alamcculloch:
When I was doing the sailing instructor bit we had a ski boat that was fuelled by LPG . The lump was a 6 litre Chrysler ,to pull skiers up you nee a lot of power for fast acceleration. When it was out of service the boss hired in a petrol boat and it used so much fuel that it wasn’t economical to run it at all.
A good example of how the intended use of the vessel, determines the engine, and so the fuel. Fast boats need a fast spinning propeller. Due to inertia laws, a fast spinning prop cant be very big, and a smaller prop needs to spin faster to displace the required amount of water. Hence fast revving petrol engines for speed boats and larger cruisers that need to be got on the plane (keep googling Carryfast). So fast boats, favour petrol engines, or LPG to keep the cost down. Slow boats, can do with an economical diesel engine, because their props can be bigger, and therefore don’t need to spin fast.