What you have to do is prop the boat with your usual load in it, say two people.
You have to get the engine to rev to its maximum rpm at full throttle which i think is about 5300rpm.
For example if you have say a 13" pitch prop and it revs to 5100 you wany to go down a pitch to 12" to give you and extra 200rpm.
The rough guide is about 200-300rpm for each pitch of prop.
A smaller prop will give you more rpm and a bigger pitch will give you less rpm.
You also have to consider pick up speed etc but thats another story!.