Mercury 225 Pro XS User Manual page 39

GENERAL INFORMATION
If full throttle operation is below the recommended full throttle range, the
propeller must be changed to prevent loss of performance and possible
engine damage. On the other hand, operating an engine above the
recommended operating RPM range will cause higher than normal wear
or damage. Generally, there is a 200‑300 RPM change between propeller
pitches.
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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.
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