MerCruiser 350 Magnum EFI GEN+ Alfa One, Operation And Maintenance Manual

Page: 17 / 136

15
CA12
How Elevation And Climate Affect Performance
Elevation has a very noticeable effect on the wide-open-throttle pow-
er of an engine. Since air (containing oxygen) gets thinner as eleva-
tion increases, the engine begins to starve for air. Humidity, baromet-
ric pressure and temperature do have a noticeable effect on the
density of air. Heat and humidity thin the air. This condition can be-
come 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 en-
gine 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 propel-
ler will, in effect, become too large. Consequently, the engine oper-
ates 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 switch-
ing 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, en-
hances overall reliability and durability of the engine.