Even with the smaller dia I'll bet that a 19 pitch prop is simply not going to be a good prop for your engine. Way too much pitch. I'm thinking more in the 15 - 17 pitch range. And my bet would be on the 15P. Aluminum would be a 17... SS would be a 15. 
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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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