Mercury FourStroke 2.5, Руководство по обслуживанию, Страница: 31 / 205

General Information
Page 1C-6 90-8M0065421 NOVEMBER 2012
2/2.5/3.5 FourStroke Top View
a - Tiller handle
b - Throttle friction knob
c - Throttle twist grip
d - Recoil starter handle
e - Fuel tank
f - Fuel cap
g - Fuel cap vent
Conditions Affecting Performance
Weather
It is a known fact that weather conditions exert a profound effect on the power output of internal combustion engines.
Established horsepower ratings refer to the power the engine will produce at its rated RPM under a specific combination of
weather conditions.
Corporations internationally have settled on adoption of International Standards Organization (ISO) engine test standards, as
set forth in ISO 3046 standardizing the computation of horsepower from data obtained on the dynamometer. All values are
corrected to the power the engine will produce at sea level, at 30% relative humidity, at 25 °C (77 °F) 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 3 to 5 km/h (2 to 3 MPH) in some cases. Nothing will regain this
speed for the boater but cooler, dry weather.
Pointing out the consequences of weather effects, an engine running on a hot, humid day may encounter a loss of as much as
14% of the horsepower it would produce on a dry, brisk day. The horsepower that any internal combustion engine produces,
depends upon the density of the air that it consumes. The density of air is dependent upon the ambient air temperature, the
barometric pressure, and the humidity (water vapor) content.
Accompanying the effects of weather inspired loss of power is a second, but more subtle loss. Consider a boat rigged during
cooler, less humid weather with a propeller that allowed the engine to turn within its recommended RPM range at full throttle.
Higher temperatures with high humidity weather will consequently decrease the available horsepower. The propeller, in effect,
is too large for the atmospheric conditions and the engine operates at less than its recommended RPM.
The engine rated horsepower is a direct relation to the engines' RPM. An engine with too large a propeller will have a further
loss of horsepower and subsequent decrease in boat speed. This secondary loss of RPM and boat speed can be regained by
switching to a smaller pitch propeller that allows the engine to run at recommended RPM.
For boaters to realize optimum engine performance under changing weather conditions, it is essential the engine has 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, the engine will be
operating in an RPM range that discourages damaging detonation. This enhances overall reliability and durability of the engine.
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