Johnson 115 prop size

[capttom]

I just bought a new prop for my 1975 115 Johnson motor. It is on a 17 ft boat. The prop is a Michigan wheel alum 13 x 19. WOT is only 4500.<br />The next size prop I see is a 13 1/4 x 17. Do you think this prop would be the right choice. <br /><br />Thanks

 

[Dhadley]

Re: Johnson 115 prop size

Unless the boat is an absolute barge, that should be close. Lets make sure the motor is running correctly and getting to wide open before exchanging props. Whoever you bought it from should work with you until you get the right pitch. <br /><br />At only 4500 you are about 1300 rpm short. What was on it before? What did it turn before?

 

[capttom]

Re: Johnson 115 prop size

It was a 13 x 19 before and was running at about 4800. I didn't realize that I should be running closer to the 5500rpm range until I started checking on this message board. I owned this boat and motor for 23 yrs and it runs like a clock year after year.

 

[Dhadley]

Re: Johnson 115 prop size

Hummm....interesting. Are you sure the tach is right? What brand is it?

 

[capttom]

Re: Johnson 115 prop size

not sure if tach is right, The tach is also 30 years old. I would like to get a new Tach, everything else on the boat has been replaced except for the tach. Most tachs I see today are dash-flush mounted, mine is mounted on top of dash, if anyone knows where to purchase a tach that mounts on top of dash, let me know.

 

[LubeDude]

Re: Johnson 115 prop size

Im not a prop guru, but it sounds like you may have some other problem unless like mentioned, your boat is extremely heavy, (a barge), If so, you may even have to go to a 15P. Even the 17 isnt going to get you to the desired RPMs.

 

[capttom]

Re: Johnson 115 prop size

the boat weighs about 950lbs.

 

[LubeDude]

Re: Johnson 115 prop size

That certainly isnt heavy, even loaded with gear. Did you remember to take the trailer off when launching? Kidding <br /><br />I think it would be good to take a compession test and get back to us, maybe you need a good decarb. Something just dosnt add up. A 17P prop should be about right for your aplication, and your 19P shouldnt be that far down in RPMs, or like Dehadly said, your tach isnt accurate.

 

[capttom]

Re: Johnson 115 prop size

I think I will start off with getting a new tachometer, and see where it is at that point, and I will do the decarb thing, but I doubt that's the problem, because I do that almost every year anyway.

 

[LubeDude]

Re: Johnson 115 prop size

Sounds like a good place to start.