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pontoon prop

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  • pontoon prop

    I am repowering my 20 foot 1999 Monark pontoon boat with a 1989 Force 85 HP, It currently has a 1999 Force 40 hp on it both have power tilt and trim.
    The new motor comes with a Michigan prop P539 13 1/4 x 17 stainless prop 3 blade. I think that this prop will probably be too much prop for this pontoon. I am at 1445 foot elevation in central Texas. The pontoon probably weigh about 2500 pounds.
    I cannot find pontoon boat recommendations on any of the prop web sites. What size prop would you recommend for this engine the boat is rated for a 90 hp engine.

  • #2
    Trial and error may be your best bet. See how your pontoon performs with that prop. You'll need a tachometer. If the boat isn't equipped with one, photo tachs are fairly cheap, and useful to have in your shop.

    The rule of thumb is 200 rpm change for each 1" change in prop pitch at WOT. Adjust your prop pitch until you hit your target. I think O/Bs are in the 5000-6000 max rpm range.
    '96 Larson 174 SEI BR. 4.3LX 4 bbl Weber s/n 0F786604. Alpha One Gen II s/n 0F709315


    • #3
      I have a tach on the boat now and am not sure if it will work on the new motor. I found out that the earlier Forces use a 20 pole stator so I may have to buy a new tach. I may get a photo tach just to have around.


      • #4
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        I think the 17" being too big is a safe bet, but it's not so far off you can't use it's rpm data while running that prop to help get the right one. The question will be how much too much? The only way to figure that out will be with reliable tach readings. 13" or 15" could both be what you end up with.