Mercury 35/40HP 2 Cylinder Outboards Service Manual PN 90-42794--1, Page 14Get this manual
How Weather Affects Outboard Performance Rated HP Horsepower Loss Due to Atmosphere Conditions
} Summer HP
,n9 Too Large for Summer-Horsepower
Accompanying this weather-inspired loss of power is second but more subtle lossAt rigging time in early spring, the engine was equipped with propeller that allowed the engine to turn within its recommended RPM range at full throttleWith the coming of the summer weather and the consequent drop in available horsepower, this propeller will, in effect, become too largeConsequently, the engine operates at less than its recommended RPMDue 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 be propped 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