Reduce Pitch on prop, Can it be done?

[Poorman]

88 115 yamaha 2 stroke
89 Tropy 170 Alumacraft. Dry weight 875
24 gal fuel tank that I always keep full before hitting the lake.
Currently running a 21 pitch SS 14X21 max 5100 rpm by myself and a 20 gal bait tank @ 48 mph gps speed WOT. If I trim it on out I get a bad porpoise.
When I add someone else it drops considerbly to 43 mph WOT. Roughly 4700 rpm or less.
I have a spare SS 13 1/4 X 21 that needs reworked.
My question is , can it be reworked to a 19 or 20 pitch? This will get my RPM up and maybe stop the porpoising and allow a better cruseing speed without running wot.
Thanks in advance for any help or advise.

 

[hwsiii]

Re: Reduce Pitch on prop, Can it be done?

If the two props are not the exact same model of prop it will be a crapshoot, beause of the differences in geometry of the two props. Theoretially the 13.25" diameter prop should turn more RPM than the 14" simply because it is a smaller diameter, but that doesn't take into account the potential differences in blade geometry. I show your motor to have a WOT RPM between 4,500 and 5,500 RPM and your gear ratio to be 2 to 1, if this is wrong then what I am saying is wrong.
Yes the pitch can be lowered on the 13.25" to a 20" Pitch and theoretically that would gain you about 215 RPM and then the difference in diameter also increases the RPM. You should be right at maximum RPM with that change if 5,500 RPM is your Maximum WOT.

H

 

[Poorman]

Re: Reduce Pitch on prop, Can it be done?

Thanks, With the higher RPM is there a chance I want lose any top end speed?

 

[hwsiii]

Re: Reduce Pitch on prop, Can it be done?

You have a veryyy good hance of losing no speed, but it may not stop the porpoising because porpoising can be caused by many different reasons.

H