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Thread: mileage

  1. #1
    Charterboat Operator kodiakcombo's Avatar
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    Default mileage

    just purchased a aluminum boat off of craigslist. previous owner had no idea about the setups mileage. it has a 1989 merc 100 2 stroke 4 cylinder with a 4 blade stainless impeller jet, which pushes a 18 foot self bailing semi flat bottom. he said it weighs about 900# any guess what this thing will burn an hour in the salt?(need to run 68 nuatical miles)
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  2. #2
    Moderator bkmail's Avatar
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    Default My opinion...

    My guess would be 2-3 mpg roughly...
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  3. #3
    Charterboat Operator kodiakcombo's Avatar
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    Default made it!

    I ended up flying home and then firing up the seasport and driving into Kodiak(passagshak, half way point) and towing the skiff home, the milage for the seasport was 5.6-6.3 gallons an hour 18-20 knots. I went for little cruise with the new jet skiff and learned that I would have never made it home! any little sliver of eel grass causes it to cavitate and a little ripple added to that I would have still been traveling or out of gas! what is considered too much cavitation?
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  4. #4

    Default Fuel

    I would guess in a stock cofiguration with a prop it used 10 gallons per hour. Wit a jet you should increase that by 30% so maybe 12.5-13 Gallons per hour. It also lost 30% of it's original speed when changed over to a jet.
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  5. #5
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    Default

    well too much cavitation means that the jet foot is too far above the bottom of the boat or there is a hull imperfection that is making the water deflect from jet itself.. You might look into a hydraulic transom jack that you can adjust while running, putting the foot lower in the water when you need to for cornering and bringin up for long straight stretches.. this will also increase your speed for straight sections and manuvering for corners. In my eyes, unless you are in a 6" + chop the motor shouldn't cavitate..

  6. #6
    Charterboat Operator kodiakcombo's Avatar
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    Default lift plates or jacks

    I will look for a jack plate, thanks for the "bar" 6 inches? I was in a bit smaller ripple but I think I also had eel grass, the boat really scoots, but as it runs for a while it cavitates, so it is the eels grass? I think, cant wait to try her out in the river!!
    Providing trips for multilpe species for over 20 yrs
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  7. #7

    Default

    Jet or prop the motor should burn the same amount of fuel, gallons per hour, the difference will show in miles per hour. In other words; range decreases running a jet vs a prop given the same fuel capacity.

    Also as a rule of thumb, a 2 stroke burns approx. 1 GPH per 10 HP so the 100hp 2 stroke would be expected to burn approx 10GPH.

    My 55HP 2 stroke burns approx. 6GPH whether I run the prop or the jet unit. The prop being more efficient through the water, grants greater range, speed and carrying capacity (weight)at the same fuel consumption per hour.

  8. #8
    Member WalknBob's Avatar
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    Default

    Quote Originally Posted by 338WM View Post
    Jet or prop the motor should burn the same amount of fuel, [B]Also as a rule of thumb, a 2 stroke burns approx. 1 GPH per 10 HP so the 100hp 2 stroke would be expected to burn approx 10GPH.

    At what RPM? No engine burns the same amount of fuel across the rpm range. If that were so then it wouldn't make any difference what speed you ran you'd be getting the same GPH.

  9. #9

    Default

    Quote:
    Originally Posted by 338WM
    Jet or prop the motor should burn the same amount of fuel, [B]Also as a rule of thumb, a 2 stroke burns approx. 1 GPH per 10 HP so the 100hp 2 stroke would be expected to burn approx 10GPH.


    At what RPM? No engine burns the same amount of fuel across the rpm range. If that were so then it wouldn't make any difference what speed you ran you'd be getting the same GPH.
    Ok, I will bite.

    This time I will use the word "generally" since approx. and rule of thumb didnt clarify my statement well enough.

    Generally when speaking to fuel consumption by a 2stroke outboard it is considered near or at WOT (typically 5-6k RPM) since, generally, the boat is on plane and the intent of the operator is to cover distance as quickly as the boat and water conditions allow. There are of course exceptions such as, weight in the boat and say current, traveling up or downstream of a river.

    Take weight as example: when I go hunting my boat is heavily loaded and I travel upstream against a 6-7MPH current. Both factors work against the outboard regardless of RPM. As I travel the boat gets lighter and gets higher on step due to weight loss through fuel consumption, this in turns creates greater efficiency in terms of speed and and MPG but does not affect GPH as the RPM are consistent.

    One also needs to keep in mind that a boat generally is powered to match the hull and load capacity so that the motor is run at or near WOT during typical use or traveling, exceptions would be trolling, sight seeing, a leisurely cruise, or say, river (road) hunting to name few.

    So when ever I plan a trip I know from use over time that my outboard will consume 6GPH and I carry fuel accordingly. After some time in the water the original poster "kodiakcombo" will now what his new set-up will require.

    Sure one can operate at a lower RPM, but why grind around at 2500RPM acting like a barge unless you are towing a wakeboarder behind the boat ? A boat is always most efficient when on step, the motor may burn more GPH but my time is worth far more than a few bucks in gas. Maximum distance over a given time is generally the goal when operating a boat or other vehicle.

    My intent was to offer a baseline expectation for him in answer to his question, hopefully I have done so.

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