How Elevation And Climate Affect Performance - Mercury V-6 series User Manual

CA12

How Elevation And Climate Affect Performance

Elevation has a very noticeable effect on the wide-open-throttle
power of an engine. Since air (containing oxygen) 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.
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 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.
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