CONDITIONS AFFECTING OPERATION
56 eng
Ventilation
Ventilation occurs when surface air or exhaust gases surround
the propeller, causing propeller speed‑up (slippage) and a
decrease in boat speed. Excessive ventilation is annoying and
usually caused by:
• A drive unit trimmed out too far.
• A damaged propeller or gear housing, allowing exhaust
gases to escape between propeller and gear housing.
• A drive unit installed too high on the transom.
Propeller Selection
IMPORTANT: Choosing the correct propeller allows the engine
to run at its specified maximum wide‑open‑throttle RPM. Use an
accurate service tachometer to verify engine operating RPM.
It is the boat manufacturer and/or the selling dealer's
responsibility to equip the power package with the correct
propeller(s). Specified engine wide‑open‑throttle (WOT) and
operating RPM range are listed in Specifications.
IMPORTANT: All Mercury Racing engines have an RPM
rev‑limiter that is set to an upper (or limited) RPM. This limit is
slightly above the normal operating range of the engine and is
designed to help prevent damage from excessive engine RPM.
Once the RPM drops into the recommended operating RPM
range, normal engine operation resumes.
Select a propeller that allows the engine to operate in the upper
half of the recommended full throttle RPM range with the boat
normally loaded (refer to Specifications).
If full throttle operation is below the recommended range, change
the propeller to prevent loss of performance and possible engine
damage. On the other hand, operating an engine above the
recommended operating RPM range causes higher than normal
wear or damage. "Propeller lines normally are designed so that
the next size pitch will change engine RPM by 300 to 500. For
every 1" of pitch change, the effect will be approximately 200
RPM."
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