Also, along with everything that TheFermanator just said, don't forget that different materials will also effect engine RPM's. You mentioned both Aluminum and Stainless Steel props.. Under load, while being spun against the resistance of the water, they both will flex, but at different rates. The aluminum more so than the SS. The amount of that flex also effects the final RPM's vs SOW of the boat.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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