Mercury Mariner Service Manual 6, 8, 9.9 210CC Sailpower, Page 10Get this manual
Summer conditions of high temperature, low barometric pressure and high humidity all combine to reduce the engine powerThis, in turn, is reflected in decreased boat speedsas much as or miles-per-hour in some cases(Refer to chart, right Nothing will regain this speed for the boater, but the coming of cool, dry weatherIn pointing out the practical consequences of weather effects, an enginerunning on hot, humid summer daymay counter 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 producesdepends upon the density of the air that it consumes and, in turn, this density is dependent upon the temperature of the air, its barometric pressure and water vapor (or humidity) contentAcompanying 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 propelier willin effect, become too largeConsequently, the engine operates at less than its recommended RPM Due to the horsepowerRPM characteristics of an engine, this will result in further loss of horsepower at The propeller with another decrease in boat speed This secondary loss, however, can be regained by switching to smaller pitch propeller that allows the engine to again run at recommended RPM For 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 detonationThisof course, enhances overall reliability and durability of the engine Conditions Affecting Operation