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The Powerplant

Boat Maintenance Special:

By Capt. Bill Pike

Prime Mover

The heart of any powerboat is her powerplant. Without it, even slo-mo’s a no-no. And there are any number of comparatively complex books, Web sites, and magazines on the market these days that’ll tell you how to extend the life of your diesel engine or engines. But oddly enough, the most important and effective of the whole shebang seldom gets emphasized.

“If you wanna make a diesel last,” the chief engineer of an oil-field anchor boat once told me, “warm the damn thing up before you put it under load.”

Check your oil levels when the engine is cool for an accurate reading.
Check your oil levels when the engine is cool for an accurate reading.

The reason? Pistons in a cold engine are slightly out of round or, to put it another way, the sides of a given piston vary slightly from a perfectly circular circumference. Moreover, cold pistons tend to be slightly contracted in terms of size. When you crank your engine, however, all this changes. Each piston—made of aluminum in most cases—assumes an increasingly circular shape and expands with considerable rapidity, at least by comparison with the cast-iron or steel cylinder walls encompassing it. Then, if throttle is poured on, design tolerances are quickly exceeded and aluminum is literally shaved off to fall into the lubricating oil below. The result is not good—the engine loses compression over time, fuel consumption goes up, and you begin to, as the mechanics say, “burn oil.”

Two more quick tips before motoring on. First, lots of guys will tell you to let your diesel idle dockside for 15 or 20 minutes after you’ve run it hard—the idea being that idling somehow cools things down in an ideal way. Certainly this is all true, if we’re talkin’ a turbocharged engine where idling dockside after a long run allows the diverse metals in the turbo to cool without distorting or warping and also prevents oil from literally “cooking” and glazing bearings and other components. But on a turbo-less engine like the old, naturally aspirated Ford Lehman in Betty Jane’s engine room, dockside idling after a run is pointless.

And the second tip? You’ve undoubtedly been told by some beastly soul over the years that you should check your engine’s oil only after it’s been thoroughly warmed up—such readings, the thinking goes, give a more accurate picture of what goes on during normal operation. This is bad advice, however. The dipsticks in most marine diesels are marked in accordance with a commissioning agent’s input during installation. Not only does the marking take into account the pitch of the engine (typically upwards, of course), it also typically represents a cold engine with all the lubricating oil in the sump.