Correct prop for 115 Johnson

[Carphunter]

Is there some sort of equation to figure out exactly which prop to use on certain set-ups? I have a 1981 Dynatrak by Ebbtide, 16.5' fish and ski, with a 1981 Johnson 115 v4. I have been told that I need a 13-1/4" dia. by 19 pitch prop. I was wondering how to figure this myself.<br /> Also, I have a 4-blade, 12.8" dia. by 17 pitch prop on the above set-up now, the rpms are much higher than with the 13-1/4" by 19 pitch prop. When someone asks me "what are your max rpms", which information should I give, the 4-blade rpms or the 3-blade rpms? I apologize if these questions seem stupid, I have just recently purchased the above mentioned boat. I had a small Seacrest boat with a 25 evinrude for 12 years, and it was much easier to work on than "The Big Johnson"!<br /> Thanks in advance

 

[BKool68]

Re: Correct prop for 115 Johnson

You want your max RPMs to be at about 5800. If the 4 blade prop lets your motor exceed this it is under pitched for your boat and I think that is the case. A prop will act as a rev limiter and keep you from harming your motor. I believe a 19 or 20 pitch will put you in a safe operating range. You should note your top speed and RPMs for each prop and post your results so we can be of better help.

 

[Carphunter]

Re: Correct prop for 115 Johnson

Thanks, BK'68!<br /> My tachometer has been broken, receiving my new tach this week and going to run some tests this weekend. I will post the results.