Generic prop question.

[DaveG55]

Going from a 14.5/19p aluminium to a 14.5/21 SS prop what kind of decrease in wot can I expect?
I know rpm drops 200 per inch of increased pitch but what effect does changing prop material have? My cars memory wants me to believe that an Al to SS is good for another 50 drop. Yes, no, maybe?

 

[MikDee]

Maybe? Usually when going SS you drop down in pitch. If you're at the bottom of your recommended rpm range, then drop down in pitch, if your at the top, get the 21p SS. You might be faster, but with a bit less torque for water sports.

 

[bruceb58]

Since a SS prop has thinner blades, the exact same prop shape will have more RPMs with a SS. That of course is a BIG IF that the shapes are otherwise identical.

 

[Frank Acampora]

IF---If the stainless is the same manufacturer and blade design: Going up two pitches will decrease RPM by 400 BUT the stainless would normally run about 200 RPM faster than the aluminum so you would expect a net difference of 200 RPM less.

HOWEVER: Stainless design varies a lot and you may find that a 21 pitch stainless will drop RPM more or may even increase RPM. Speed expected can not be determined except by experimentation.

Example. On a 125, Rpm with a standard 19 pitch aluminum was 5500 RPM. A stainless 21 pitch cleaver prop gave 5900 RPM while a stainless ballistic 21 pitch prop gave 4500 RPM same engine, same boat.