Indmar Standard 360 Operator's Manual page 38

SECTION TWO
Elevation and weather also have a very noticeable effect on the
wide-open-throttle power of an engine. There is a 3% power
output drop per 1000 feet of altitude above sea leavel. Since
air (containing oxygen) gets thinner as elevation increases, the
engine begins to starve for air. Humidity, barometric pressure and
temperature have a noticeable effect on the density of air since
heat and humidity thin the air. This phenomenon can become
particularly apparent when an engine is propped out on a cool dry
day in spring and later, on a hot, humid day in summer, does not
have the same performance.
Although some performance can be regained by dropping
to a lower pitch propeller, the basic condition still exists. The
propeller is too large in diameter for the reduced power output.
An experienced marine dealer can determine how much diameter
to remove from a lower-pitch propeller for specific high-elevation
locations.
Indmar suggests that if you do require a different propeller,
consult the dealer you bought the boat from. The dealer is best
equipped to help with the selection of the correct propeller for your
application(s).
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