Lower pitch for more RPM's....not on mine!

[kryan2]

1984 Pontoon 28' with a 1984 75hp 3 cyl Johnson outboard.

old prop is a 15 pitch 3 blade and most I can get is 4480 on the RPM gauge. If I trim it up it starts to cavitate.

"Prop Shop" said we will go with a 14 diameter 13 pitch 3 blade....OK! Installed it and couldn't get the RPM's over 3000 and the whole boat had a vibration, absolutly no power at all. I thought I dropped a cylinder.

Put old prop back on and everything was back to normal. Any suggestions on how to get RPM's up?

 

[Sea Rider]

Re: Lower pitch for more RPM's....not on mine!

Is old prop 15 pitch 3 blade same diameter as the new one ?

Happy Boating

 

[cribber]

Re: Lower pitch for more RPM's....not on mine!

Your numbers aren't adding up... you could hit 4480 with the 15" pitch and you dropped to a 13" and lost 1400 rpm? Dropping to a 13" pitch should have gotten your RPMs up to around 4800. You need to go back to the prop shop and tell them your findings. Is there a part number on the hub you can look up to verfiy what you've got?

 

[steelespike]

Re: Lower pitch for more RPM's....not on mine!

Diameter and pitch seem reasonable Is it a new prop? Has it been doctored or repaired?
Rule of thumb a 2" pitch change will result in 300 to 400 rpm increase in your case.
I sure hope the shop took it back.Sounds defective to me or completely wrong for the application.

 

[pootnic]

Re: Lower pitch for more RPM's....not on mine!

Vibrating,no power and not doing what a smaller prop should do for you,something isn't right.
You sure your old prop is a 15 pitch?
I don't own or have much to do with pontoons but 28' with a 75 hp,15 pitch seems big...
I'd say either the "new"prop is way out of balance and maybe not so new,or the old prop maybe a 12 pitch?