Raw-Water-Cooled Engine

[Mark Matthews]

What are the worries of buying a boat with a raw-water-cooled engine?

Mark Matthews responds:
A lot of older engines are raw-water-cooled. I myself happen to own one—one that's now 31 years old. Let me begin by saying that there is no shortage of potential problems with raw-water-cooled engines. For starters, because the engine takes water from the outside to cool itself, small twigs, sticks, marine life, and other flotsam can lodge into the strainer, clogging it, and causing the engine to overheat. Scale is also a concern. Ocean water, which is corrosive by nature, will build up mineral deposits over time inside the water jackets and other water passages that are crucial to keeping the engine from overheating.

The biggest problem though has to do with regulating the engine temperature. As the water temperature varies, so does its effectiveness to cool the engine. And many raw-water-cooled engines lack a way of governing the flow of water. Those that do have such devices are prone to fouling due to scale and other contaminents.

We have got some good miles out of our Volvo MD2B; we sailed from San Francisco,CA, to Charleston, SC, and used it a lot. Sure there was a blown head gasket, an errant oil temperature light that came on while we transited the Panama Canal, and more hand-cranking than I care to remember; and there was also the leaky water pump, which eventually leaked into the oil and made life miserable for a bit. But several engine gurus we met along the way told us d that we'd be hard-pressed to find a new engine today that will last this long. Today's engines are easier to start, but operate at higher rpms, which in the long run may mean they won't last as long. With the right maintenance, a raw-water-cooled engine may serve you well.

Oil changes are key as is regular service of the fuel filters for these beasts. You can send an oil sample away for analysis, which will tell you a lot about what's going on inside the engine. In any event, if you're unfamiliar with engines, it's a good idea to get the engine looked at by pros. You can then query them on whether the engine will be giving you headaches in the future.

Additionally, if yours is the same model as mine, I should tell you that along the way we learned that the engine is relatively simple to disassemble and reassemble, should you be inclined to take off the exhaust manifold and inspect for scale—it's a relatively straightforward procedure given some patience, time, and determination. I'm hoping, mine will run forever.

To demystify the process, I'd refer you to either Don Casey's This Old Boat or Nigel Calder's Boatowner's Mechanical and Electrical Manual.