Increasing RPM

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wright74012

Cadet
Joined
Jun 23, 2011
Messages
25
Just recently switched out the engines on my 25' Pontoon. I have a Johnson 115hp that I can't get over 3000 rpms? When it was on the other boat it was fine. I don't know if I am have fuel problems or what. I think the Max RPMs should be around 5000 or 5500 if I am not mistaking. Anyone got any suggestions?
 
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esco

Cadet
Joined
Jun 29, 2011
Messages
13
Re: Increasing RPM

Could be a couple of things, give us a little more detail and lets see what we can come up with...

What was the other boat you're refering to in regards to size and weight compared to this new 25'?


Are you able to get over 3k with motor out or the water?

Are you using the proper sized fuel lines?

What about the prop, can you give the details on it...

Pitch & Diameter
Your boat will respond to different pitch propellers the way your car responds to different gear positions in your car's transmission. Moving to a higher pitch propeller will result in a decrease in your full-throttle RPM, just like the effect when your car's transmission shifts to a higher gear. The reverse is also true: move to a lower pitch propeller and your full-throttle RPM will increase. Most propellers are sized by two-inch intervals, example 17", 19", 21", etc, and a 2" pitch change will result in an average 300 to 400 full-throttle RPM change.
 
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wright74012

Cadet
Joined
Jun 23, 2011
Messages
25
Re: Increasing RPM

The boat that I took it off of was a old maybe 16' Kingfisher bass boat and very light.

The 3K was with the motor in the water, I did not pay attention to the RPM when I ran it out of the water.

Fuel lines- I did not change them, they are the same as what was connected to my old motor.

Prop- No details, I will have to look. But I used the same prop that was on the bass boat that I took it off of.
 
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wright74012

Cadet
Joined
Jun 23, 2011
Messages
25
Re: Increasing RPM

Looks like the prop that is on it now is a 20. I don't know what the other propeller is that came off with the old motor that was on the pontoon, either it is not there or too much paint on it.
 
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Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
28,550
Re: Increasing RPM

So you replaced a light fast planing hull with a heavy semi-plaining hull, and used the same prop. That will cause low RPM every time. Sounds like you should try a prop with about 16" pitch, and see what you get. You may need to go lower in pitch, but as the RPMs rise, power will increase, so it is not a linear calculation.
 
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bob johnson

Rear Admiral
Joined
Feb 25, 2009
Messages
4,306
Re: Increasing RPM

I bet he has to go all the way to a 13 pitch prop

i run a 115 on my 20 foot MOD V that only has a 12 degree hull and floats real shallow...and I need a 15 pitch to run 5800 rpms...that pontoon has to be a dog compared to my hull

put a 13 pitch on her and see what she will run


bob
 
JB

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: Increasing RPM

Take this discussion to the Prop Forum. I will move it.
 
K

Kaplooi

Seaman
Joined
Jun 2, 2009
Messages
64
Re: Increasing RPM

Your engine thanks you :). I think running an engine at high load, flat out and getting much lower than the rated RPM is very hard on piston rings, bearings, etc. Your fuel economy will decrease running at your new WOT obviously, but that's the price you pay to have full access to your power-band.
 
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