If changing conditions cause the RPM to drop below the recommended range,
such as warmer, more humid weather, operation at higher elevations,
increased boat load, or a dirty boat bottom/gearcase, a propeller change or
cleaning may be required to maintain performance and ensure the outboards
durability.
Check full‑throttle RPM, using an accurate tachometer, with the engine trimmed
out to a balanced‑steering condition (steering effort equal in both directions)
without causing the propeller to break loose.
Effects of Elevation and Weather on 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
These conditions above reduce air density to the engine, which in turn lowers
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 8,000 feet will have over a
30% power loss while a loss of engine power on a hot and humid day could be
as much as 14%. These losses apply to normally aspirated and supercharged
engines.
Compensating for power robbing conditions:
• Switch to lower pitch propeller.
• Change gear ratio.
Some boat performance can be improved by dropping to a lower pitch
propeller, but engine performance will still remain lower. In some cases, a gear
ratio reduction may be more beneficial. To optimize engine performance, prop
the engine 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.
Other advantages to propeller or gear ratio changes:
• Reduces the possibility of detonation
• Enhances overall reliability and durability of the engine
OPERATION
eng 75
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