“Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover.” – Mark Twain

Running a Diesel at Idle


In a discussion on the sailboatowners.com forum, I had posted that it was bad to run a diesel at idle.  I was quickly rebuked on this point by some knowledgeable/experienced boaters, including one very well respected moderator/featured contributor.   I have also seen this subject come up on the Cruisersforum.com.

First thing, lets establish the definition of idling because this was of some issue.  In the case of this discussion it would be running your motor in neutral. Even if you bring the RPMs up to 1,000 you are still idling your motor.  For instance, the idling range for my Universal M25-XPB is 1,00-1,200 RPM.

Pages from Universal Deisel Operation Manual_February 2010

From the Universal Diesel Operations Manual – February 2010

Now, I have always operated under the notion that idling is bad for your diesel engine.  For instance, running your diesel at anchor to charge your batteries and heat hot water.  My owner’s manual tells me such:

Idling from Universal Deisel Operation Manual_February 2010

But here are some quoted responses I got during the discussion on Sailboatowners.com.

From jviss:

For millions of sailboats running the main propulsion diesel is de rigueur for charging batteries, including everything from affordable, 20-something footers to 1/2 million dollar plus coastal cruisers (from Morris Yachts, for example). So while you may have a theoretical point, in practical terms it is meaningless. To paraphrase you: in general, it doesn’t matter.

I don’t idle mine, I usually run it at 1000 RPM or slightly higher. I don’t get enough of a difference in output at higher RPMs to make it worth the extra noise. At idle (700 RPM) it doesn’t make enough voltage to charge beyond a float level.

The only potential negative effects are glazing of the cylinder walls over time, that may cause some smoking. Mine has not exhibited this in 29 years of operation (at about 3000 hours). And, if it becomes necessary it is correctable: Westerbeke used to run generators on a dyno at 80% load with a teaspoon of feldspar in each cylinder to roughen up the cylinder walls and stop the smoking! (maybe they still do). You can break the glaze with a hone pretty quickly when it’s time, like every 40 years or so.  Probably won’t even need new rings.

A big charging load, taking conversion efficiencies and drag into account is about 3 or 4 HP, which is about 15 to 20% of max rated output for this engine; so, hardly idling.

A bonus is that it heats the domestic water, too…

From Maine Sail:

My buddy Darren owns a good sized excavating, irrigation and landscaping company in Colorado and we talk diesels quite a bit. Just got to see him last week.. He has a good sized fleet of them and a couple of them have over 20k hours with no rebuilds. The last time I spoke with him about engine longevity he had one Yanmar block and one Mitsubishi block with over 20k hours. He bought both of these machines used with about 5k hours on them back in the late 90’s.

Most of his smaller engines are either Yanmar or Kubota but he does have a few Mitsubishi’s too. His engines run all day and never shut off and they idle for long, long, long hours. Now granted these are not in a marine application but in well over 500,000 hours of combined run time on his fleet he has yet to rebuild an single small diesel engine. All his machines run Shell Rotella and it gets changed regularly. Of course he buys his oil in 55 gal drums and I buy it by the gallon…

If heavy equipment running Yanmar, Mitsubishi and Kubota blocks can rack up10k to 20k hours, while doing hundreds and hundreds of hours of idling per year, with no rebuilds then a well maintained marine diesel should be able to do the same.

When we had the discussion about not letting diesels idle a few years ago he just laughed about the glazing the cylinder walls. His sarcastic comment was something like “Sh&t I better let my guys know not to let them idle”. Course he’d already been doing it for 20 some odd years, with no failed engines or rebuilds needed, so he was surprised to find out his engines were going to die soon…

Our engine has idled perhaps half or more of its 3600+ hours. It burns ZERO oil, has cross hatching in the bores that looks like new and she purrs like a kitten. We have Sea Frost and often sail with the engine idling or will let it idle to chill the plate if we are alone and not disturbing others. Our boat also did a five year 24/7 on-the-hook almost circumnavigation. She had no generator and only the factory alt and a single solar panel. The batteries lasted six years and were still kicking.

