1998 Mercury 9.9/15HP 4-stroke outboards factory service manual, Page 27Get this manual
GENERAL INFORMATION
Due to the horsepowerrpm characteristics of an engine, this will result in further loss of horsepower at the propeller with another decrease in boat speedThis secondary loss, however, can be regained by switching to smaller pitch propeller that allows the engine to again run at recommended rpmFor boaters to realize optimum engine performance under changing weather conditions, it is essential that the engine have the proper propeller to allow it to operate at or near the top end of the recommended maximum rpm range at wide-open-throttle with normal boat loadNot only does this allow the engine to develop full power, but equally important is the fact that the engine also will be operating in an rpm range that discourages damaging detonationThis, of course, enhances overall reliability and durability of the engine