Diesel engine underload

We’ll be running our main engine at fairly low load for several weeks while we cross the Indian Ocean. Since this likely is below the manufacturer’s recommended range, we’ve been asked if we’re concerned about underloading the engine.

It is true that manufactures have a preferred and fairly narrow range over which a diesel engine is operated. There is good evidence they are correct that this is “best” in the absolute sense of the word. Some of the highest hour engines I have seen were running in near constant load generator applications. From an engine manufacturer perspective, their job is to tell you how to get the absolute longest life from your engine.

Recreational boats are definitionally used to have fun. Being able to cross oceans and see different parts of the world is fun. So, the ideal engine for a boat like a Nordhavn 52 will not be much bigger than 100hp if you focus on multi-thousand mile ocean crossings. We are, for example, currently operating at 80hp. However, few of us will spend much of our lives crossing oceans and having a boat where the top speed is just a shade over 6 kts kind of sucks. At least for us, as much as we like going 6 kts right now and know we are on track to successfully go over 3,000nm without running out of fuel, we also like running at 9 kts and getting to the destination in time to watch the sun set over a glass of wine. We did this a few days ago on our multi-day run to Dampier, Australia. The former needs 100hp and the latter needs more than 200hp. The ideal boat for most users will be able to operate reliably over a wide range of power levels and the ideal engine will last longer than most owners in this usage model.

More diesel engines are destroyed by overload than underload. It’s not a common problem in trawlers but overload still does happen. Where it is super common is in the planing cruiser market. Manufacturers prop to achieve high advertised speed. Owners then move their personal effects on board, install bait tanks, fill the fuel tanks, forget to the clean the bottom, and invite their friends to cruise with them. Consequently, over-propping is common and many Bayliners and Searays are trailed by thick clouds of black smoke. Looking at the Bayliners, their poorly-treated Cummins and Hino’s mostly soldier on. Some fail early and, if any engine has a weakness, overload is truly the fastest way to find it. One model of Cat used by Searay was particularly suspectable to overload-related failures and they were replaced by the hundreds. It was an absolute disaster that was hard on both Cat and Searay but I’m told all the lawyers were fine.

Overload is potentially fatal. Underload is less than ideal and requires more care. My general rule is take very good care of your boat and all mechanical systems, change your oil and filter often, run it up to full power periodically, ensure the water AND oil get to full operating temperature, and then have fun with your boat rather than letting it control what you do.

Our previous boat had a pair of 270 HP Cummins. We ran them near max to get to a cruising area and then cruised the area at displacement speeds where each engine was producing 20 to 30hp. Fuel economy was stupendous and needed to be since we only carried 220 gallons. We loved that boat, cruised all over BC, wrote a cruising guide, and put 4,100 hours on each engine. The engines continue to operate beautifully today and I expect they will continue to do well for a decade or more. They have spent much of their life under light load but they have been well-maintained, have lasted 15 yeas so far and will go well past 20 years.

The Deere 6068AFM75 in Dirona has averaged around 44% max output through its life. That’s actually a fairly high number given that you spend time moving around marinas, warming up, cooling down, putting through locks, etc. 44% is on the high side but we are currently running at only 80hp and will run at around that light power level for the next 3 weeks. The coolant is currently 178F and the oil is 194F. Slightly warmer oil wouldn’t hurt, but 194F is fine. Our engine operates like this frequently. It also spends considerable time up over 200 HP for long periods of time. It serves us over a wide dynamic load range and we love the boat and the engine for being able to do that and do it well.

Our Deere has 6,015 hours on it now and, in the unlikely event that it does die early, I’m confident it won’t be from underload. Personally, it’s nowhere close to the top of my worry list. We are going to unapologetically keep running near maximum output when we want to get in before nightfall and running near minimum output when we need to cross long distances.

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15 comments on “Diesel engine underload
  1. Mark Nowlan says:

    A Happy New year to you both (including Spitfire). I’ve been following you for some time and am thoroughly enjoying not only the education but the scenery. I do have a question for you though, in regard to both over/underloading and prop pitch…..had or have you considered a variable pitch prop…i’ve absolutely no experience with them in a marine setting but i have noticed some European makers using them regularly. What’s your “spin” on that one?

