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How to improve performance on a 2005 Yamaha 15 hp 4 Stroke

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  • How to improve performance on a 2005 Yamaha 15 hp 4 Stroke

    I replaced a 15 Hp Mercury (year 2001 long shaft) with a 2005 Yamaha (year 2005 long shaft), and it seems the Merc. was a lot faster (on a Lund 14 ft aluminum).
    The Yamaha has a prop 9 1/4"x11, and unfortunately I failed to look at the prop size
    of the Mercury and already sold it.
    I've heard both arguments that to improve speed/performance I should go to a higher
    pitch prop, and also that dropping to a lower pitch prop would raise the RPMs, thus
    increasing the speed. Of course this scares me, because I could blow up the engine.
    Does anyone know what the 'normal' prop size is that comes with a 15 Hp 4 Stroke
    Outboard ? Any comments on whether I should move to a higher or lower pitch prop ?
    How about 3 vs 4 blade ?
    Thank you in advance
    Rds
    Cabovic

  • #2
    Re: How to improve performance on a 2005 Yamaha 15 hp 4 Stroke

    Small 4 strokes sometimes struggle to pertform like a 2 stroke.
    Actually you shouldn't change props without the help of a tach.Also a gps for speed is a big help.
    Just from checking the iboats prop chart it appears to me a 9 or 10" is probably standard.
    An 11 may be too much.
    Describe your overall performance in detail. timed hole shot with one aboard and top speed.
    Use the trim setting and weight distribution to try to get the best hole shot and top speed.

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    • #3
      Re: How to improve performance on a 2005 Yamaha 15 hp 4 Stroke

      Thank you, good advise. With the 15 hp Merc. 4 Stroke I could get up to 20 or 21 mph, and with the 15 hp Yamaha I only get about 12/13 mph. Both are long shaft, both 4 stroke.
      When you say 11 pitch may be too much.... do you mean this would make the boat go slower ? but faster out of the hole ?
      thank you for your candid comments..... I'll try to set up a tach next time out.
      Tx again
      Rds
      Cabovic

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      • #4
        Re: How to improve performance on a 2005 Yamaha 15 hp 4 Stroke

        In order to help we need a lot more onfo.As impiied in my previous post.
        Without knowing what boat and total weight your pushing there is no eway to tell where you are at.
        Assuming standard prop was 9 or 10" an 11" would be slower out of the hole.
        If your only doing 12 or 13 now it may not even be planing, could be a motor issue?
        May be faster if the motor can turn it fast enough.If you read the stickies at the top of the page and the helps on the prop store page it will help you understand better.
        Keep in mind forcing the motor to run below its rated wot(wide open throttle)rpm
        with a prop can cause premature repair problems.And as you noted by you running higher than rated max rpm can cause a motor come apart.

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        • #5
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          Re: How to improve performance on a 2005 Yamaha 15 hp 4 Stroke

          We really need more info to help without boat info typical load as tested there is no way to tell what you need.

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