Correct prop

[iceman121]

I have a 1979 Johnson Javelin that always starts and runs like new. I am running a brand new 17 pitch prop on it. I have this motor on a 17 ft alumacraft trophy. The boat will run about 32Mph but the RPMS top out at about 4400. What do you recomend. Thanks

 

[wilde1j]

Re: Correct prop

If the motor is already set up correctly (not too deep, right kickout, etc.) and is running OK, I would try a lower pitch prop and try to get RPM up to 5500 minimum @ WOT. (Like ~ 13" pitch) I don't know your HP, but you may be underpowered.

 

[iceman121]

Re: Correct prop

I am sorry it is a 85 Hp motor it has strong compression and is set up right... not sure what prop is recomended for this motor.

 

[wilde1j]

Re: Correct prop

There is no "recommended" prop. They are always determined empirically by trial and error until the RPM is right for a particular hull.engine combination. Your motor is turning way too low @ WOT.

 

[iceman121]

Re: Correct prop

So my question is should I change the pitch on my prop?

 

[iceman121]

Re: Correct prop

If I make an adjustment which way should I go?

 

[wilde1j]

Re: Correct prop

I would try a 13" pitch prop, which is likely the lowest std pitch for your motor.

 

[steelespike]

Re: Correct prop

Something isn't right a 13" pitch is something you would see on a pontoon.
Your speed is in the ballpark but I think it should be closer to 38 with the rpm 5,000 to 5,500..Are you sure the throttle plates in the carb are opening all the way.The 17 might be a little too much but I think a 15 would be the smallest that would go on that boat/motor setup.

 

[iceman121]

Re: Correct prop

Do you think the 15 would make that much of a difference? How would I know if the throttle plates are opening?

 

[steelespike]

Re: Correct prop

Your are about 1,000 rpm too low and going down to a 15 will only add ABOUT another 400.
With the motor off and in forward you need to open the throttle and look into the carb throat.You may have to remove the air box.
Describe the overall performance, how is the hole shot. what happens with say 4 or 5 people in the boat?Where is the antivent plate(just above the prop) in relation to the bottom of the boart.. below,even, above? a picture with a straight edge along the keel from the bottom to the plate showing the position would help.