    • It is a good question. The reason why automobiles are getting more transmission gears over time is becuase getting an engine running at it’s most efficient RPM for the current load can make a big difference in fuel efficiency and even help with engine longevity. In the marine world, the same rules apply and the three approaches I’ve seen applied are: 1) multi-gear transmissions (not that common), 2) multi-engine configurations where more egnines are used during higher load demand periods (commone with diesel electric configurations), and 3) variable pitch props. This latter solution is actually a pretty good one and is used on many larger boats and it is also used on the Nordhavn 56 motor sailer.

      The only downsides of the variable pitch prop solution is they are kind of expensive running around $70k for a boat of our size and they require some thought to use. This latter concern is a nit but the operator needs to be aware of pitch and watch engine exhuast temperature and/or loads and keep it where it needs to be. The only real dowside, is the expense. It would take a long time to save enough fuel to pay for the variable pitch prop. Other than cost, it’s a very nice solution.

      • Mark Nowlan says:

        …i figured you would have some good reasons. Now…if money wasn’t an issue (wouldn’t that be nice) I’m guessing there’d be some systems that could monitor those parameters and warn or automatically set pitch?

        On second thought….I guess that just went from a 70k “option” to 150k! disregard!

        Again, thanks for the insight and education….


        • I’ve never seen a automation system to keep the variable/controllable prop near optimum pitch but it wouldn’t be that difficult to one to build. Monitoring engine RPM and exhaust temperature is easy. If the pitch control systems supported some form of API, what you want to do could easily be done. It would be a smart move by Hundested to offer an option that pluged into the J1939 connection on the engine and provided auto-pitch control, operational override, and alarming. It woudl surprise me if your suggestion doesn’t get implemented eventually. It just makes too much sense.

          • Mark Nowlan says:

            Thanks again…it’s certainly an interesting discussion, I was also thinking more along the lines of maintenance of such a system and potential life span vs initial cost. The question actually came up when i was watching your trans atlantic passage and the great job you were doing with fuel monitoring…. and would such a system make those long passages more within range of say a vessel with a different fuel load or better yet, Dirona with the existing fuel?

            thanks again…i do appreciate your thoughts

            • A controllable pitch prop would make the engine more efficient at lower operating speeds so, yes, that option would effectively increase the range of the boat on a given fuel load. The fuel savings alone are unlikely to justify the additional expensive of variable pitch props unless the boat is used extensively for a long period of time but I agree that increasing range might be sufficient justification on a small long range boat.

              The option offers greater efficiency, longer range, and better engine loading at low operating speeds. The downside is expensive and complexity.

    • Mark Nowlan says:

      Sorry, meant to say controllable pitch prop…

      • Colin Rae says:

        Hi James, what about changing props for the really long runs? Jim Leishman goes into some detail about this when discussing Jim and Suzy Sink’s voyage (Salvation II, a N46) in Voyaging Under Power. There are disadvantages ( mainly cavitation) at higher RPM but definite advantages (I think it was 30% greater range?) to a different prop. I think they went from 4 blade to 3? It could either give you incredible range with bladders or very long range without bladders! Downside would be storing an extra prop, haul out at each end to swap props unless a diver could do it.

        • You absolutely could do it and it would deffinitely extend range. I would expect the gain would be considerably less than a 30% gain but even 10% would still very interesting. The prop is a beast though and I have nowhere to put a spare. It’s almost a hard across and quite heavy (32 x 23.5 x 5 blade). On our last boat, we did have two spare props and we considered it on this boat but it’s just so big I don’t know where to put it.

          My take is that we have the capability of doing 4,000 nm right now and extending much beyound that is not something that feels all that important. It’s oene of those cases where the additional cost and hassle doesn’t seem worth it since we have found other solutions.

