1995 Mariner Mercury Outboards Service Manual 50HP 4-Stroke, Page 18Get this manual
How Weather Affects Engine Performance Rated HP
In pOinting out the practical consequences of weather effects, an enginerunning on hot, humid summer daymay encounter loss of as much as 14 of the horsepower it would produce on dry, brisk spring or fall dayThe horsepower, that any internal combustion engine produces, depends upon the density of the air that it consumes and, in turnthis density is dependent upon the temperature of the air, its barometric pressure and water vapor (or humidity) contentAccompanying this weather-inspired loss of power is second but more subtle lossAt rigging time in early springthe 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 willin effect, become too largeConsequently, the engine operates at less than its recommended RPM