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    RPM cruising speed

    From all I've read, so much doesn't jive with my real world observation. These kinds of topics often get so out of hand that I wouldn't bother bringing it up on some of the larger sites, as it will surely devolve into some weird metal alloy discussion from the second cousin of the man who maintains the nuclear fleet and knows best guaranteed :-)

    To me cruising RMP is 24/7 setting for running continuously. I'll always run the same after that for any trip being 2 hours or days. So far on my setup 1300 RPM seems good, give or take a 100. Looking at the fuel map/torque curve and RPM for my engines help me figure that out.

    What I find fascinating is all the discussions about loading your engines to 75% (or variations of that scheme) or you will do untold damage. Now i'll admit to knowing very little about mechanical devices except from the using them side. My engines are also used in trucks (Cummings 450 diamond), and other things and are rated at 2600 wot.

    A Mac truck cruising down the highway is sure as hell not doing 1900RPM ever. A loader is not screaming at high RPM either, the commercial boats I worked on did not run at high rpm, heck my personal truck has max RPM of 6500, but cruises on highway at 1000RPM, using a fraction of available power. Most heavy transport trucks I know that are decently maintained get 1M miles out of them.

    Either I'm missing something, or some strange information has been perpetuated for some unknown reason.

    Any of you guy's running your engines at say half max RPM or there about?


    #2
    You can’t compare road use of a diesel with marine and generator use. In road use you have constantly varying rpms and load, as the truck goes up hill the engine gets heavily loaded, going down hill it’s unloaded. With a generator you have constant rpms but you have changes in load. If the generator is over sized such that it never gets loaded the pistons get glazed over time. This is why you often see boats with 2 different sized generators. In camps you will see heaters on the roof of the camp, these keep a load on the generator during low demand times. Problem with boats is they sit at the same rpm and the same load for ever. If the load is sufficient then no issue. If the load is not sufficient then the pistons will get glazed over time. Now the big debate is always how much load is sufficient? Can’t answer that question for you. Can only say that most people recomend running the diesel hard for 20 minutes out of every 24 hours.
    Azzurra
    Seattle, WA
    Ocean Alexander 54

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      #3
      I run my engines to make whatever speed and fuel economy I want for the journey that I am on that day.

      I find that my Cummins 330 engines are a bit rough if I run them down around 1200 RPM, but they smooth up nicely at 1350-1400 RPM. This is also the point where my Balmar alternator has sufficient RPM to charge the house batteries plus keep up on the boats loads.

      This gives me a Great fuel economy and about 8 knots of speed.

      If I need to get somewhere a bit quicker I can go as high as 1800 RPM and still be running the boat in displacement mode. I get about 10 knots at that rpm and my fuel economy drops close to half.

      I do not know where this all comes into play regarding loading, but as Dave indicated loading related issues can be avoided by pushing the engines a bit once in a while.

      KEVIN SANDERS
      4788 LISAS WAY - SEWARD ALASKA
      where are we right now​​​​​​???​

      https://share.findmespot.com/shared/...j23OquWOj2N3Xe

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        #4
        That seems to be the cruz of the matter. Yes idling for ever and very low RPM will cause glazing. I seem to run mine at about the same as you 1300 or so. I think it's not an issue, specially as the temp is in range. As in all things however, what 'i' think and reality can be a lot different so I'm hoping to make sure before I hurt stuff. I get it with the generator, but again how much of a load is the question. I've got 2 32KW generators. The man who built the boat had one hell of an air-conditioning system in there as he lived in tennesse and did not like to sweat I think. So these bastards put out 150amps all day. I've got an electric furnace that can suck up 35amps when it's cold enough, then I've added infrared heater, looking at getting a couple more of those for upstairs deck just to add a bit of load, plus the engine heaters and all that, but i'll never get to 75% load. Not too worried on those since before they give me issues I might be dead.

        Regarding load, in anything but flat water, I would expect to keep the rpm steady, the load goes up and down with current, wind, waves.

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          #5
          The load doesn’t go up and down with current, wind or waves. The boat goes faster or slower but the load is the same. Not saying 1300 rpms is wrong. If the boat is at full temp everything is probably fine. If worried then just run her at 2000 rpms for the last 10 min of the trip.
          Azzurra
          Seattle, WA
          Ocean Alexander 54

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            #6
            Thanks, not worried only trying to get better understanding. . I think I got it now.

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              #7
              I run my boat at 1750 rpm. Why? Because the boat goes 10 kts and burns 10 gph. I am comfortable with that speed and consumption. I can go 15 kts but my consumption goes up to 30 gph. If I am trying for maximum range I go 1300 rpm. Anything lower makes an insignificant fuel savings for me. I carry 650 gallons, this gives me a theoretical range of 1000 nm. Useful range is 850-900 nm.
              Azzurra
              Seattle, WA
              Ocean Alexander 54

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                #8
                It all depends on the engine size and design. Old and older diesels for commercial work generally were unphased at what rpm/load they were run at. In commercial fishing, trollers might run at slightly above idle for days at a time. Or just as likely draggers could run at max rpm for days. Natural engines, non turbo, sometimes were never overhauled in decades.
                I also run at 10 knots and burn about 8.5 gallons. I can cut the fuel in half, but like to figure eta in a easier number for an old mind. When I bought the boat it burned 14 gph, mains needed an overhaul and po ran a generator 24/7. With changes I still have 120/240 AC w/o a generator running.

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                  #9
                  That lines up with my recollection. I appreciate the feedback.

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