How Weather Affects Engine Performance - Mercury 45 Jet 50-55 Service Manual

How Weather Affects Engine
Performance
It is a known fact that weather conditions exert a profound
effect on power output of internal combustion engines.
Therefore, established horsepower ratings refer to the
power that the engine will produce at its rated RPM under
a specific combination of weather conditions.
Corporations internationally have settled on adoption of
I.S.O. (International Standards Organization) engine test
standards, as set forth in I.S.O. 3046 standardizing the
computation of horsepower from data obtained on the dy-
namometer, correcting all values to the power that the en-
gine will produce at sea level, at 30% relative humidity at
77 F (25 C) temperature and a barometric pressure of
29.61 inches of mercury.
1-20 - GENERAL INFORMATION and SPECIFICATIONS
Summer Conditions of high temperature, low barometric
pressure and high humidity all combine to reduce the
engine power. This, in turn, is reflected in decreased boat
speeds--as much as 2 or 3 miles-per-hour (3 or 5 Km
per-hour) in some cases. (Refer to previous chart.) Nothing
will regain this speed for the boater, but the coming of cool,
dry weather.
In pointing out the practical consequences of weather
effects, an engine--running on a hot, humid summer
day--may encounter a loss of as much as 14% of the
horsepower it would produce on a dry, brisk spring or fall
day. The horsepower, that any internal combustion engine
produces, depends 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) content.
Accompanying this weather-inspired loss of power is a
second but more subtle loss. At rigging time in early spring,
the engine was equipped with a propeller that allowed the
engine to turn within its recommended RPM range at full
throttle. With the coming of the summer weather and the
consequent drop in available horsepower, this propeller
will, in effect, become too large. Consequently, the engine
operates at less than its recommended RPM.
Due to the horsepower/RPM 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 a smaller pitch
propeller that allows the engine to again run at recom-
mended RPM.
For 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 a normal boat load.
Not 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
detonation. This, of course, enhances overall reliability and
durability of the engine.
90-817643R1 DECEMBER 1996