Prop Pitch

[Grumman338]

I have a 19’ Foot Grumman with 15HP Johnson w/ adj lift. Primary used for Moose hunting in Rivers in Alaska. Currently has factory Pitch 9.5x10 prop. Fought about going down to 8 pitch. Any thoughts ?

 

[airshot]

Depends on current max rpm... you need to know what that is ! If you allow the engine to over rev the max rpm...poof, your motor will blow up..less pitch means higher engine rpm and vice versa...

 

[Grumman338]

The owners manual gives full throttle 5500 to 7000 RPM
And 6250 RPM at shaft full throttle

 

[airshot]

The owners manual gives full throttle 5500 to 7000 RPM
And 6250 RPM at shaft full throttle

Doesn't sound right !! I believe that motor probably has a 5500 max wot, would not be 7K or you would be picking up pieces. once your sure motor is running at its peak then get a prop with correct pitch for you to reach your wot recomended rpm

 

[Grumman338]

Doesn't sound right !! I believe that motor probably has a 5500 max wot, would not be 7K or you would be picking up pieces. once your sure motor is running at its peak then get a prop with correct pitch for you to reach your wot recomended rpm

I am reading out of operators manual on engine specs 1984 version

 

[airshot]

I am reading out of operators manual on engine specs 1984 version

Gotta be a missprint, no way it can run 7k and stay together. I am reasonably sure it is 5000-5500 max rpm. But you still need a tach to measure that rpm.

 

[airshot]

I do stand corrected...it seems there were a few models with a tuned exhaust that was rated at 7K rpm...my apologies..It seems some models used for racing were let out to the public during that time, guess that changes everything! Not sure I would want to run mine that high, but the factory says it will handle it!! Really need an accurate tach now....no matter, still need to adjust pitch to max wot....makes me kinda wish I had one now!!

 

[thill]

Some of those small motors revved really high. Let me take a look in a service manual....

I think Brumman338 is correct. In my service manual, the recommended test wheel speed (which simulates running the engine on a boat with a prop) for a 15 HP, 1987 is 6,650 RPM, using test wheel 386537.

So a max engine speed (red-line speed) of 7K is probably right.

 

[Grumman338]

Yes I guess I have the higher RPM one !
I noticed the recommended spark plug gap is .040” so that confirms it’s the hot rod.

 

[DaleH]

Gotta be a missprint, no way it can run 7k and stay together. I am reasonably sure it is 5000-5500 max rpm. But you still need a tach to measure that rpm.

RPMs: Well, I have an '83 15hp and the service manual and this is what is says:



Ideal Prop Pitch: This used to be a 'sticky post' in the Motors Forum, but it got lost once Jim sold off the Site ... but go here and enter in ALL of the specifics for YOUR hull, motor and gear:

https://turningpointpropellers.com/PROPWIZARD/index.html?tan=

 

[thill]

Glad you found the right line. I only saw the test wheel rpm, and had to get back outside. Good job!

 

[Randaln]

Wonder how much factory shortened the exhaust tuner to get the higher rpm ?

 

[Grumman338]

After some minor tweaking my 15 HP Johnson really does good on my 19’ canoe. I will be carrying a load thought about going down in pitch on prop from standard 10 to 9 or 8.

 

[Grumman338]

I decided on a 9.25” x 9 pitch. Just a tad more pulling power under load.

 

[airshot]

Easier on your motor as well....

 

[Grumman338]

I got my set of props 8 / 9 / 10 and 11.
Thanks all ! I will probably never run full throttle in my canoe , never had that I can remember with my 15 HP.