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1984 Yamaha 40 hp prop on 20' pontoon

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  • 1984 Yamaha 40 hp prop on 20' pontoon

    Hi Guys,

    Took the above pontoon out yesterday, and now that everything is cleaned out and running well I tested the top speed of the pontoon.

    GPS verified 15mph on a 10 3/8 x 13 aluminum prop. Everyone I talk to seems to think the prop is too 'steep' for the pontoon and that I should be running in the neighborhood of a 10 pitch. I'm not sure of the weight, exactly -- We just 'redid' the entire thing -- I would guess the pontoon would weigh in the neighborhood of 1500 lbs or so, maybe a little more. By the time you put a handful of people on it, it's gotta be close to 2200 lbs.

    As I read the 'sticky' at the top of the page, I'm still lacking on information -- it's an outboard with no tachometer. Not feeling real comfortable about pulling the hood off and reading the rpms with the handheld meter -- there's so much water splashing around back there that I'm sure I would fill the carbs... Worked really hard (twice) to remove the junk from them to begin with.

    I'm just wondering, does this seem like it should be faster than 15mph? I understand that I'm probably a little under-powered, but most of the reading I've done here lately seems to think in the neighborhood of 20 mph or so...

    Wish I had more info for you,

    Jason

  • #2
    Re: 1984 Yamaha 40 hp prop on 20' pontoon

    Your right.Without the full info I would say a 9" prop perhaps a Solas Amita 4 blade.
    3313-118-09 Usually a 4 blade will resist venting and have better slip numbers.
    If you have no venting issues now after you get the right prop you could try raising the motor to a point just before venting becomes a problem.Final result should be around 20 mph.
    Carefully adjust trim for the least venting.
    As with any boat try to balance the weight fore and aft.

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    • #3
      Re: 1984 Yamaha 40 hp prop on 20' pontoon

      I agree. I was running a 13 on our old pontoon but a lesser pitch prop seemed to get us through the water quicker! Hmmmm.

      V

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      • #4
        Re: 1984 Yamaha 40 hp prop on 20' pontoon

        Interesting Volphin, makes me think I'm getting some pretty good 'slippage' in the water (maybe cavitation or venting is the real term... The motor sits pretty deep in the water, don't know if that helps or not.) If I had a prop with a lesser pitch, maybe it would cut down on this, due to the prop being a little bit bigger as well.

        Thanks for the help, Steel...

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        • #5
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          Re: 1984 Yamaha 40 hp prop on 20' pontoon

          Pontoons do produce high slip numbers even when propped right.
          No doubt the 13 has high slip and besides your rpm is probably way low
          producing low speeds.

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