2007-2009 Yamaha F15/F20 Outboard Service Manual, Page 309Get this manual
3PROPELLER SELECTION Yamaha outboard motors are fitted with propellers chosen to perform well over range of applications, but there may be uses where propeller with different pitch would be more appropriateFor greater operating load, smaller-pitch propeller is more suitable as enables the correct engine speed to be maintainedConversely, larger-pitch propeller is more suitable for smaller operating load
1) WOT (WIDE-OPEN-THROTTLE) OPERATING RANGE
At WOT operation and under maximum boat load, the engine speed should be within the upper half of the WOT operating rangeSelect propeller which fulfills this requirementWOT operating range: 5,000 6,000 rpm
CAUTION:
Do not exceed the WOT speed range, as an overheat or engine damage could occurIf the engine speed exceeds the WOT speed range, replace to larger pitch propellerConversely if the engine does not reach the specified speed range, smaller pitch propeller is suitable