Hmmm... I know this isn't scientific, but your 15 pitch alum prop and your 13 pitch SS prop are generally considered to be the same for ballpark numbers. Reason being the alum prop flexes more than the SS. A general rule of thumb is a SS 2 inches of pitch lower than an Aluminum will give the same performance. So you seeing nearly the same numbers between the two isn't that far off. But pay attention to what TheFermanator said about your engines power band. If you're actually that much lower in your rpm's then something isn't right.
Since your speed is about what I'd expect for a 140hp engine my first guess would be your tach is off. If you can, try a different one for a test without doing anything else to the engine and see what you come up with.
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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
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