Ventilation; Propeller Selection - Mercury 496 MAG Bravo Owner's Manual

CA10

Ventilation

Ventilation is caused by surface air or exhaust gases which are introduced around the propeller resulting in
propeller speedup and a reduction in boat speed. Excessive ventilation is annoying and usually caused by:
•
Drive unit trimmed out too far.
•
A missing propeller diffuser ring.
•
A damaged propeller or gear housing, which allows exhaust gases to escape between propeller and gear
housing.
•
Drive unit installed too high on transom.
CB822

Propeller Selection

IMPORTANT: Installed propeller must allow engine to run at its specified maximum wide open throttle
revolutions per minute (rpm). Use an accurate service tachometer to verify engine operating rpm.
It is the responsibility of the boat manufacturer and/or the selling dealer to equip the power package with the
correct propellers. Refer to Specifications for engine WOT and operating rpm range.
IMPORTANT: The engines covered in this manual are equipped with an rpm rev-limiter that is set to an
upper (or limited) rpm amount. 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 will allow the engine power package to operate at or near the top end of the recommended
WOT operating rpm range with a normal load. High rpm, caused by an excessive trim angle, should not be used
in determining correct propeller selection.
If full throttle operation is below the recommended 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 and/or damage.
After initial propeller selection, the following common problems may require that the propeller be changed to a
lower pitch.
•
Warmer weather and greater humidity cause a loss of rpm.
•
Operating in a higher elevation causes a loss of rpm.
•
Operating with a damaged propeller or dirty boat bottom causes a loss of rpm.
•
Operating with increased load (additional passengers, pulling skiers) causes a loss or rpm.
For better acceleration, such as is needed for water skiing, use the next lower pitch propeller. Do not operate
at full throttle when using the lower pitch propeller but not pulling skiers.
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