  2. Brett Griffin says:

    Hi Jennifer and James, I like your article on engine loads. I used to run a couple of game fishing boats and we trolled for hours at just above idle much of the time and then used to run home at higher power.
    From your log entries the weather so far looks fantastic and I just love the color of the deep open ocean. The detail and setup of your instrumentation is amazing.
    I think from what I see that you have mainly Furuno electronics. I was wondering what sort of sonar equipment you have and if you use it regularly out in the open ocean? What sort of depth you get bottom to and if you may ever see fish or other returns out there? On my last boat I had a type of side scan sonar (Lowrance Structure Scan) and it was truly amazing what could be seen.
    Also do you troll and catch many fish? I would guess wahoo and dorado would be the main species encountered in the warmer mid ocean regions.
    Anyway enjoying watching your progress and wishing you currents from astern,



    • Brett was asking about fishing on Dirona. We should but we don’t. We tried fishing on the run south from Hawaii and ended up giving up a lot of gear but not catching any fish. Not having even a glimmer of success has made neither of us super motivated to get out there. We need to spend more time with somenone like yourself with game fishing experience.

      The boat instrumentation is a boat wide NMEA2000 bus with a lot of Maretron sensors connected. We really like Maretron — it’s amazingly flexible and we use it all over the boat. We particularily like the precision of the fuel pressure measuring sensors. We get +/-3% on nearly the 8,35 gallon side tanks. It’s really good to know with precision how much fuel is on board.

      We have also extended the NMEA 2000 system with our own software that stores all ship board telemetry in a relational database every 5 seconds. We have massive amounts of data on just about everything. It’s this database that drives the real time display of boat position, current weather conditions, and fuel economy on the web site.

      For bridge navigation equipment, it’s nearly all Furuno. We chose Furuno because it’s the equipment of choice for the Alasakan fishining fleet sailing out of Seattle. We know they like quality and reliability and that drew us to Furuno. The equipment has performed super well and we make the same purchase again in a minute. The Furuno RADAR systems are standout performers. They are incrediibly easy to use, can resolve boats near shores, and ovverall are standout performers. The RADAR alone would bring me back to Furuno.

      The other notable Furunon “feature” is the North American customer service. All modern integrated nav systems are complex and the true test of any electronics vendor is how they deal with the post install teething issues. It’s very rare for a multi-vendor (or even single vendor) integrated electronics system to be installed and there not be some issues. On many installs, these early install issues just never get resolved. We appreciate Furuno USA being willing to spend thetime and get them solved quickly and for always being available.

      You were asking specifically about sonar systems on Dirona. I have heard great things about side scan and forward looking SONAR systems but they are expensive and some report they are very limited in their look ahead ability. We ended up going with a Maretron and Furuno Depth sounder with side scan or forward look capability. If we werre doing it again, we might look closer at that choice.

      • Brett Griffin says:

        James if you want to catch the odd pelagic fish for the table I’d keep your fishing gear nice and simple. Probably a couple of Shimano TLD25 reels on simple short 25kg rods. Spool them with 24kg monfilament line with snap swivels on the end and attach heavy single strand stainless trace tire of around 60kg and 3 foot long direct to a simple bibless lures something like the Halco 130. When you set the drag up on the reals just set the strike drag which is the first stop on the drag lever to about 1/3 of your line strenght i.e. 8kg strike drag for 24kg line. Once you let the line out to your desired distance behind the boat (50-80 yards) set the reals on strike drag with the ratchets on and wait to hear the strike. Wahoo are suberb eating fresh and Mahi Mahi aren’t bad either. A good tackle shop should help you set them up,



        • I appreciate the fishing advice Brett. For this trip, we’ll depend upon what I refer to as the “remote catch” program. This is where friendly fisherman down in Tasmania catch Salmon for us and we keep them in the freezer to eat underway. So far, the remote team has been pretty reliable and they lose less fishing gear than I used to lose :-).

  3. Jacques Vuye says:

    Couldn’t agree more.
    There is probably more trouble to expect from the diesel in a **generator** aggregate that continuously runs at light loads.
    And Dirona is certainly closer to a “mobile home” than a “drag racer”.
    And I’m sure you both enjoy the low dB level of a lightly loaded main!

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