i'm currently running 30x17.5 (5) blades and can spin them to about 5900-6000rpms at about 100-102mph, i would like to lower my rpm to about 5600rpm where my peak hp is. should i go to a 31pitch and will it effect my speed?

Every inch of pitch is equal to 200 RPM. You should increase 2 inches of pitch to achieve 5500-5600 rpm.

Julie

i have a set of 32x18 4-blade which i can only get to 4800-4900 at 92mph, i'm just afriad of tweeking these 5-blades two inches

Performing a pitch change of 2 inches is not advisable! Max of one inch of pitch and if you have a heavy vessel or high HP then it is not advisable at all. Obviously your two sets are very different propellers.

Julie