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  1. #1

    Unhappy MPG - Anyone have experience on 871's

    I've recently bought a 49 yachtfish with twin deisel 871 325hp naturals. I'm bringing the boat up the east coast and am trying to extimate gas consumptioin. I've had 3 different numbers from 3 different people. PS engines in very good shape and boat is a 74.

  2. #2

    Re: MPG - Anyone have experience on 871's

    it all depends on what speed you want to cruise at !

    hull speed (around 9kts) should result in about 9 to 10 GPH. or just under 1 MPG. slow down a little more, 8kts... and you should get a good 1 mpg. anything above hull speed and you're going to use a lot of fuel pushing a big wave.

    i dont' know if the 49YF will get on plane with naturals. if it does, that will probably drop your mileage to .5 mpg...
    Pascal
    Miami, FL
    1970 53 MY #325 Cummins 6CTAs
    2014 26' gaff rigged sloop
    2007 Sandbarhopper 13
    12' Westphal Cat boat

  3. #3

    Re: MPG - Anyone have experience on 871's

    At 1400rpm with 8v71n's our 53'MY burns about 10-gal/hr combined. The numbers have been checked over a 700-mile run down the Pacific coast from Anacortes, WA to San Francisco. We averaged about 9 - 10 knts depending on sea conditions. Most of the trip the wind wave hts were about 3 -4', on top of 3 - 5' swells.

  4. #4

    Re: MPG - Anyone have experience on 871's

    Stay off plane and you'll do a little better than 1 MPG. If it planes, it'll be less than 0.5 MPG. If you run at 7-8 knots, you can get almost 2 MPG, but it's not good to run the engines that slow all the time without running them up to operating temps for 30 minutes per day.

    Doug Shuman

  5. #5

    Re: MPG - Anyone have experience on 871's

    your gas milleage should be infinite.....

  6. #6

    Re: MPG - Anyone have experience on 871's

    Quote Originally Posted by krush View Post
    your gas milleage should be infinite.....
    Smart Azz! Actually if he tried to burn gasoline in those 8v71's, I doubt there would be any miles to divide that first gallon into. Zero divided by one is zero.
    Sky Cheney
    1985 53EDMY, Hull #CN759, "Rebecca"
    ELYC on White Lake--Montague, MI

  7. #7

    Re: MPG - Anyone have experience on 871's

    I've attached a set of power/fuel curves for 8V71Ns. Hope this helps.
    Attached Files

  8. #8

    Re: MPG - Anyone have experience on 871's

    I've seen those curves for various DD engines and I am embarrased to admit that although I spent years building performance gas engines and personally installed/tested them on dynos and printed out the data, I don't have a clue what that lower DD curve is telling me. I'm assuming that the BHP and SHP are simply what they sound like - the difference between the power available at the flywheel and the power available at the propshaft. But what does the lower curve tell me and how is it useful?

  9. Re: MPG - Anyone have experience on 871's

    Use the lowest curve of the four to determine fuel consumption versus RPM via the right hand fuel consumption data. It's the "fuel consumption- propeller load" we all care about.

    The upper pair of curves are HP versus RPM, using the left hand HP data.

    My DD chart says: (regarding the lowest of the four curves) "PROPELLER LOAD- Indicates horsepower absorbed by a typical propeller and the corresponding fuel consumption throughout the speed range."
    Rob Brueckner
    former 1972 48ft YF, 'Lazy Days'
    Boating isn't a matter of life and death: it's more important than that.

  10. MPG - Anyone have experience on 871's

    There was a thread last winter with MANY posters about this subject. Most owners posted that in the 45 to 58 foot range they got about a mile per gallon at around eight knots...turbos,naturals,all seemed pretty close. In another thread a theoretical claim was made that turbos should be about 10% better but even if true this could be masked by many other factors. No one claimed better mileage at higher RPM's and speed. About a half mile per gallon in the 1800 RPM range is typical; run at 2000 RPM and it's lower still.
    Rob Brueckner
    former 1972 48ft YF, 'Lazy Days'
    Boating isn't a matter of life and death: it's more important than that.

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