Every outboard and sterndrive motor has a WOT rating, as established by the manufacturer. The WOT rating is the RPM range the motor should achieve when running at wide open throttle. If the propeller has too much pitch, the motor will not reach its optimum RPM at WOT, while a prop with too little pitch will exceed the recommended RPM limit at WOT.
RPMs – How to adjust RPMs with the correct style, diameter, pitch propeller. If you increase pitch, all things be equal, you will decrease RPMs with the opposite if you increase pitch. So, if your RPMs are not at the upper end of the wide-open throttle range recommended by the motor manufacture (Without Over Trimming), changes may need to be made.
The Old Wives Tale - Every inch of pitch equals about 200 RPM is not as reliable as one may think. Another method (more reliable) to calculate RPMs gain/loss is by percent; however, this applies to the same style prop from the same manufacture where the diameter remains constant regardless of pitch.
For Example: A boat owner/customer is under revving at (5000 rpms) with a 3-blade 19 pitch prop on a 4/stroke outboard (rated for 5000-6000 RPMs), short 1000 rpms from achieving 6000 so this is where math comes into play – You divide 5000 (RPMs achieving with a 19-pitch prop) by 6000 (RPMs you want to achieve) and you get 83.33% or 17% short of wide-open throttle range of 6000. Decrease your current pitch prop (19) by 17% and you get a 15.80 pitch prop to achieve 6000 rpms. So that translates into a 16-pitch prop of the same style and manufacture. So, the 200 RPM rule just does not add up, subtracting 5” of pitch from a 19 down to a 14.
Take another example: Tohatsu 18hp 4/stroke with a 10” pitch prop only doing 5000 RPMs, should be closer to 5800 – using same procedure as above, at 5000 RPMs, you are 14% short of achieving at least 5800 – Reduce pitch of current prop (10 Pitch) by the same 14% to gain RPMs and that = 8.5” pitch prop.
The Old Wives Tale method (200 RPMs = 1” pitch) would require a 6” pitch prop, too low in pitch and you would over rev.]I hope this has helped you learn about RPMs as it relates to prop pitch and have a grrrrreat time on the water. For reference please click on your motor manufacture below.