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Max RPM 3600/Max Speed 18mph

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GovtHR

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We recently purchased a 2022 20ft S with a 2021 Mercury 90 4stroke (20 inch). We bought it in December from an individual who purchased it new in February 2022. Boat's in great shape and engine runs good, however, I can only get 3600rpm according to my gauge and 18mph on the Garmin. I appreciate any ideas as to what is causing the slow rpm and speed.
 
I previously had a 22 ft SLX with a 90 at about 5800 rpms was able to get 28 . What is your prop size ? Are your toons clean ?
According to the recommendation chart that was provided to my dealer ,20ft with a 90 HP should be 13 1/2 X 15 .
 
I don’t know prop size except that it’s a 3 blade that came on it. I’ll check.
Toons are very clean.
So here’s two things I considered- since it’s a 20 inch shaft not 25, would that matter?
Also, to our knowledge the motor was never in a rental program so could still have governor or rpm limiter on it?
 
The dealer or person you bought it from should know ,if it was a rental or has a governor . If you haven't done so already, transfer your warranty's at an authorized Bennington dealer.
 
GovtHR said:
I don’t know prop size except that it’s a 3 blade that came on it. I’ll check.
Toons are very clean.
So here’s two things I considered- since it’s a 20 inch shaft not 25, would that matter?
Also, to our knowledge the motor was never in a rental program so could still have governor or rpm limiter on it?
Click to expand...
I did have a 4 blade on my 90 ,what size? I don't remember what I had for breakfast.
 
What speed and RPM's are you getting with the new prop?
 
My 90 was on a 22ft SLX . I got 28 mph at about 5800 rpm's , but I did have a 25 inch shaft .
 
What hole is it mounted on?
 
I have a 21 SLX which is actually 21'10" with a 90 yamaha on it and I get to at least 5900 RPM and when I trim it I get 6000. Generally run about 26-27 MPH depending on the wind LOL. I do have the long shaft yamaha though.
 
I may still have an issue that needs to be resolved. I'm still only getting 25 mph and 5000, trimmed. I'll check mounting tomorrow.
 
Sounds over-pitched, particularly with that four blade prop. Could be prop style and cupping (assuming it’s mainly a prop issue).

SS or aluminum (assuming aluminum, but checking here)?

I would personally go with a three blade aluminum and a different pitch. But minimally (perhaps either way really) if sticking with a 4 blade, I think you want to go down about 2-4 pitch sizes and get those RPM’s up. In so doing, see what that translates into performance wise if looking to dial that in more. Leaving what, about 1k rpm on the table right now…

Also, what’s the prop diameter. Chance that is also the problem.
 
The four blade improved rpm and speed over the 3 blade that was previously on there. (I think it had a 19 and we dropped to a 13).
It's aluminum.
Yeah I think we should have about 1000rpm left to gain and 4-5mph.
 

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I checked the hole where it was mounted. It could be dropped one more, as it's in number 4 of 5 now.
 
Sounds like you are on the way to a solution, but I'll share my experience with nearly same problem maxing out at 3000 RPM - in my case a mouse had built a nest in the intake so the motor was air-starved!
 
GovtHR said:
The four blade improved rpm and speed over the 3 blade that was previously on there. (I think it had a 19 and we dropped to a 13).
It's aluminum.
Yeah I think we should have about 1000rpm left to gain and 4-5mph.
Click to expand...
We run the Quicksilver version of the same prop you have. They typically have lower slip than most other designs. Don't know what gear ratio your motor has, but we get more RPM (~5400 depending on load) and lower speed (~23 with moderate load). Our RL model is relatively heavy and only has 23" tubes, so speed will never be very impressive. It would only make 25-26 in a very light operating condition with clean tubes and the parachute down.

Your numbers seem to indicate you could benefit from a Spitfire/Nemesis 11p (last I checked, there is no 12p). That will put RPM in the top part of the range with better acceleration (and towing). But, likely no increase in speed. I expect with a heavier load or a turn, your motor would fall below 5000 RPM, so a lower pitch would help that too.

If the previous prop was a 19p, it must have had terrible slip...like more than 40%. We saw that too with basic 3-blade Yamaha props of 13 through 15p except for one 15p that held the RPM to less than 5000. Recommend you stick with pontoon rated props for combinations like ours. You may or may not need to drop the motor down. I've used all settings from full down to up about 4" and full down is the best for our pontoon...your mileage may very...

BTW: sometimes run the Yamaha Talon 12p for heavy loads and towing. It will keep RPM in the top of the range (above 5700, as I remember), but slippage is higher. Like the Nemesis 13p way better for every-day, light to moderate loads.
 
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Great! Thanks for all of your responses and helpful information. I have learned quite a bit.
 
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