been on it for quite a few years now and it is invaluable.

To your problem, the advise above is good. However, those older engines will produce soot. Sometimes it is just something you have to put up with. I have a pair of Cats and the transom gets a little on it from time to time, especially if I am running hard in some rough water. Does your boat run up to max rpm? If you are over propped or the bottom is fouled the engines could be working harder and will produce more soot as well.