1981 18' Century Resorter
Marine Engines, etc.
I have a good friend who was badly burned in a marine engine explosion. (The wood Century she was operating was completely destroyed in the incident.)
Here's the MerCruiser engine in my boat:
Think about this. When there is a gasoline leak in a car, the gas leaks to the ground, and explosive fumes aren't likely to form. But, in the confines of a boat's hull . . . Well, you see what I mean.
Is there really a such a big difference between marine and automotive V-8s? Yes! Differences between the two are so striking, one ought never place an automotive engine on stringers where a marine engine used to be.
The biggest difference is, marine engine cylinder blocks are based on heavier-duty truck blocks, with four-bolt main bearing support of the crankshaft. A car only uses about 15 of its one- or two-hundred horsepower to sustain a speed of 55 mph. Conversely, a boat (like an airplane) is almost always under load. It's like hitching a 10,000 pound boat and trailer on the bumper of a car and trying to climb the Rocky Mountains at 80 miles an hour.
Besides the severe duty cycle, there are other important differences. A marine engine's core plugs (also called freeze plugs) are corrosion resistant bronze. The camshaft is ground to different specifications, most often to maximize low end torque instead of high rpm horsepower. Valve overlap (the time when both intake and exhaust valve are open) is shortened in order to minimize the chance of water being sucked out of the exhaust and into the combustion chamber. Gaskets are premium quality for better sealing and corrosion resistance.
Most important of all the marine grade components are the starter, alternator and distributor. All three are fitted with special screens that quench internal sparks that might otherwise vent into the atmosphere and ignite gasoline fumes present in the engine compartment. For the same reason a marine carburetor bowl vents its overflow to its throat, instead of externally to the atmosphere. Marine carburetors meet USCG specifications for safety.
The bottom line, an unmodified automotive engine is totally inappropriate for a boat motor. Its torque curve won't meet the needs of a boat, its light-duty components won't long survive the rigors of marine usage, and you risk blowing yourself out of the water (like my friend did years ago.)
Another pretty good web page by Grant MacLaren