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Fuel consumption guidelines

Discussion in 'Engines' started by Nuts n Boats, May 26, 2020.

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  1. Nuts n Boats, May 26, 2020
    #1
    Nuts n Boats

    Nuts n Boats New Member

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    I have owned boats since i was 10 some 65 years ago. I have always tracked the fuel consumption of my own boats under varying conditions. But I have never seen a discussion or forum about the relative fuel consumption of different diesel engines.

    Now looking at a boat with Cat C12 s and the Cat manuals say something like 20 litres (5 gals) per hour at idle and 50 litres (13 gals) per hour at 1250 revs, for each engine which I find very high consumption.

    What am i doing wrong here.? Any C12 owners out there top give me a hint on consumption, please.
  2. Capt Ralph, May 26, 2020
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    Capt Ralph

    Capt Ralph Senior Member

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    It takes torque and horsepower to move a hull thru the water. Some hull designs go easy, some not so easy.
    Then the loading, then this that and another thing. So, It's hard to give an easy answer.
    The later engines these days are built on features that differ from mfg to mfg.

    The comment I would like to state, on a turbo diesel, any late model model, 1/10 of a gallon of fuel, per HP, per hour /2 (diesel turbo) for a single engine is a good rule of thumb to work by.
    Just asking for a C12 consumption? better results will come from Cat on a loaded HP/Fuel curve.
    So the C12 can offer 340 to 490 HP at max curve. That would be roughly be 17 to 25 GPH per engine, max curve.
    The hull design will offer the better MPG as the operator keeping the throttle down.
    I would say your numbers are not far off. In gear, moving forward, I can see a C12 requiring 100 hp on a large hull and at 1250rpm making 260hp.
    Remember, HP is required just to make the engine run, turn the water pump, fuel pump, alternator, stabilizer pump and more parasitic hp loss. On a large hull, in gear 100 hp is pretty darn good.

    Now, Is there a particular hull design you would like to discuss per C12?
  3. Capt J, May 26, 2020
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    Capt J

    Capt J Senior Member

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    Down at lower RPM's you're not going to see the miserly fuel consumption of a 2 stroke D.D. if that's what you're thinking. The old 71's and 92's sipped fuel at a 1000 rpms and under due to their blowers and 2 cycle nature. The 4 stroke diesels just arent nearly as efficient at low rpms. For a C12, it's been a long time since I've run one, but at idle they are not burning 6 gallons per engine in gear........at 1250 maybe. At 1000 rpms, you're probably going to be around 8-10 GPH for both engines
  4. Capt Ralph, May 26, 2020
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    Capt Ralph

    Capt Ralph Senior Member

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    Yes, but the engine tune and hull design are needed for further or closer numbers.
  5. BRyachts, May 27, 2020
    #5
    BRyachts

    BRyachts Member

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    All engines will have different efficiencies due to age, design, etc etc.
    The general rule I go by is 20 HP/Gal/hour. That will get you in the ball park.
    So, if your burning 10 gal/hr, you are pulling approx 200 HP out of that engine. Fuel in = HP out
    How much HP you need to pull out of that engine to move the boat is a factor of hull shape/weight etc.

    Typically when I launch a new boat, especially a new design, the engine mfg will, on sea trials, measure fuel in/fuel out, to get exact fuel burn. When factoring in their known fuel efficiency of that engine design, they can determine the exact HP the hull/props are demanding, and we can adjust prop pitch to get engine to operate in it's designed output. Once prop pitch is done, and other parameters like exhaust temp and back pressure, engine room ventilation etc are within parameter the warranty will be approved.
  6. Nuts n Boats, May 27, 2020
    #6
    Nuts n Boats

    Nuts n Boats New Member

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    Its apparent from the replies that simply enjoying boats for 50 years has not meant that I am well educated about engines.

    I thought that the same engines at the same revs would consume the same fuel whether it was a work barge or a yacht, and I can see thats wrong.

    That being the case, how do Caterpillar produce a consumption table according to RPM? Do they do some sort of average?

    For those of you willing to advise, these Cat 12s will be in a semi planing hull displacing 75000 llbs, called a Marlow 57, and are 13 years old with 2500 hours.

    Really appreciate the inputs from all.
  7. Capt Ralph, May 27, 2020
    #7
    Capt Ralph

    Capt Ralph Senior Member

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    Have you taken delivery of her yet?

    I am not Marlow heavy nor a fan. Now that you have mentioned the M word, I'm sure others will be chiming in soon.
    Some of us will have some pop corn ready.
    There should be HP per propeller load curves available on CAT's web site. This would be the best factory numbers based on a dyno emulated load.
    Based on an exact hull will require an experienced M operator.