When this came up on the Cruisersforum.com, the following replies were posted:

From Jd1:

Everything I have read and have experienced supports the notion that a diesel is best run in the 75% to 90% (or thereabouts) power output range. An occasional period of idling will not be detrimental if followed by a period of running at a good load (for example starting the engine and letting it idle for a while to warm up before heading out). The frequent in and out of the harbour short hops are murder to the poor diesel.

I can not explain why automotive diesels, which spend a lot of time idling, survive. I could speculate though that they would last a lot longer if run like a semi trailer road transport truck – much harder and much longer.

A modern diesel should last 20000 to 30000 hours yet they get replaced in boats at a fraction of the expected service life. IMHO that is, amongst other things, related to the unfavourable working conditions of a sail boat auxiliary engine.

Diesels like to run fairly hot (180 – 190 F range). Running colder and/or idling a lot causes more wear and tear, carbon buildup and soot buildup.

I am sorry that I can’t produce a link to something official looking.

From nes:

I don’t know if this applies to the small diesels (non turbo/super charged), but the diesel generator that I have a lot of experience running had 36 cylinders displacing 645 cubic inches each, and it had an interesting gear driven (at low load) turbo (at full load) charger/compressor.

This diesel had very specific directions against running at low load (less than 30%).

The reason given for the load restriction was lower cylinder pressure at low load. The piston rings are designed to operate at rated load combustion pressures, this ensures normal lube oil consumption rates. Light and no load operation promotes “souping”, which is excessive oil escaping past piston rings into combustion chamber. Souping causes smoking exhaust and potential exhaust manifold fires.

The recovery direction, if the engine was run at low load, was to follow that with at least a half hour at greater than 50% load.

Based on what I have seen running this engine, I would suggest that it is not a good idea to do extended runs at low load.

[Interesting side note, jviss and Jd1 both have Catalina 36s.]

So by posting this (and cross posting it to several different areas) I am hoping to get some feed back that will help me determine if idling is bad for my diesel.  If you comment on this, please try to provide something to support your opinion.

Fair winds,


Cross posted on Sailboatowners.com and Cruisersforum.com.

15 thoughts on “Running a Diesel at Idle

  1. Hi Jesse,

    This is a shot in the dark, but did my crew hang with you and your crew last November at the Soggy Dollar (and then recently bump into you in Newport)? It’s a small world if so. I came across your post on SailboatOwners.com and, well, here I am asking.

    Rich & Ruthie

    • LOL. Yes, that is us.

      Small world.


      • Too funny! Ruthie and I have been sailing Spirit out of Wickford, RI this past season and plan to do so again next year. It would be great if we could cross tacks somewhere along the line. I’ll be following your blog in the meantime since we seem to share a similar long term plan.

        Regards, Rich

      • That’s great. We will definitely be down that way next year. We are planning a cruise to Block Island. When we get the plans better together will have to see if we can get our schedules to match. It will probably be near the end of June and will include stops in Cuttyhunk, Block Island and Newport or Jamestown.

        What kind of boat is Spirit?


      • Awesome. We missed out on getting to Block this year and it’s on the “must do” list for next year. Wind and schedule never seemed to cooperate. We managed Cuttyhunk and the Vineyard…and Newport is a second home port. Heading there this weekend, actually. I’m sure we can work something out to catch up if you head south next season.

        Spirit is a 1982 Tartan 37. She’s old, but still a fine looking lady…if I do say so myself. 😉


  2. Pingback: Solar Panels – Flexible Panels vs. Hard Bimini & Rigid Panels | s/v Smitty

  3. Sea Raven was a charter boat when we bought her, the Captain told me how the owner would never let him run the engine above 12-1500 rpm, its a 3000 rpm max engine and I generally run it at around the 2200 rpm mark. When the engine was taken out to be reinstalled correctly (a whole other blog post waiting to happen!) the exhaust was caked thick with carbon, to the point there was a gap about the size of a dime left for the air to flow through. My mechanic explained this was due to constant running at low loads, lesson learned!

    • So I have been meaning to get back to this with you. By the way, I hope all is well with the wife and little one. Your blog has been as quiet as mine but you have a better reason. 😉

      I could see how the CONSTANT running at 1,500 RPM or less could do that. But the fix for that would be to get out and move under max throttle for a decent period of time (say an hour or two) to blow out that carbon. Right?

