CONDITIONS AFFECTING OPERATION
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• High humidity.
The conditions listed above reduce air density to the engine which in turn
reduces the following:
• Boost pressure on supercharged engines
• Horsepower and torque throughout the RPM range
• Peak RPM
• Cranking compression
EXAMPLE: An engine running at an elevation of 2,438 m (8,000 ft) will
have over a 30% power loss while engine power on a hot and humid day
can be reduced by as much as 14%. These losses apply to both normally
aspirated and supercharged engines.
Compensating for power robbing conditions:
• Switch to a lower pitch propeller.
• Change the gear ratio.
Some boat performance can be regained by dropping to a lower pitch
propeller, but engine performance will remain lower. In some cases, a
gear ratio reduction may be more beneficial. To optimize engine
performance, prop the engine to allow it to allow operation at or near the
top end of the recommended maximum RPM range at wide open throttle
with a normal boat load.
Other advantages to propeller or gear ratio changes:
• Reduces the possibility of detonation.
• Enhances overall reliability and durability of the engine.
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