Mercury 40 Operator's Manual page 27

INSTALLATION
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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.
PROPELLER REQUIREMENTS - BIGFOOT MODELS
Bigfoot models require the use of specially designed rubber hub propellers to reduce both instances and severity
of gearcase clutch rattle. The use of the other propellers, although not detrimental to either performance or
durability, could result in clutch rattle.
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