Prop Suggestions 1981 Johnson 25 - 15ft Jon Boat

[CullenTN]

I got an 81 Johnson 25 on a 15ft Jon Boat that I've built a full deck and everything in and wondering if there's a different prop I can put on there to make it a little faster.

It's fast for what it is but any little extra bit of power & speed I can squeeze out of it would be great.

Anybody know if there is a different type of prop I can get on there that will give me a little extra speed? The one on there now look like a standard 3 blade and its a 9.2 X 12

 

[steelespike]

Re: Prop Suggestions 1981 Johnson 25 - 15ft Jon Boat

It appears the motor uses the old style gearcase which limits prop availability and setup some.
Initially a brief look indicates a 12" is the max.You really need a tach and speedo. or gps to tell how well your setup works.A jons design can makit hard to get up free of the water and some have 4 or 5 keels that don't help the speed.
In an ideal setup a 25 can produce speeds over 30.
What prop to get will depend on your present performance.An induction tach like the Tinytach will work or maybe an optical tach. And of course an OMC tach.

 

[CullenTN]

Re: Prop Suggestions 1981 Johnson 25 - 15ft Jon Boat

Thanks for the reply I'll get a tach and already have a GPS. So I guess I'm measuring speed at different RPMs? This will tell whats the best prop then?

 

[John_S]

Re: Prop Suggestions 1981 Johnson 25 - 15ft Jon Boat

Thanks for the reply I'll get a tach and already have a GPS. So I guess I'm measuring speed at different RPMs? This will tell whats the best prop then?

You can do it at different rpms, but the most important is at your wot rpms, using your standard load. That along with engine recommended wot rpm, gear ratio, and current prop info, people can make valid recommendations for a prop.

 

[Rsdoull]

Re: Prop Suggestions 1981 Johnson 25 - 15ft Jon Boat

A tach is the key to your puzzle. What you are trying to do is find a prop with the most pitch that allows your engine to hit the manufacturers recommended max rpm when you have the throttle wide open. Aside from some other wacky variables, that will produce the most speed possible from your specific motor, hull and load combination.

In your case, I would guess that you now have too much pitch and your motor is not hitting max rpm at open throttle. You may actualy need a prop with less pitch to gain more speed (yes that sounds backwards...)

I have a flat bottom jon, 1648 and a 25 hp 2 stroke yamaha. I added a floor, gear box, electronics and a bunch of gear. Then the motor was losing rpm with the stock prop when I had a "duck hunting load" (another fat guy, couple dozen decoys, guns, shells, food). I dropped from over 30 mph to about 22. The stock prop will get about 28 mph with a fishing load (me, rods, tackle boxes). So I bought a prop with about 2" less pitch and I run that in the Fall with heavy loads. Gets the rpm back up and runs up to about 28 mph again.

Get a tiny tach and start from there. Run flat out and see what rpms you can get. If you hit or go over the max recomended rpm, try a prop with more pitch. If you are short of the recommended limit, try a prop with less pitch.

If all else fails, buy a 30 hp!

Rob