How Elevation And Climate Affect Performance - Mercury Alpha Owner's Manual

INDEX
CA960

How Elevation And Climate Affect Performance

Elevation has a very noticeable effect on the wide open throttle
power of an engine. Since air gets thinner as elevation increases, the
engine begins to starve for air. Humidity, barometric pressure and
temperature do have a noticeable effect on the density of air. Heat
and humidity thin the air. This condition can become particularly
annoying when the propeller testing was done on a cool, dry day.
Then later; on a hot, sultry day, the boat doesn't seem to have the
same performance.
Although some performance can be regained by dropping to a lower
pitch propeller, the basic problem still exists. In some cases, a gear
ratio change to more reduction is possible and very beneficial.
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 in some cases. 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. With the drop in available horsepower, this
propeller will, in effect, become too large. Consequently, the engine
operates at less than its recommended rpm. This will result in further
loss of horsepower at the propeller with another decrease in boat
speed. This secondary loss, however, can be somewhat regained by
switching to a lower-pitch propeller that allows the engine to again
run at recommended rpm.
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