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Calculating Hull Speed Fuel Consumption

Discussion in 'Technical Discussion' started by MBevins, Jul 1, 2008.

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  1. MBevins, Jul 1, 2008
    #1
    MBevins

    MBevins Senior Member

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    All the discussion about fuel prices got me to thinking (which is never a good thing)
    I know what my fuel consumption is at cruise. I run 6-71 DD's, at 2100 RPM I cruise at 20Knts and burn about 33GPH. I have calculated my hull speed to be about 8knts. The question is can does somebody know how to extrapulate the above numbers to tell me what my calculated fuel burn would be at hull speed.
    I know I could go out and run the boat for an hour bit that's not the point. In the end I want to figure my max range based on both scenerios.
  2. CODOG, Jul 2, 2008
    #2
    CODOG

    CODOG Senior Member

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    Theres not enough information to estimate the consumption at 8 knots (hull form, rpm at 8 knots, engine power / fuel burn curve for example) ...even if there was, it would still be an estimate. To be honest, if you have the boat, the time and the ability to measure fuel flow / return, you are in best position to answer your question :)
    In the meantime, if you can source a specific fuel consumption graph for your engines (normally in the form of a power curve with a nominal prop law curve superimposed, with specific fuel consumption zones under the curve) and you log the rpm at 8 knots, you will be able to approximate the fuel burn at that rpm from the graph.
  3. Capt J, Jul 3, 2008
    #3
    Capt J

    Capt J Senior Member

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    Again, I agree there is not enough info. We need to know HP/model of the 671's and RPM @ 8 knot hull speed. I do know that 12v71TI's that I run, at 1,000 rpm's they're burning 5 gph each. It's a 75' Hatteras and at 1,000 rpms it does 9.5 knots, so almost 1mpg. I would say generally if you're at 1,000 rpm's with 671's TI's, you'll be around 2.5 gph each motor (1200rpm's and that consumption will more then double), at 800 rpm's 1.5-2gph each engine. Judging by a 33gph burn you probably have TI's.

    Look at power and motoryacht or any boat test with your model 671's with a fuel graph and that will be pretty close. A similar vessel even better.

    Also Detroit's do not like to idle for very long if they're TI's or TA's. The airboxes will start getting dirty and so forth. I would recommend running at cruise for 30 minutes every 4-6 hours.

    If you plan on doing so, going to one size smaller injector and having the rack adjusted as well as cutting pitch on the props will lower your fuel consumption considerably. I ran a 58' hatteras with 8v71TI's with the 115 mm injectors and it burned 50 gph (both engines) at 1950rpms, when we switched to the 90 mm injectors fuel burn dropped to 30 GPH (both engines) and the vessel lost 1.5 knots at cruise or 10% speed. Smaller injectors also helped the motors to run much cleaner at low rpm's for long periods of time.
  4. K1W1, Jul 12, 2008
    #4
    K1W1

    K1W1 Senior Member

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    Hi,

    Good info Cemberci.

    I just sorted the results into columns as I was having trouble making sense of the numbers as posted.

    It looks as though going by the first posters already known fuel use that his 6 71's might be knocking out quite a bit more power compared to the example you have posted.

    I will be interesting to see what output he has if he ever posts back and answers the earlier request for the power info.