New prop for early 80's Johnson 15 HP

[Big Daddy Braddy]

I have this engine on a 15' River Hawk. It is very similar to a Gheenoe. I just got this rig and don't have any gps speeds or RPM, but the engine still has the factory prop, a 9.25X10. I assume I could really benefit from a new prop, but i need recommendations. Top speed would be my highest priority, with handling next, and fuel consumption last. The boat weighs 250, as well as myself and my partner, totally rigged I would say close to 1100 pounds with a tournament load. Ideas?

 

[steelespike]

Re: New prop for early 80's Johnson 15 HP

Welcome ,If you read the stickies at the top of the page and the helps on the prop store page it will help you understand prop selection better.
Its unlikely anyone has the exact setup so a prop selection would be pure speculation.
The only sure way is to get us your max rpm and speed lightly loaded with your present prop.
At 1100 lbs you surely will need less pitch. Depending on the style of the bottom I would wonder if you can get it to plane when loaded.The motor must be ableto operate within its rated rpm range both for performance and health.

 

[Big Daddy Braddy]

Re: New prop for early 80's Johnson 15 HP

Well, I was able to use a GPS to get a top speed of 16 MPH, and I have a tinytach on order, but I have another question. I see some articles say the max rpm is 7000, others say it is 6000. How do I know for sure?

 

[Big Daddy Braddy]

Re: New prop for early 80's Johnson 15 HP

By the way, I found out it is a 1984 model.

 

[steelespike]

Re: New prop for early 80's Johnson 15 HP

My info indicates 5500 to 7,000. The 9.9 and 15 used the same prop size 9 1/2" X 10".
The gheenoe is not intended to go fast it appears to be very canoe like with a broader bottom and slightly wider outboard type stern.Max speeds with a 10 hp are reported to be about 18 mph.
A 10hp is capable of speeds above 21mph on a more typical type boat.
Your speed may depend on its similarity to the Gheenoe. I would suspect that 18 is well beyond what the Gheenoe is intended for.So your 15 may not increase your speed much.When alone you may need a tiller extension to get you farther forward.Also the higher the motor can be mounted will help speed as well.I would think a stock prop would work fine.
Fuel use at wot will be about 1.5 gph(gallons per hour) whether pushing a Gheenoe or a house boat.Miles per gallon depends on the speed.Reduced throttle reduces gph.
usually best practical cruising speed is somewhere above planing speed.Probably impossible to tell without a flowmeter.

 

[rscottmeyer]

Re: New prop for early 80's Johnson 15 HP

I have a Riverhawk B-52 (15') w/ a 1978 Johnson 15. I went with a 9" pitch Solas 3 blade, and I get 6800 on the top end with just me (245 lbs) and a full fishing load including trolling motor, battery, etc. Top speed is GPS 22 - 23. With me and my 300 lb brother, rpms drop back to 6400 - 6500 and around 18 or 19 mph.

In 78, they had not yet moved to the shorter exhaust tuner like yours, and it doesn't have the .125" leaf shim like yours... and so you should have about 10% more power. Your are good to 7,000 rpms. 9" would be a good choice unless you are a lightweight and will run a lot w/ a light load. Of course, you could back off just a touch if running with no weight and keep your ability to get out of the hole. 9" made a BIG difference in how hard mine accelerates.

 

[Big Daddy Braddy]

Re: New prop for early 80's Johnson 15 HP

Wow, that's great info! I never thought someone with almost the same boat would see this! I weigh 245 also and my partner is about the same so that may work fine. Do you think a 4 blade would make any difference? Do you normally run it a lot at 6800? Is your fuel 50:1? If I went with a 10 pitch that would put me closer to 6200 wouldn't it? Would it be faster? Sorry for all the questions!

 

[steelespike]

Re: New prop for early 80's Johnson 15 HP

The 78 is rated at the powerhead if yours is 85 or newer it is rated at the prop also the mechanical differences mentioned.
Yes the 10 would likely reduce your rpm.I would run the stock prop and get some rpm and speed figures.If you like the performance and the motor is above that 6,000 figure it won't hurt to run the stock prop.If it seems to struggle with a load and can't get the rpm up there then
a 9" may be in order.

 

[Big Daddy Braddy]

Re: New prop for early 80's Johnson 15 HP

A 2" increase in pitch (for example, from 21" to 23") typically results in a decrease of approximately 300-400 RPM.

This is from the prop finder page, does it apply to the smaller engines like mine? If this is correct, I could go with a 4 blade 10" and be just about where I want to be, based on the fact that he has almost the exact same setup and weight that I do. I want to get to about 6250 which I believe is the recommended WOT rpm. I just dont have the money to buy two or three different props and I don't know anyone that has some I could try.