Propeller selection for 40 HP Yamaha

[tim carey]

I just bought a 2001 Yamaha 40HP motor that was on a 13 1/2 Whaler, I plan to run it on a 15' tri hull. I need help determining the size and pitch of propeller for general overall performance. It currently has a 992409 16P prop I think from a Mercury on it. I am looking at putting a 11 X 15 on it for overall performance, O don't have the motor on the boat yet. Thanks in advance.

 

[johnnygjr]

Your prop selection is determined by wide open throttle RPM, found in owners manual

 

[ClassicAQ]

First knowing the rpms @WOT with the 16" pitch will help to determine if a different pitch is needed. Refer to your engine manual for spec rpms @WOT. The rule of thumb used to approximate rpms with a change of pitch is 150-200 rpms per 1" of pitch. Increasing pitch decreases rpms, and vice versa. There are also other variables that influence rpms, such as engine height and prop alloy (SS or Al). Choosing the perfect prop is more of an art than a exact science.