Indmar 5.7L Vortec Operator's Manual page 45

An engine that is under-revving is lugging. Lugging
places tremendous loads on the pistons, crankshaft and
bearings and can cause detonation, piston seizure and
other engine damage.
Elevation and weather also have 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 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
Indmar Marine Engines
spring and later, on a hot, humid day in summer, does
not have the same performance. Although some per-
formance 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 spe-
cific high-elevation locations.
Indmar suggests that if you do require a different pro-
peller, 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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