How Many Hours on Your Diesel?

Marine diesel mechanics have told us that most marine diesels go bad after about 4,000 hours. This is a shame, because marine diesels should be good for anywhere from 10,000 to 14,000 hours. And even more.

Usually the reason they go bad is lack of use, or misuse. Diesels are designed and built to be used often — nonstop isn’t bad — and under a big load. This is rarely how most recreational sailboat diesels are used.

In most cases sailboat diesels are used infrequently, and when they are used, it’s often only for short periods of time, and often at idle or near-idle speeds. This is the worst way in which a diesel can be used. And it’s the rare sailboat owner who warms up his diesel before putting it under load, or lets it cool down after hours of being under load.

Despite misuse, some diesels just go on and on and on and on. The 76-hp Yanmar diesel on Steve Schmidt’s SC70 Hotel California, Too has been badly abused in the sense that it’s almost never been used for propulsion in something like 30 years, but only to charge the batteries and power the refrigeration. That kind of misuse is often the death knell for a marine diesel, but Schmidt’s has well over 17,000 hours. Perhaps the boat’s saving grace is that Schmidt uses the boat — and diesel — something like nine months a year.

We have two Yanmar 56-hp diesels on ’ti Profligate in the Caribbean, and they spent nearly 15 years presumably being ‘abused’ by countless charter guests motoring madly around the British Virgins. But as long as the engines were properly maintained — oil and water levels, as well as belts and general engine condition, checked before each charter — which they were, it wasn’t ‘abuse’ at all. It was the frequent hard use that diesels thrive on.

As a result, both our 17-year-old Yanmar diesels have over 10,000 hours. But they run great, and burn almost no oil at all. In fact, they burn less oil than do the same diesels on Profligate, which have half the hours and don’t get as much use each year.

The engine-hour question came up after Jim Fair of the Berkeley-based Outbound 46 Chesapeake, which Jim and Linda Powers just spent nine years sailing around the world, reported he removed his Yanmar diesel in San Diego for a rebuild. And after just 3,700 hours.

“It’s been burning about a quart of oil every 24 hours,” reports Jim. “I’m not really sure of the cause, as it could be the seals on the turbocharger or the valve stems pitting on one of the cylinder walls. The rebuild guys will bore out the cylinders and install oversize pistons, along with a lot of other stuff. The engine is getting about as major of a rebuild as you can get, short of replacing the engine.”

Fair thinks the decline of his Yanmar took place over about a four-year period. The sequence of events was: 1) A small loss of coolant in the header tank, which he thinks was really air going into the header tank and forcing coolant to overflow into the overflow tank. 2) An incorrect diagnosis of the radiator cap’s being bad. 3) A leak in the heat exchanger. 4) A leak in the hot water tank. 5) Some oil consumption. 6) A catastrophic leak in the head gasket, at which time it was finally diagnosed correctly as a blown head gasket.

So what’s the story with your marine diesel? Have you been treating her right? Has she been treating you right? How many hours does she have, or did she have before major work needed to be done?