prop size ? how to get it right.

[rpmunroe]

Well my luhrs just hit the water with its new Diesels (yanm 6lp) it origionaly had 454 crusaders 1 3/8 shafts and 19X1 9 props. currently it is spinning a 18X21 prop. it needs to be pitched up a bit as I am getting to many rpms. But I have room for a bigger Prop.

so the question is ? ? as a rule of thumb up 2 degrees in pitch means down 300-400 rpm what about up an inch in size? so instead of a 18x21 a 19x21? what will that do? thanks in advance

[modunlavy]

RP,
Adjusting pitch is easy...just do a ratio, it works well

pitch now pitch need
------------ = --------------
rpm now rpm desired

A rule of thumb (and its not much more than that) is 1 inch of diameter replaces 2 inches of pitch...

Was the angle of the shaft changed during the repower??

Michael