CONDITIONS AFFECTING OPERATION
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• Operating in a higher elevation
• Operating with a damaged propeller or dirty boat bottom
• Operating with increased load (additional passengers, pulling
skiers, etc.)
Conditions That Lower Engine Performance
The following conditions lower engine performance and cannot be
compensated by the engine fuel or electronic management
systems:
• Above sea level elevations
• High temperature
• Low barometric pressure
• 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.
To compensate 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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