Mercury 300 XS OptiMax Manual page 45

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 outboard's
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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