Effects of Running Diesel at Low RPM

[Fanfare]

I have your engines. Lately we run 1950 rpm to go on plane for deep water runs. But in shallow or sheltered water we run about 1300 which gives us 8.8 kts, just below our hull speed of 9.1. I have 170 degree thermostats. At this cruise speed my engine temperatures run 165 degrees. We have traveled hundreds of hours at this speed and have had no problems in about 25 years. I do try to blow the engines out for a few minutes each day if I don't have to watch my wake. Coming back from the Bahamas I always have at least 55 nmi back to Florida which at 1950 rpm seems to purge anything in the oil. In the Intracoastal I have to run about 750 rpm for the wake. This may not be the best but has never hurt my engines.

[bobk]

I have a 48 MY with stabilizers, 6V92 425 HP. I have about 4000 hours on them at mostly 1000-1100 rpm with no issues. I do use Howes MPK for cetane booster and 170 t-stats.

Bobk

[Seapig100]

I have been thinking about this thread and thought I would add my 2 cents. IMO we are doing zero damage to our engines running them at 1200 or 1300 rpm. It takes quite a bit of power to move these boats at 8 or 9 knots. I would actually argue that this is the ideal rpm to run at. In a over the road truck it's not uncommon to go 6 or 7 hundred thousand miles even a million miles is not that rare. Let's assume an average speed of 50 mph that would be about 20,000 hours before major overhaul. How many boats get 20,000 hours between major overhauls? A car or truck uses a fraction of the available power to cruise at 70mph a car uses about 30 hp to cruise that fast. My car has 300 hp so it's using 10 percent of its available power. How much is a truck using 125/150 hp? And today's truck have upwards of 600 hp available. My Detroits have 325 hp and I read that maybe 60 or 70 hp is used to move at 8 knots. This is plenty to keep the engine healthy if it's run up occasionally.

[Glory]

DDs 8V71 manuals indicate that for some uses, such as large generators, 160 degree thermostats are to be used, so I'd say DD designed the engines to be run at 160 degrees for long periods of time. Generators can run 24/7 and only have large loads for a few hours of those 24, while the RPMs will stay constant. I'd have no problems running 900 RPMs and 160 degrees for long periods. I ran at 900 or so for days with no ability to run them up in the Trent Severn canal.

Doug

8V71Generator RPM is not going to be 900. Generator manufacturers normally suggest loading the unit.

[Pete]

In most all discussion on this subject, the scavenger air volume does not get included in the consideration. On DD 2 cycles the blower is gear attached to the RPM of the engine. Thus when you slow the engine down you also diminish the cooling air going through the engine. This makes the comparison to over the road usage invalid in my opinion. In over the road applications, the engine runs in the 1400 to 1600 range, producing on average very little horsepower. Lots of air going through the engine with very little fuel, and stress.

By comparison, many boaters will over prop to save a little fuel, utilizing more fuel with lesser volumes of scavenger/cooling air, increasing the stress at low RPM. IMO if you want the engine to go further between rebuilds, you underprop it and let it breathe/cool better as it produces the same horsepower at a higher rpm. And this needs to be done where the engine temperature is managed in the proper operating range. Over the road applications manage proper engine temperature at light loads, so we know it can be done.

And for the overprop/lower rpm/save fuel crowd, I suggest they complete the thought to overprop/lower rpm/save fuel/rebuild sooner.

Pete

[Seapig100]

Up until about 15 years ago every truck in our fleet were powered with Detroits. Many of the trucks had over 150,000 miles. These are fire trucks they spend the majority of their life idling. I know of one major overhaul that took place and it's because the oil pan got ripped off. This is not high miles but the hours are through the roof. Turbo rebuilds absolutely like clock work at 100k miles. This however is because they were shut down hot. So the life of a fire truck is run like hell for 3 min then idle for 20 minutes to a half hour then drive back like a normal vehicle and shut her down. They just kept going. Running at 1400 rpm puts me at 10 knots just a bit to fast. I guess I can probably do 1300 instead of 1200 but not sure it will make a measurable difference. I know for sure running 1800 to 2000 will just rack them out so slow it is for us.

[krush]

By comparison, many boaters will over prop to save a little fuel, utilizing more fuel with lesser volumes of scavenger/cooling air, increasing the stress at low RPM. IMO if you want the engine to go further between rebuilds, you underprop it and let it breathe/cool better as it produces the same horsepower at a higher rpm. And this needs to be done where the engine temperature is managed in the proper operating range. Over the road applications manage proper engine temperature at light loads, so we know it can be done.

And for the overprop/lower rpm/save fuel crowd, I suggest they complete the thought to overprop/lower rpm/save fuel/rebuild sooner.

Pete

"Overprop" debate at low RPM is irrelevant as the engine is still very lightly loaded.

[Pete]

Disagree. The engine will benefit from the additional scavenger air flow for whatever power is produced. Also, the overproping folk always recommend an occasional run up in the rpm to "clean out the carbon" and the engine will be well under the designed scavenger air flow during this operation.

Pete

"Overprop" debate at low RPM is irrelevant as the engine is still very lightly loaded.

[Top Hatt Craig]

Here's some verbiage from the Wartsila 9L26 (inline 9!) heavy marine engine. I wish DD could be as specific. . "Operation at low load and idlingThe engine can be started, stopped and operated on heavy fuel under all operating conditions. Continuous operation on heavy fuel is preferred rather than changing over to diesel fuel at low load operation and manoeuvring. The following recommendations apply:Absolute idling (declutched main engine, disconnected generator)● Maximum 10 minutes if the engine is to be stopped after the idling. 3-5 minutes idling before stop is recommended.● Maximum 6 hours if the engine is to be loaded after the idling.Operation below 20 % load● Maximum 100 hours continuous operation. At intervals of 100 operating hours the engine must be loaded to minimum 70 % of the rated output.Operation above 20 % load● No restrictions." kinda apples and oranges but what the heck.

[krush]

Disagree. The engine will benefit from the additional scavenger air flow for whatever power is produced. Also, the overproping folk always recommend an occasional run up in the rpm to "clean out the carbon" and the engine will be well under the designed scavenger air flow during this operation.

Pete

Even when "overpropped", at 1200 RPM, the engine will still be able to make much more HP than the prop is absorbing. And, when "overpropping", one should also derate the engine by lowering the MAX RPM operated at OR (more better) lower the governor high-idle setting.

Also, most of our boats have turbo chargers. Loading the engine makes the turbo pump more air.