Optimax 115 prop question

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OldDude

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Forgive my ignorance. We have a 2008 189 sport we bought in December 2020. It had two props when we bought it.

The motor is an Optimax 115. Top speed with my wife and I is 42 most at 5700rpm. The prop currently on the boat is a Mercury Marine Vengeance 48-16988. 18p. This shows to be a 13inch prop with an 18 pitch. The extra prop is a Precision Propeller turbo M13 1/4 x22.

What is the difference? What performance change would I see if insewitched props?

Is there anything special I need to do to make the switch?

Thanks.
 

Larry Williams

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The "pitch" is a number that respresents how many inches your boat would move forward with one complete turn of the the prop assuming NO water slippage. 18" means 18 inches of forward movement with one full revolution of the prop. This assumes that the prop would not "slip" in the water- this never occurs in reality. the 22" pitch indicates the same- in theory, the 22 would be faster on you boat. If you are hitting 5700 RPM with the 18 pitch, depending on your boat, the 22 might cause your engine to not reach the maximum rpm. Running below the max RPM, at full throttle, due to the prop pitch being to high. is not recommended due to the strain it places on the engine drive components. I owned a 115 Opti and an 18' aluminum bass boat that performed very will with a 22' pitch prop. It would reach about 51 MPH. The weight and hull design of a boat make the difference. Heavy boats generally require a lower pitch prop number and consequently go a little slower. It's good to have a spare prop- even it the boat didn't perform well with it, it could get you home in an emergency where you damage your prop.
 

cceguy

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Larry,
I'd like to thank you (and many others) too. I have learned a lot from this forum. I have not found the answer to this question, which goes back to prop selection. Is there a difference between the high limit of WOT and maximum rpm? If so, I haven't been able to find max rpm in any specifications.
 

OldDude

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What difference does the diameter make. The one on my boat shows to be 13in and the extra shows to be 13.25 in
 

Larry Williams

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I can say that you should not exceed your WOT Throttle specification for RPM. Your engine manual will state the RPM that you should not exceed. The wrong prop (pitch too low) might allow yoy to throttle past the max RPM you can control that with your manual throttle. With a pitch too high your engine won't reach maximum throttle and that's not good either. generally diameter variance is ok as long as sit fits the space allowed for prop rotation. Diffferent Pitch props can vary some in diameter depending on the particular engine.


HP/ModelYearWide Open Throttle RPM Range*
Mercury Optimax* WOT RPM Range Chart
1152007~5000-5700
115 PRO XS2015~5000-5750
 

OldDude

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I get the RPM limit alarm at 5800. The 3 x18 prop hits the RPM limit pretty quickly. The motor still wants to go....

So the diameter doesn't really make any difference? As long as it clears?
 

Larry Williams

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I would run it and see what it will do. Can't hurt anything to try it out. Seems that like you may be able to turn the 22 ok. The 22 may slow your start but it may be fine to run. It should get you close to the max RPM but not exceed. If it struggles to get the RPM it might be too much pitch for the weight you pushing. I'm not familiary with your boat.
 

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