Right PROP

Ed Cook

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I just Purchased a 2019 S20 and have, I think, a 15 pitch prop on the 90 HP Mercury.
After the break in time WOT is just 4400 RPMS. And 21 MPH. I think the Mercury should turn 5500 - 5800 WOT. Should I change props? If so what pitch do you think would get me to top RPMS.
 
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Try a 13 pitch. Welcome to the family.
 
If I remember correctly, each inch of pitch is about 200 rpm. Lower it and you gain, increase it and you lose.
 
If I remember correctly, each inch of pitch is about 200 rpm. Lower it and you gain, increase it and you lose.

Is this at WOT? If so, working backwards I have WOT 5500 rpm / 200 = 27.5p???? Or is there another throttle position where this rule of thumb applies.
 
Is that with any trim or motor tucked all the way down? 4400 seems very low even for a 15 pitch. 90 merc 4-stroke rpm range is 5000-6000, you want at least 5700. If you have the merc 90 optimax, rpm range is 5000-5750
 
I just Purchased a 2019 S20 and have, I think, a 15 pitch prop on the 90 HP Mercury.
After the break in time WOT is just 4400 RPMS. And 21 MPH. I think the Mercury should turn 5500 - 5800 WOT. Should I change props? If so what pitch do you think would get me to top RPMS.
For comparison - I have a 20SLMX 2017 with a 13 pitch / 4 blade / 90 mercury CT /25" toons w lifting strakes - WOT is around 5200 and top out at 30 MPH alone in good conditions - cruise 14.5 MPH at 3500 RPM with 3 people
 
Is this at WOT? If so, working backwards I have WOT 5500 rpm / 200 = 27.5p???? Or is there another throttle position where this rule of thumb applies.
Jim, I'm not the prop expert but this rule of thumb generally applies to WOT. The OP states a 15P with WOT 4400 RPM. As such, by moving to a 13P he could stand to gain about 400 RPM or about 4800 WOT, which still is quite low for his engine. As such, I suspect there are some other issues at work besides being over-propped.

And as you've deduced, dividing it out doesn't work but I'll leave that up to the smart folk on this site to explain that. :)
 
Michiman, I see how the rule is applied, makes sense. I also am no prop expert, maybe his motor is mounted to low??
 
I can't even tell you what is on mine right now in fact. Ignorance is bliss for me!!! Seems OK so I don't fiddle.
 
Thanks guys going to try a 13 4blade and see it that helps. Hate it but it’s getting to the end of boating season so it’s about time to put her in the garage
Thanks
 
Thanks guys going to try a 13 4blade and see it that helps. Hate it but it’s getting to the end of boating season so it’s about time to put her in the garage
Thanks
Ed, post some pictures for us!
 
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