GENERAL INFORMATION
Page 1C-2
90-826883R2 JUNE 1998
Conditions Affecting Performance
Weather
It is a known fact that weather conditions exert a profound effect on power output of inter-
nal 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 con-
ditions.
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 compu-
tation of horsepower from data obtained on the dynamometer, correcting all values to the
power that the engine 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.
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 combus-
tion 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 wa-
ter 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 en-
gine to turn within its recommended RPM range at full throttle. With the coming of the sum-
mer weather and the consequent drop in available horsepower, this propeller will, in ef-
fect, become too large. Consequently, the engine operates at less than its recommended
RPM.