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Thread: fuel flow questions

  1. #1
    Member propgrinder's Avatar
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    Smile fuel flow questions

    I've asked a similar question in the thread about AK Mining/Diving but I'm just way too curious for my own good so here goes:

    I hear guys talk about 3-5gph at 25-30mph with inboard jets on 1800 pound boats. What I don't understand is how that kind of fuel flow can be possible with such an inefficient mode of thrust. I've always heard that it's about 1gph per 10hp WOT and that a propeller is much more efficient than a jet.

    I burn about 3gph at WOT (about 25mph empty) with my little 35hp Go-Devil which uses an industrial Briggs and Stratton 2-cylinder engine. And on an 18'x48" Jon boat that weighs about 600 pounds. I thought that was pretty efficient.

    So, is this a situation of "that's the gph the boat dealer told me I'd get" or is it actual numbers measured by honest people with accurate fuel flow meters?

    Not trying to be a wise@ss. Just searching for the truth.

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    I get about 5gph at 4500 rpm in the speed range you mention 2175 XS. This from a flow meter.

  3. #3
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    Akrascal, I run the same boat/motor. On my meter (Smartcraft) 4500 RPM is more like 8 GPH. Maybe our meters are calibrated differently?

  4. #4
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    propgrinder. I found this fuel usage chart for the motor that is in the Jetcraft J-Max boats. I don't know how accurate the chart is.

    My dad has a 1875 XS, and with a light load he cruises at 8000 RPM, I don't know his speeds.

    Yamaha 160hp 4 cylinder 4 stroke 1052 cc

    RPM GPH
    3000 1.0
    4000 1.2
    5000 1.8
    6000 2.6
    6500 3.3
    7000 3.8
    7500 4.3
    8000 4.8
    8500 5.7
    9000 6.9
    9500 7.9
    10250 10.5

    I have a 2175 XS. Running empty, me and my dog and some fishing gear, I can cruise 5-5.5 gph. Speeds 20-26mph, depending on water depth and current.

    With a few people and gear for the cabin for the weekend, I cruise anywhere from 6-8, again, depending on water depth, current, wind, ect. I average 6-7gph.

  5. #5
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    Hmm strange... haven't been out this year, maybe I'm remembering it incorrectly.. will check next week.

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    "I've asked a similar question in the thread about AK Mining/Diving but I'm just way too curious for my own good so here goes:I burn about 3gph at WOT (about 25mph empty) with my little 35hp Go-Devil which uses an industrial Briggs and Stratton 2-cylinder engine. And on an 18'x48" Jon boat that weighs about 600 pounds. I thought that was pretty efficient".


    Your numbers are right in line with the industry standards of approximately 1 gallon per hour for each 10 HP when running at wide open throttle.
    But remember, weight hurts. Most people quote fuel burn rates remembering the best possible numbers they ever received, ie, very light and very slow. Some boat shapes allow the boat to get on step at slower speeds helping the fuel numbers.

    Power and gas consumption with any propellar outboard with be within the standards of 1 gallon used for each 10 hp of motor. However, it seems the newer motors receive better fuel numbers than older 2 strokes. It takes a certain amount of gas to produce a certain amount of hp. If someone has numbers to good to believe, then chances are they are pulling your leg.
    Tennessee

  7. #7
    Member AKBassking's Avatar
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    Default GPH

    I have a 250 Evinrude with a jet and i get 8GPH at 4200RPMs

    ALASKAN SEA-DUCTION
    1988 M/Y Camargue YachtFisher
    MMSI# 338131469
    Blog: http://alaskanseaduction.blogspot.com/

  8. #8

    Default Mercury carburated 2-stroke 150 HP V6 w/4blade SS impeller

    On 20ft Wooldridge Sport with Flo-scan fuel gauge

    4000 rpm gives 24 mph @ 8 gph with equivalent of about 7 adults on board

    5500 rpm gives 35 mph @ 15 or 16 gph with same load..........
    don't do that for long these days.......

  9. #9
    Member propgrinder's Avatar
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    The last two posts are kind of what I thought were normal gph figures.

    thanks all.

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    My 23hp Mudbuddy burns 2 GPH at 3900 RPM, about 15mph up the Tanana and 24ish down, on a 1448 hull, with about 400lbs in the boat.

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