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Thread: Outboard Gas Mileage

  1. #1
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    Question Outboard Gas Mileage


    Need some opinions on gas mileage. I have a 1979 Merc 20 HP pushing a 15 foot trihull (1980), which weights about 1,000 lbs. The best I can tell I am getting about 2 miles per gallon. This seems to be a lot of gas for this motor, at WOT I am going about 21 miles per hour. I rebuilt the carb, but maybe I have a leak, any thoughts?

  2. #2
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    Default

    The fuel comsumption you describe given the age of the motor, weight of the boat, and running at WOT does not sound unreasonable. I had a 16' Glastron with a 1977 85hp Evinrude and got about 4 mpg at cruising speed. A friend had a 65hp Mercury on a trihull just a little larger than the one you describe and I could go twice as far as he could on the same amount of fuel.
    Keith
    2008 NWR Bash Crappie Champion
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  3. #3
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    Default small motor

    the small motor has to work very hard to push a heavy boat,there fore using more gas....
    same goes with a 6 vs 8 cyl gas engines in full sise trucks.....i had an 88 ford with the 6 and got about8-10 mpg , traded it for an 87 with a 351 and got 12-14.....

    just my theory

  4. #4
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    I don't think that unreasonable for the weight of the boat. I had a 1971 20hp Merc on a 14" modified V. I don't know how much it weighted, but not that much. Total maybe 800 pounds. I could go a round trip of about 20 miles to the swamp and it would use about 4.5 gal... I think that it may have done about 20 mph at the best.

    Now I've got a 16' with a Yamaha 40 4 stroke that weighs about 1500 pounds total. Same trip is 2.25 to 2.5 gal.. And it'll do 33mph wit 2 in the boat.

    I spun a prop the other day and had to go back to the prop that came on the boat, a 12 pitck. I lost 6 mph and gained 600 rpm and used 3.5 gal of fuel... A big loss all the way around... I kept playing with the pitch until I found the best in the RPM range and MPH that made things fall in place... I now run a 14 pitch and the motor doesn't sound like it coming apart from high RPM's.

    What pitch are you using. I would have to chech my son nopw to see what's on it. Pitch, size and the angle of the motor to the boat has a lot to do with everything.

    Gerald K4NHN
    Cayce, SC


    Quote Originally Posted by PanMan_VA
    The fuel comsumption you describe given the age of the motor, weight of the boat, and running at WOT does not sound unreasonable. I had a 16' Glastron with a 1977 85hp Evinrude and got about 4 mpg at cruising speed. A friend had a 65hp Mercury on a trihull just a little larger than the one you describe and I could go twice as far as he could on the same amount of fuel.
    Gerald K4NHN
    Cayce, SC

  5. #5
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    I replace my connector, hose and bulb last month and it seems my gas milage increased plus it sure does run better.
    Proud Member of Team Geezer
    Southern Sickle Jigs Pro Staff

  6. #6
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    Default Prop Pitch

    I don't know what the pitch is, judging from the dog eared look, it may be original. Accerlation seems to be about right, it takes probably 5 to 6 seconds to plane out and the top end RPMs do not seem excessive, although I have not put a tach on it. That may be the answer that I need, to attach a tach and run at WOT. If RPMs are low or high the pitch may need to be adjusted.

    Am I thinking about this the right way?

    Thanks for everybody's input.

  7. #7
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    I'm gonna have to agree with fish247,I think your workin that 20 pretty hard. Also if your prop is dog eared (never heard it called that before) your losing alot of performance there, and if it's out balance it's hard on your gear case. Have to remember (dog eared) I like that one...

  8. #8
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    Default Mph

    Ever think about looking at this as fish per gallon. Might even be worse.
    Joe

  9. #9
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    Trihul
    I agree with izzysfishin, if that prop is that bad, preformance is going to be real bad .. Is that a 2 or 3 blade prop.. I don't know if a prop from a 71 model will fit what you have or no. I think that the size and pitch is stamped into it some place.. I don't have one handy to look at, but I will Sunday. I may have a 2 blade that you could try and see if that helps. But if yours is like you said "dog eared" you need to get another one or have that one rebuilt.. You always need a spare anyway.

    Gerald K4NHN
    Cayce, SC
    Gerald K4NHN
    Cayce, SC

  10. #10
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    Default

    Doesn't sound bad to me, probably above average. My former 200 Merc got about 1mpg till she blew. My new Suzuki 4-stroke gets 6-8 mpg.
    Quit Wish'in and Let's Go Fish'in
    Darryl Morris

    FAMILY FISHING TRIPS GUIDE SERVICE
    501-844-5418 --- [email protected]

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