Diesels are different beasts from gasoline engines. Diesels will last longer if they are run closer to their maximum output. Do not lug a diesel
, meaning making it so that it can never reach its maximum RPM. In fact, most diesel manufacturers that I know of will not honor a warranty on an installation
where the engine cannot reach its maximum governed RPM. The other factor is that a diesel must be able to reach its maximum RPM in order to eliminate carbon accumulation in the combustion chamber.
Its a flawed thought to think that a Diesel runs best if you gear
it like the fifth gear in a car. Doing this will significantly shorten the life of your diesel. Its difficult for some people to understand this concept
and therefore don't agree with it. But all you have to do is check with any diesel engine manufacturer to confirm this.
This book was helpful when I repowered a twin screw workboat. It goes in to hull
shapes, engines, transmissions, gear ratio's, propeller
sizes, desired speeds etc. The formulas in the book allowed me to nail the correct size propellers the first time.
The Propeller Handbook: The Complete Reference for Choosing, Installing, and Understanding Boat Propellers
Also, you really do want to have a right and left hand outboard
turning propellers. Its more efficient and it makes maneuvering your boat in tight quarters much easier. Many modern transmissions can be run backwards or forwards, its just a matter of which way you attach the shift linkage.