Prop Pitch Vs RPM

[mwhitten]

I have a 24' Smokercraft pontoon. I'm trying to correctly prop my boat. It weighs 2100 dry. Has a 115 Mercury / 4 stroke. Rpm range is 5500 - 6000. I had a 15" pitch prop with WOT at 4800 RPM - 28 MPH. I was told I was loading down the engine. I went down to a 11" pitch prop getting 5800 RPM at WOT - 21 MPH. Am I working in the right direction?

Michael

 

[Dhadley]

Re: Prop Pitch Vs RPM

That's not the direction I would have taken but with a pontoon that may be the only option. When looking for more rpm, we drop pitch only as a very last resort. Not the first change.

Are you using props designed for a pontoon or just the standard props?

 

[mwhitten]

Re: Prop Pitch Vs RPM

I stayed with Quick Silver. I did not want to risk the warranty. It was a suggested purchase from my local repair shop that sold me the boat. WOT was with the engine trimed out. I have never owned a displacement hull. In addition I had the underside skinned.

 

[Crownie2]

Re: Prop Pitch Vs RPM

My concern is that your top speed dropped so much, which doesn't sound right.

How about trying a Black Max XC 13" pitch? These have a 14" diameter, which may help. See the merc web site for more info...

Bob

 

[mwhitten]

Re: Prop Pitch Vs RPM

The 15" pitch prop would not allow me to reach peak RPM. Is it true that this is running the motor under a load? If so, will it cause harm to the engine and poor fuel economy? :/

Michael

 

[Dhadley]

Re: Prop Pitch Vs RPM

You don't have to stick with Merc props. Using another brand prop will not void your warranty.

With the 15" prop did anyone try moving the motor up a bit? That will gain rpm but on a pontoon it may vent too much. You'll just have to try it and see.

It makes sense to me that you lost speed as you gained rpm. Going from a 15 to an 11 is a fairly big jump.

Crownie is right -- the larger diameter blades should help on a pontoon. Some companies make props specifically for pontoons. Large blades with little rake.

 

[mwhitten]

Re: Prop Pitch Vs RPM

The 15" pitch gave me a comfortable speed range 0 - 28. Is there any harm having a motor WOT at 4800 when it is rated for 5000 - 6000?
Thank you guys for the advice.

Michael

 

[Dhadley]

Re: Prop Pitch Vs RPM

I wouldn't do that, it creates all kinds of internal heat and stress. The 4 strokes have to be run pretty hard at break in to seat the rings or it'll start making oil (gas in the oil). I'd get it set up as close to spec as possible.

Have you tried raising the motor with the 15?

 

[mwhitten]

Re: Prop Pitch Vs RPM

No. Currently the way it's mounted the foot is the only thing clearing the hull. The motor transom looks like a triangle between the pontoons. At the base of the triangle is the foot of the motor. I always trim the motor.

I hope that made sense

Michael

 

[Dhadley]

Re: Prop Pitch Vs RPM

There should be some mounting holes in the motor that will allow you to remount the motor higher. That will decrease drag and allow for higher rpm. To a point. But that's where we'd start with the quest for more rpm. Depending on the type prop, going higher may or may not work but it's the logical place to start.

 

[mwhitten]

Re: Prop Pitch Vs RPM


I was out on the pontoon yesterday. The mounting holes are 1" apart and I can move the motor up 2 if needed. Should I start by moving it up one hole? Does anyone know what mounting bolts should be torqued at?

Michael

 

[Crownie2]

Re: Prop Pitch Vs RPM

If I were doing it, I'd move one hole at a time. If you move two and that works OK you will always wonder if moving one would have been better. In addition, thrust is very important for a pontoon and you don't want to reduce it by going too high because top speed is not a factor here.

Also, your dealer should have some knowledge about what height works best and may be able to advise you.

IMO you should run at about 5,500 top and still may end up with a 13" pitch - but try the raising idea first.

Good luck.

Bob

 

[mwhitten]

Re: Prop Pitch Vs RPM


Thanks for the help. Everyone has been a great help.

Michael