Effects Of Elevation And Weather On Performance - Mercury 200 Verado FourStroke Operation And Maintenance Manual

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 may lower engine performance if the electronic
management system is unable to compensate:
• Above sea level elevations
• High temperature
• Low barometric pressure
• High humidity
These conditions above reduce air density to the engine, which may lower 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 engines and may
apply to 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 changes:
• Enhances overall reliability and durability of the engine
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