Prop for a 115 Johnson

[matt9224]

I have a 1974 Johnson 115 on a 15ft Sport bass, I have no idea how to choose a prop for a boat. It currently has a 3 blade stainless prop but the boar will only go 35mph and I was hoping for somewhere around 45-50 range. Any help would be greatly appreciated.

 

[sutor623]

Re: Prop for a 115 Johnson

I have a 1974 Johnson 115 on a 15ft Sport bass, I have no idea how to choose a prop for a boat. It currently has a 3 blade stainless prop but the boar will only go 35mph and I was hoping for somewhere around 45-50 range. Any help would be greatly appreciated.

Your gonna hear it from someone so it may as well be me. What are the RPMs that you are turning at WOT? And how is your throttle linkage, is it allowing you to fully open up the butterfly valves in the carbs? And what pitch prop is it?

 

[sutor623]

Re: Prop for a 115 Johnson

Looking at the prop calculator, and assuming that you have the standard 2:1 ratio, and 10% prop slip, Id say you have a 15p prop if in fact you are spinning at the upper limits of 5800RPMs. The only way that boat will hit the 45-50MPH mark is if it can spin a 19p prop at 5800RPMs.

 

[steelespike]

Re: Prop for a 115 Johnson

Sutor has got you started but::
To keep it simple,read "read this first" at the top of the page and get us as much info as possible.
It is the only way to tell if your setup is working and if a prop is the solution.Something like a Tinytach will get your rpm and
and a gps for accurate speed.