The Offshore Voyaging Reference Site

What Marine Engine Duty Ratings Mean To You

Earlier in the year, John and I were writing about marine engines, and among the questions that keep coming up is:

“Just what is a ‘commercial’ or ‘continuous’ duty rating on an engine, and should I get that?”

Let’s tackle that by starting with the reason that these ratings exist at all and then move on to how to select the correct rating for a displacement boat, power or sail.

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Hi Matt,
Nice post on the duty cycle of engines. For anyone that has ever opened an engine to remove piston and having to turn it by hand, it is easy to understand that it is almost magic that an engine doesn’t wear that much when pistons are travelling at many meter/second metal on metal. It would have been interesting to provide some comparison of HP/liter of displacement to show the real difference between engines of different duty cycles and different technology. Also a reminder that in our modern sailboats we don’t necessarily need those old clanky 2 cycle diesels or 800 lbs behemoth for 80 hp.

thanks !


Hi Matt,

Great article but I do not quite agree in your statement that in continuous duty engines’….any RPM setting other than redline is going to represent a loss of efficiency’; My understanding is that the efficiency can be easily determined by the Best Speciffic Fuel Consumption values provided by the engine manufacturer. For example for the marine engine MAN D2866 LXE 40 (190 KW @1800 rpm) the Best Speciffic Fuel Consumption value is about 195 g/kWh at approximately 1300 rpm at full load and for Deutz BFM 1013M marine engine Best Speciffic Fuel Consumption value is around 210 g/kWh (at 1600 rpm for all engine ratings (redline 2300 rpm). So these continuous duty engines (among many others whose curves I have seen) do not seem to be ‘rigged so that the most fuel efficient setting is the full power setting’ ?


Peter Rousseau

Hi Matt,

What do you think about a traditionally reliable automotive diesel, like the Mercedes OM 600 series (47 kW at 3300 rpm, 60 kW at 4000 rpm) being marinised for a 13-ton sloop? The Ford Lehmans made a successful transition from Peugeots to marine duty, but as you mentioned, the demands of cars and cruising yachts are different.

John Harries

Hi Peter,

Matt does not monitor older posts, so you are stuck with me on this one.

I think the answer is that part of the marinization process also includes, or at least should include, making changes to the fuel pump to better match the needs of a boat.

For example I’m going to guess that the Ford Lehmans in boats and that in Peugeots have very different fuel pump settings.

And I know that this is so for example with our Perkins M92B, which has a completely different fuel system from it’s industrial shore based cousin, even though the two engines share the same block and most other parts.

So, turning to your question about the Mercedes, doing a good job of turning into a good marine engine would probably, or at least should, involve dropping the peak RPM and HP down to the point that the engine would be reliable when run at say 80% load continuously.

So, taking wild guess said engine might end up at say 60 hp peak if properly optimized, or even a bit less.

But even so, that would still be more horsepower than would be optimal for most 13 ton boats. See this chapter for why:

Eric Klem

Hi Peter,

The major difference between the applications is the type of load which brings about several associated changes, the big one being heat management. In designing an engine, the engineers have several constraints that they need to work with and the big ones are heat management, cylinder pressure and emissions regulations. Also, they have different priorities, in the automotive world, weight is really important where it is much less so in the industrial and marine markets.

In a boat, the average load is much higher so the engine reaches steady state at this high heat output state whereas in an automotive application, the engine typically does not reach steady state at high heat even cresting that giant mountain pass. The good news is that there is an enormous cooling potential due to the thermal capacity of water. With the addition of a heat exchanger and a second water pump, this is taken care of. The issue is that there are often other things that are different as well. If the engine has a turbo, the marine version is typically water cooled while the auto one is not. It is a real challenge to manage the heat around the oil and bearings in a turbo. Also, the exhaust manifold is often water cooled in a marine engine.

On the fuel side, there are a few different changes. The governor is usually different as the auto one is load sensitive while the marine one is constant speed to limit hunting. The pistons and injectors are sometimes different but not always. Also, timing is often different.

It should be obvious that the transmission will need to change. People do occasionally use automotive manual transmissions but they are not well adapted and need a separate thrust bearing.

You can absolutely do the conversion but it usually doesn’t make sense, you would be better off buying a marine engine and selling your engine. I had a pickup with 260k miles on it where the Cummins 6BT ran like the day it was made and the truck was totally rusted out and I looked hard at converting the engine and then building a bartender boat to put it in. The parts that I would have had to change/add were the exhaust manifold, turbo, hx, water pump, etc. The additional parts that it would have been wise to change included governor, injectors, pistons, timing. If it hadn’t been for the cost of the turbo, I probably would have done it but that put me over the top and I sold it to someone who put the engine in a Ford pickup.


John Harries

Hi Erik,

Thanks very much for fielding that one.

Hi Peter,

You can take what Erik says to the bank, he knows of what he writes.

Peter Rousseau

Many thanks, John and Erik.

I’d come across a 13-ton sloop with a Mercedes diesel, but when I went to research the engine, I could only find Mercedes-manufactured marine engines from the 1950s and 60s. That led to the realization of a marinised diesel, and then the question was had the owner done the conversion himself, or purchased it already converted from a firm like DMV. Either way, I knew I needed to find out the feasibility and the potential pitfalls of such a conversion, both of which John and Erik provided. Thanks again.