Optimum RPMs For a 115hp Johnson Motor

[tdoyle100]

I have an 18 ft Glen-L cabin cruiser that I've just spent the last 2 years restoring. I just put a mid 1980s vintage 115hp Johnson V4 on it and have had it out on the water a couple of times. It comes up on plane and runs very well (about 30mph) at 3200rpm. I have been told that I would be at WOT between 3200 and 3500rpm, but after reading many of the entries and responses on this bulletin board it seems that it should be somewhere between 5000 and 5500 at WOT. Can anyone shine some light on this for me? I realize that a lot has to do with water conditions and prop size and the like, but this seems to be quite a discrepancy. This is my first power boat, and I'm more than a little clueless.

 

[DJ]

Re: Optimum RPMs For a 115hp Johnson Motor

TPD,<br /><br />The "operating" wide open throttle range is: 4500-5500.<br /><br />The V-4's run best when propped to run at about 5500 at WOT. You can twist them a "little" faster, but I prefer not to.

 

[JB]

Re: Optimum RPMs For a 115hp Johnson Motor

Howdy, TPD.<br /><br />I agree with DJohns. You should be able to turn 5500 at wide open throttle (WOT).<br /><br />Now the question is whether you have the wrong prop or a sick engine that pretends to be okay.<br /><br />Because your GlenL cruiser is probably quite a bit heavier than that 115 was originally propped for, I am going to guess that you need a prop with at least 2" less pitch.<br /><br />Good luck and let us know what you find.

 

[tdoyle100]

Re: Optimum RPMs For a 115hp Johnson Motor

Ok, Well I finally got the boat up to full throttle with the tilt down. It would only get up to 4200rpm at about 32mph. My prop is stamped 13 x 19. <br /> <br />I assume that I have too much pitch. It seems to me after reading all the various articles in this forum though, that my diameter is probably too small too. Any suggestions on what I should replace this prop with would be appreciated.<br /><br />Thanks<br /><br />TPD

 

[angus63]

Re: Optimum RPMs For a 115hp Johnson Motor

Check out www.theoutboardwizard.com for recommended prop sizes for bass boats to cruisers. My guess is no more than 16 pitch otherwise your staining that puppy. Good luck. P.S. I had a 24' fiberform with a retrofit 115 evinrude on a bracket and ran a 15" pitch to cruise @ 20 kts @ 4200 (wide open was about 4850 when loaded).

 

[tdoyle100]

Re: Optimum RPMs For a 115hp Johnson Motor

You guys were right. I went with a 13 3/4 by 15 prop, and now I have all the RPM I need, and I can get out of the hole a LOT faster. My top speed is only about 35, but that's about as fast as I want to go in an old wooden boat. Puget Sound is notorious for its large, mostly sunken logs. Thanks agian for the advice.