Effect of prop diameter

[phipps]

I had a 11 3/4 x 17 prop on my 83 Evinrude 50 and was getting about 3500rpm so I know I need a lower pitched prop, probably a 13x11.

I had a 12 1/4 x 15 prop given to me so I decided to try it to see what difference there was. I took the boat out today and it was still only getting to 3500-3600rpm. I did notice it seemed a bit faster.

I was expecting a jump in rpm but didn't really see one so I was thinking maybe it was the difference in diamter and thus this question: does diameter affect WOT RPMs?

 

[tpenfield]

Re: Effect of prop diameter

The larger diameter prop will try to move more water per revolution, and accordingly will require more power than a smaller diameter prop of the same pitch.

the fact that you went down in pitch and up in diameter left you with only a slight increase in RPM, as they pretty much offset each other.

so, yes, diameter is a factor to consider.

 

[phipps]

Re: Effect of prop diameter

Thank you for an easy to understand explanation. I'm slowly learning this boating stuff!

 

[airshot]

Re: Effect of prop diameter

May I also add... be sure your motor is running correctly, would be a shame to spend money on a prop only to find an issue with the motor itself. That RPM sounds quite low and a very big swing in pitch will be required it seems. Perhaps a tuneup should be done first then re-evaluate your prop setup.

 

[phipps]

Re: Effect of prop diameter

Perhaps a tuneup should be done first then re-evaluate your prop setup.

The carbs have just been rebuilt, new plugs, new fuel lines, replaced 1 coil due to a crack. The motor starts right up, idles good and throttles up good. Everything seems to be running good. I talked to a local Evinrude dealer and he suggested a 13x11 prop.

This is on a pontoon boat, if that makes a difference.

 

[phipps]

Re: Effect of prop diameter

How would I go about confirming everything is running the way it should before buying a prop?

Would I just try revving the motor in neutral?