Marininized engines meant for tractors.
Tractor engines are huge. Not just in size but weight too. A engine capable of producing 90 hp weighs in at 1,000 lbs and produces 2,000 rpm. Engine weight means nothing for a tractor. Often you can see tractors adding weight to the front of their vehicles.
Marine engines max out at say 3,000 rpm and then use reduction gears to turn the propeller at 1,000 rpm. Engine weight is important. approx 200 to 300 lbs for a 50 marine horse power engine or so. Too much weight slows down a boat. No one wants that. Owners want an engine that will last for thousands of hours. MOST tractors while pulling say plows don't go faster then 2.6 knots. They also have a big fan to cool the radiator.
Tractors use aftercoolers to keep engine heat down as well as water cooling.
Marine engines can use sea water to cool the engine. And where theres sea water, there's plenty of it.
Marine engines and air plane engines, and even tractor engines are similiar because they both use reduction units to slow down rpm from the engine to the propeller, this increases low end torque. Weight and size are top designe priorities.
But most airplane engines are air cooled, not water cooled, and they run on high octane, not low octane diesel.
I agree with
jssailem
and max hull speed with rated horse power. The larger boats tend to have a faster hull speed and so can benefit from a larger displacement engine right? Or faster hull speed from planning or hydrofoiling.
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