Hey P19
According to Merc marine, adding 1 inch pitch will reduce RPM by 150 - 200.
Info from really useful ask mercury site (ok not yammi - but same principles)
http://sites.mercurymarine.com/portal/page?_pageid=126,49901,126_49907&_dad=portal&_sche ma=PORTAL
ps what you doing with your redundant bow tank - need one for my R21