15 HP Johnson Prop Question

Please Help!

I have a 1982 15HP Johnson that I need to replace the prop on. The prop is slipping. I've been to the local "prop shop" and it's going to be $40 to fix it. I can buy a new one on iboats for $60 including shipping. I figured I would go ahead and spend the money for a new one. The old one is 9 1/2" diameter and 10" pitch. The only thing listed on iboats is a 9 1/4" diameter. How much, if any is this 1/4" diameter difference going to effect my performance? I use the boat less than 12 times a year. Mostly duck hunting. I fish with it as well, but use my trolling motor only. Also, can anyone reccomend what size pitch I should be using. I've been told by a "buddy" that I should go down in pitch, because I'm usually pushing a lot of weight. Usually 2 men around 225lbs each and an additional 100-125lbs of weight. The old prop seemed to work ok. This is my first boat, so I'm new to all of this. Please help.

Thanks!

Re: 15 HP Johnson Prop Question

Welcome,Prop diameter typically follows the design,only considered in certain specific conditions.If the prop is intended for your motor it will work fine.
Original equipment was a 10" pitch.Its difficult to suggest any size change without a tach reading,And a tach and gps is nice to have. If you want to really fine tune things a tiny tach is reasonable and easy to install.Could improve performance and economy though a little motor doesn't use that much anyway.If the motor seems to get up promptly and planes I would stick with the stock prop.Problem with going to less pitch is the possibility of over reving especially with a light load.
With a tach you can adjust your speed accordingly.The 15 is free reving 5,500-7,000 rpm.

Re: 15 HP Johnson Prop Question

I posted this info below for another guy a few months ago, this might help.
My performance from the factory prop to the 9.25x9 4blade was a great improvement on my 14' klmath. However with one guy in the boat at sea level you have to back off on the throttle so you do not overrev. the engine.

I have some very helpfull advise. My old kicker was a 2001 15hp 2stroke johnson, same as evinrude. I put a tach on the motor and 4 blade 9.25x9 solas prop.
It was on a 21foot starcraft cuddy cabin boat. Boat + trailer weight is 4,000lbs on the axles, so I estimate boat weight is 3800-4200 with 4 people on board. I run the boat in the pacific ocean and have it there for safety and kicker to keep me off the rocks if my main motor quits.

with the 4 blade 9.25x9pitch the motor runs 4500 rpms wot (500 rpms below spec.) spec. is 5000-6000 rpms. and it would push the boat 5mph in the ocean (gps speed).
I also run the motor on a 14ft. Klamath and at 4000 feet elevation and 2 people it runs 5200rpms. So the prop is great for my use.
Solas makes a 10x7 4blade for the 15hp motor which would probaly get the motor in 5,000 rpm range.
I bought my prop at cabelas for less than $100.