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Small Motor (9.9), Heavier Boat (Than Before)

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I held onto an older Johnson 9.9 horse motor when I "upgraded" to a larger boat because occassionally I fish lakes restricted to that size motor.  From my understanding, most stock props (I'm sure this is what mine is) are optimized for certain speeds and/or certain sized craft.  Would it likely be better to locate and install a different prop (different pitch and diameter) for this heavier boat?  Or should I just live with what I have?

 

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It would depend on if your motor is reaching max rpms or not. But on a motor that small it doesn't make much difference and it's easiest to just live with it. I have a 9.5 that goes on a 12ft aluminum Jon and also a 14ft fiberglass Jon with the same prop

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9.9 on a 12 ft aluminum jonny runs like a frigging rocket.

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I wouldn't change props, I doubt you would see much change.

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On something that small you could change props, but since you only run it that way occasionally, I wouldn't bother. 

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IF you don't get max rpm with the stock prop, it should be changed so it does unless you just use it at idle speed.

 

I put a standard instead of the Big Foot version 9.9 four stroke Merc on my Tracker PT 170 TX and the rpms would only get to about 4500 with the stock 9" prop. I changed the prop to a 6 1/2" and the motor is "happier".

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My "new" boat is a 17 footer aluminum.  It will never in a million years get on plane with a 9.5 but it seems the little motor revs more than it does "push" (if that makes any sense).  I don't know of any formulas but it SEEMS like a little larger prop might make better use of the available horsepower.  Or is my logic flawed?

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If you can't get a boat on plane it will only go at displacement speed (which is a consequence of the boat's length/width - rowing sculls are long and thin to give them a higher displacement speed as are oil tankers), then start pushing a wall of water as it tries to come up on plane. Pushing the water makes it incredibly inefficient, so is barely worth it for the minor speed gain and the massive wake you make. If the boat is not going to plane with the motor, it's probably not worth swapping the prop, unless you want a lower pitch to troll slower.

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You will find a sweet spot for the throttle vs forward speed, about 3/4 would be my guess. The reason is your boat doesn't plane, it's a displacement hull with the 9.9 hp engine and can't go any faster.

Tom

PS, ^^^ Didn't read the above post by Kelly, spot on!

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