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Old 10-08-2020, 05:11   #1
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Diesel Engine Life

I have heard many folks state a diesel can last 30 years (assuming good maintenance and spares availability). However, this seems like a silly measurement.

30 years for a weekend coastal cruiser is different from 30 years of circumnavigation.

What should one expect the life of a diesel to be in hours? Is there a rule of thumb for hours/year?
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Old 10-08-2020, 05:24   #2
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Re: Diesel Engine Life

Depends on the engine and how it was configured. Sport fishing boats often extract very high horsepower and can blow-out an engine in 3500 hours. I used to drive a 65-foot Pacemaker motoryacht that had been modified for use as a 20-passenger dinner-cruise boat in San Francisco Bay. It had a pair of Detroit 6-71's at about 225 hp/each and had close to 20,000 hours as I recall. I just watched a YouTube of a Nordhavn owner who was surprised the rear main seal on his Northern Lights 12kw was leaking after 6500 engine hours (I think the core engine was a Deere, but may have been a Kubota).

If I had to put a number on it, would say at least 10,000 hrs between major overhauls, though much higher is certainly possible. I have a Perkins 4.236 in my trawler, the same engine many 42-55 foot sailboats have (at least those from the 1960's-1980s when Yanmar started dominating). Sounds like most cruisers do under 50-engine hours per month so as long as you took less than 15-20 years to do a circumnavigation, you'd be fine.

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Old 10-08-2020, 06:39   #3
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Re: Diesel Engine Life

The yacht club bought a Yanmar 3GM, used, maybe sunk once, at least 8-10 years ago. No idea how many hours. Installed in an open launch. Sank again. Towed to a shop and left sitting in the open for six months. Then a very casual rebuild because nobody expected it to run. Itís still running fine. It has well over 10000 hours since the casual rebuild. YMMV.
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Old 10-08-2020, 07:04   #4
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Re: Diesel Engine Life

Years ago I was told by some expert that the hardest thing on a diesel engine was starting it. He stated that Greyhound bus engines were just left on constantly, as at idle they used virtually no fuel, and they ran almost indefinitely.
That would not bode well for the life of our diesel auxiliaries which are mostly used to get the boat out of the marina and bring it back into the marina.
Any experts want to weigh in on the question of the type of usage?
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Old 10-08-2020, 07:34   #5
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Re: Diesel Engine Life

Depends on the engine if you are talking about aux motors for sailboats they can last 5,000 to 10,000 hours. They could be overhauled and do it again after that.

Then again people have destroyed marine diesels a lot quicker than that. Water in the fuel is rough on diesel engines.
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Old 10-08-2020, 07:38   #6
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Re: Diesel Engine Life

The chief engineer of Cummins once told me that other than proper maintenance and luck, the three things that determine engine life are 1) engine hours 2) wall clock hours (a thirty year old engine will be closer to death than a three year old engine with the same number of hours and 3) fuel consumption (an engine run at "proper" load will out live one run too low or too high). Given that most of those variables are almost impossible to know unless you owned the boat since new, it is a little bit like knowing the average score of a baseball game. In general, engine hours will not be the determining factor in determining sailboat engine life. I seldom see sailboat engines with more than 5,000 hours.
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Old 10-08-2020, 15:42   #7
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Re: Diesel Engine Life

Itís very much about load and not RPM. Yanmar elbows are particularly susceptible to carbon buildup hence their high RPM recommendations which make sense. Most diesels will last longer than you want them to in a sailboat unless they are used solely for charging batteries.
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Old 10-08-2020, 16:32   #8
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Re: Diesel Engine Life

Little Diesels usually die from a lack of maintenance, people who change oil maybe every 250 hours or so and never change an exhaust elbow, elbows in particular will eventually fail and likely let salt water into the engine, and as often as not, that’s the end of that engine. They have never heard of having the injectors cleaned, and never thought about adjusting valves.
Other than maybe seals, years don’t really kill an engine, lack of maintenance does, and corrosion doesn’t help either of course, and if left to occur corrosion is related to age.
My 4JHE is 33 yrs old, has very few hours and runs like a top, uses and leaks no oil.

My little airplane’s engine is 77 yrs old, and has been overhauled likely a couple of times, but still it’s 77 yrs old, now and leaks oil, a lot of oil actually. I need to overhaul it just to fix its oil leaks.

We change oil in our Diesels to get rid the soot, soot is abrasive and your circulating a wear compound, the oil likely doesn’t even come close breaking down, you get to decide what’s enough abrasive particles to allow to build up in your oil before changing it.
In other words shorten your oil change interval and you will decrease wear in the engine. If you do not want to change oil more frequently, look into a high bypass oil filter, but don’t buy into the theory of I’ll use a high quality synthetic so I don’t have to change oil as often, because we don’t change our oil when it wears out, we change it to get the soot / carbon out, and soot build up just as fast in a high quality sun oil as it does and old fashioned single grade Dino oil.

At least older Yanmar’s have very old fashioned simple combustion chambers, they are low swirl chambers and don’t mix the air and fuel as well as more modern high swirl chambers do, so Yanmar’s are “smoky” engines, it’s just the way they are, it doesn’t make them bad, or make them not last as long, just makes them smoky. They are also made for higher Cetane fuel than what’s commonly available in the US, but oddly to me I believe California requires high Cetane Diesel, higher Cetane ignites quicker, burns faster and cleaner and less particulates than low Cetane fuel.

I assume the newer common rail motors that have to pass emissions testing have high swirl chambers to help pass emissions, but I have no experience with them.
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Old 10-08-2020, 17:04   #9
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Re: Diesel Engine Life

I think these answers sum it up nicely. The 10,000 hour number is the most common one to be bandied about. Proper care and feeding is really the subjective unknown unless you get a used engine with fastidious maintenance logs. Keeping the carbon out, by running at WOT for a short while and regular oil changes are certainly key, as well as exhaust riser and injector cleaning. It can take many, many years for a boat to get to 10k hours. I have a 20 year old sailboat with about 3k hours on it. A Volvo, to boot. (some folks pooh pooh these). But it runs like a top.
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Old 10-08-2020, 17:18   #10

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Re: Diesel Engine Life

The engine will last a VERY long time, now time between overhauls; with good maintenance and a good operator, I’m sure it’s published somewhere, but for a non turbo basic Yanmar I think 4-5k hours would be a good time to OH the engine, that’s also not running it till it dies.
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