      So in the cruising scenario, you are anchored, it’s been cloudy for a day or two and your batteries are getting near 50%. So you kick on the diesel and let it run for a couple of hours at like 1,000-1,200 RPM to charge the batteries. As long as you get off the hook sometime in the next week and motor around (most likely to empty your holding tank anyways) the diesel should be fine. Thoughts?

      • my grandfather always used to tell me you needed to take your car out for a “good blowout” every now and again, for just that reason, run her at higher revs to blow out any carbon buildup, Sea Raven just never really got that as a work horse! So from my limited knowledge I would say that yes, it would help to run it at a higher load once in a while. As for charging up, I have a small Honda 2000w generator on board that I use if we ever need to top off the batteries, can stow it away when not in use, and it uses barely any gas to run while throwing up to a 50A charge into the batteries (bulk charge cycle) and is barely above idle for the absorb and float stages. Personally we prefer this method to running the engine, at low rpms with a 40A alternator, we’re would need to run it for longer to get the same result!
        Was actually just staring at our blog, thinking I need to update, but we are doing the last few things before a 4 day passage to the mainland of Mexico so it may have to wait, our new crew member is having a hard time getting to sleep with all the activity too, so no time to type for now!

      • On the Honda generator, I was initially thinking of going this route but was talked out of it by others on one of the sailing forums. Essentially they made it out to be a total dick move to run your Honda generator in a crowded anchorage due to noise. This did seem odd to me since I have seen it several times and never thought it was a big deal. What has been your experience with running the generator? Do other cruisers really care as long as you don’t run it at night or early in the morning?

        I will likely get the Honda generator anyways (mainly so I can use bigger power tools when needed). I will probably get the Honda e2000i companion because that has a 30 amp outlet that can plug right into the boat for when needed.

        Fair winds on your passage.

      • I use the honda 2000 without the 30A plug with an adaptor on our shorepower cable, runs straight into my charger/inverter and it does get some use during winter or after a few cloudy days, we use the watermaker more now that the bubs is here! As far as it being too noisy for the neighbours, the forum crowd obviously haven’t been in many anchorages around here, normally there is more noise coming from the massive powerboat with its diesel genset running ALL night than the sailboat running the honda for a few hours. And they really aren’t that noisy at all, with the shore power hook up it will juice the batteries, make water AND run your tools, without breaking a sweat!
        And we ended up motoring most of the way on the trip, every forecast had good wind, but I guess we just missed it!

  4. Lots of good information to ponder in this post, even if there’s no conclusive answer to the diesel idle debate. The bottom line for me is that I generally don’t intentionally idle our M25-XPB for things such as charging while on the hook, hotwater, etc. so I’m not going to worry much about it. If you think too much about it or read threads about idling you can be led to believe these diesels are fragile engines and become stressed to even run them at all. My plan is to diligently keep up with the maintenance schedule and run the M25 like I’ve always run my engines, within the recommended operating ranges of the manufacturer.

  5. Hello All,

    We have recently overhauled our Cat engine generator 3406C,replaced F.O. Injection pump assembly,F.O. transfer pump,cyl.head and injectors. After that overhauling we tested run the Aux. engine and it looks fine. tested for black out test to make that the standby generator will automatically run and supply power to the board. and the issue now is that newly overhauled Aux. generator cannot hold on 50% kw load the RPM was dropped and fluctuated. And when shared parallel to the other generator the kw loads fluctuates but this time the RPM stable . Tried to replaced a new AVR but the same issue. All F.O. filters replaced. Fuel lines cleaned. But no development. With normal parameters and no dark smoke.Any ideas or advise?

  6. I work on GE 4500 hp 12 cyl engines. They soup within 1 week of idle time, oil out the exhaust, (oil out stack),which is common. Cold weather is worse. People do NOT listen. Needs to be under a load while running.

    • So Jason, my fundamental disagreement is that if you are running the engine to charge the batteries then the engine is under a load. The load is being applied by the alternator. Or by engine driven refrigeration or water maker or other devices that put a load on the engine without the prop spinning.

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