A few yrs back someone was experimenting with using the Mazda twin turbo 1.3B rotary motors and hooking up to an outdrive. Their weight is just a fraction of a BB but of course they made very little torque. Does anyone remember who was doing that and if they had any success?
Rick

Just go to an all aluminum engine for the reduced weight.
You still have a lot of horsepower and torque.

Some big HP builders are reluctant to go to aluminum blocks.. They tend to twist a little. Also... the weight savings of an aluminum block and heads is offset by the closed cooling system you would have to run if you are using it in salt water. Some builders opt for aluminum heads with an iron block. This is to keep the detonation levels down. But there is still the issue of corrosion with salt water.

just don't go in the ocean, duh...

Yeah, a couple years ago some guys tried to use a pair of import twin-turbocharged motors in a custom built Reno air racer called the Pond Racer. They had all kinds of temp problems with them and it had an inflight engine fire during a run. A highly experienced pilot by the name of Rick Bricker died in the fire after landing the aircraft.
Tried and true parts are pretty well wrung out and new stuff can bite you.

This is correct, but mercruiser is going mostly to closed cooling on all standard production engine packages, so there is no weight savings now. The 496 and 525 packages are all closed cooling systems and still cast iron blocks.