Another spun prop question

[sparkyinbc]

Hey folks. Newbie here, have a prop question that's very similar to one from Aug 19th but didn't want to hijack his thread....

I have an older (at least 20 yrs) Johnson 20hp outboard. Just put it on my new lund 1448 Jon this spring. First trip out to test it, it appears to have a spun prop (initially accelerates ok, at about 1/4- 1/3 throttle it continues to rev-up somewhat faster than normal, but without increased propulsion). I've switched the prop, gone from 10X13 to 10X15. Now it's doing the same thing, just at higher revs and faster speed.

Question: is there a torque rating for putting the prop on, and if it's not on tight enough could this be the cause?
Question: if not, what else might I be looking at....

Thanks for any advice or feedback
cheers
Sparky

 

[Bondo]

Re: Another spun prop question

Hey folks. Newbie here, have a prop question that's very similar to one from Aug 19th but didn't want to hijack his thread....

I have an older (at least 20 yrs) Johnson 20hp outboard. Just put it on my new lund 1448 Jon this spring. First trip out to test it, it appears to have a spun prop (initially accelerates ok, at about 1/4- 1/3 throttle it continues to rev-up somewhat faster than normal, but without increased propulsion). I've switched the prop, gone from 10X13 to 10X15. Now it's doing the same thing, just at higher revs and faster speed.

Question: is there a torque rating for putting the prop on, and if it's not on tight enough could this be the cause?
Question: if not, what else might I be looking at....

Thanks for any advice or feedback
cheers
Sparky

Ayuh,.... Welcome Aboard,.... Is it a pin drive, or spline drive propshaft,..??

How 'bout pictures of the propshaft, 'n the prop,... multiple angles on the prop please....

 

[sparkyinbc]

Re: Another spun prop question

It's a spline. Will try to figure out how to upload pics from my iPhone...

 

[Bondo]

Re: Another spun prop question

It's a spline. Will try to figure out how to upload pics from my iPhone...

Ayuh,... The hub is Shot,.... Nut torque, while important, ain't yer problem....

 

[sparkyinbc]

Re: Another spun prop question

Replaced the prop, same issue just at slightly higher speed. First time with this motor on this boat- cavitation plate starts 3 inches below water, is it too short for the transom maybe? Prop is approx centred at bottom of hull

 

[QC]

Re: Another spun prop question

cavitation plate starts 3 inches below water, is it too short for the transom maybe? Prop is approx centred at bottom of hull

I think you are saying the center of the prop is even with the lowest point of the hull? Sounds like a short shaft (15") OB on a 20" hull.

If so, yes, too short and could easily be the problem. Your Anti-ventilation plate (cavitation plate) should be flush to maybe 1 or 2 inches above the bottom. Prop should be pretty much 100% below hull.

 

[sparkyinbc]

Re: Another spun prop question

Damn. Was hoping it was the prop, easier to deal with.

given that I'm not in the market for a new motor, are there options other than modifying the transom that anyone can suggest? Again, thanks for the feedback guys.

 

[jestor68]

Re: Another spun prop question

Trade your motor for one with a 20" shaft, or get out the saws all. :redface: