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  1. #21
    Registered Luv-N-It's Avatar
    My Boats:
    1996 Baja 272
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    Jun 2002
    Central Ohio
    ursus, thanks for the info and insight. very well said.

  2. #22
    Registered velocity260's Avatar
    My Boats:
    2000 velocity260
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    Feb 2001
    I have a flowscan on my velocity that only goes to 20gph. With the 7.4mpi at cruise 35-40 14-16gph. Doing 50mph I burn 20gph. I put 25gallons in the tank and went for a cruise ran 50mph the whole way there and back. 30min there 30min back. I burned 21gallons.

  3. #23
    Member #2 ....Jeeezz Charter Member cigarette1's Avatar
    My Boats:
    Toy Boat - Toy Boat - Toy Boat
    Join Date
    Oct 2000
    Kicked to the curb
    We don't put BIG carbs and BIG cams and BIG pistons and BIG blowers on motors to worry about fuel efficiency.

    I usually try to figure how many miles we're going to run and then I assume if I'm going to be running mostly 3500r's, I'll calc for 1mpg, If I plan to run hard with the boys, I'll figure on .5mpg

    Hell, the gas is the cheapest part of this stupid game.

  4. #24
    Platinum Member Platinum Member mcollinstn's Avatar
    My Boats:
    1991 F311SR1
    Join Date
    Sep 2001

    correct analogy for all "typical" planing hulls. Displacement or semi/displacement hulls, though, will use more fuel @ 3000rpm then a planing hull @ 3000 rpm.

    a displacement hull's fuel usage curve will look like a curve that starts rising at low rpm and gets steeper and steeper as the rpms rise (there is a relationship between the waterline length and max efficient speed on a displacement hull that will affect this curve).

    a planing hull's curve will start out with a steeper rise, then will peak and drop significantly, only to then mimick the curve of the displacement hull. Most, if not all, planing hulls will be on full plane by 3000 rpm and will be in the "nice" part of the now-rising curve. The displacement hull will be in the steeper part of the curve.

    Since both, all if you will, motors will have the same fuel usage at WOT then there will be spots on the two curves where the difference is significant.

    Since I assume that all of us here are speaking about planing performance hulls, then the numbers for a given motor (502 Mag EFI, etc) should indeed be similar.
    I see London, I see France...

  5. #25
    AT Cult Member #3 VIP Member R Addiction's Avatar
    My Boats:
    '99 Active Thunder Tantrum
    Join Date
    Oct 2000
    All I know is that when my light comes on, and stays on, that means get fuel @#$hole. The other week the light came on and I came back to the marina and filled up the tank........turns out I had 2 gallons left . I should have gone to the next bar!!! I can make a 110-120 run at 55mph/3600rpm on just under 65 gallons. I think that comes to around 1.8 miles to the gallon. I'm happy with that.

  6. #26
    Charter Member #9/ Moderator Charter Member LakeRacer's Avatar
    My Boats:
    99 Advantage 34 Offshore
    Join Date
    Oct 2000
    Phoenix, AZ, USA
    Twin 502 EFI's in my 34 Advantage. Hooked up the MerCruiser computer to it and at 3000 rpms I was getting/using 12 gallons per/hour per engine for a total of 24 gallons/hr.

  7. #27
    Registered ragtop409's Avatar
    My Boats:
    1995 Nova Spyder
    Join Date
    Jul 2002
    Maximo FLA

    Talking GGAASS

    Not sure about the boat but the truck has a 502 carb in it and it gets 6 miles to a gallon of fuel guess thats why it has a 45 gallon tank
    Attached Images Attached Images

  8. #28
    Displacement or semi/displacement hulls, though, will use more fuel @ 3000rpm then a planing hull @ 3000 rpm.
    This is just incorrect. @ 3000 rpm an engine will use the same ammount of fuel whether it is in a planing hull, or a tug if the load is the same and so it depends on the propping. If the planing hull is over propped say wot = 4000 rpm and the tugboat is under propped, it hits the rev limit of 5200 @ 3/4 throttle then the planing hull will use more fuel, Gallons per hour because the engine is under a heavier load, period. If the engine is propped the same in other words the prop is chosen correctly so that the engine is properly loaded then @ 3000 rpm they will both use the same ammount of fuel give or take a gallon per hour.

    If you look at the fuel consumption GPH versus rpm graph it is a relatively straight line, Mileage curves as you point out our quite different for dif hull types/sizes/weights

    You are describing the MPG curves. The MPG will be signifigantly different at dif spots on the curve but GPH is a function of rpm and load not the type of hull the engine is in. In other words @ 3000 rpm an engine uses a certain ammount of fuel per hour, mileage depends on how far you travel in that hour and so hull type weight etc
    Last edited by ursus; 08-09-2002 at 09:09 AM.

  9. #29
    Relatively straightl ine fuel consumption curve for 502efi in a planning hull
    Attached Images Attached Images
    Last edited by ursus; 08-09-2002 at 09:22 AM.

  10. #30
    typical MPG versus rpm for a planing hull, the curves you were describing
    Attached Images Attached Images

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