effect of cup and rake on pitch

[PuckHog]

15' tri-hull with '78 Merc 800 80hp outboard. Current prop is a Michigan Wheel Vortex 17p.
GPS top speed was 40mph. Manual for outboard says '78 had a gear ratio of 2.3:1. WOT 5400rpm.
Prop calculator says 5400rpm, 2.3:1 ratio, 17p, .10 slip (assumed), =34 mph
at 19p =38 mph
The prop is cupped and raked from factory. I've heard cup and rake can behave as additonal pitch. Is it possible at top speed my prop is actually a 19p?
Could also be my outboard has a different lower end with a ratio of 2:1 which =39.1mph at 17p

 

[walleyehed]

Re: effect of cup and rake on pitch

Cup and Rake are all figured in to the "stamped" pitch at manufacture time. and no, not all like-pitched props run the same. A 17P in "X" brand will most likely run different than a 17P in "Y" brand.