MerCruiser Scorpion 350, User Manual

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Section 3 - On the Water
90-899883175 DECEMBER 2007 Page 41
Ventilation
Ventilation is caused by surface air or exhaust gases that are introduced around the
propeller resulting in propeller speed‑up and a reduction in boat speed. Air bubbles strike
the propeller blade and cause erosion of the blade surface. If allowed to continue, eventual
blade failure (breakage) will occur. Excessive ventilation is 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.
Elevation and Climate
Elevation and climate changes will affect the performance of your power package. Loss of
performance can be caused by:
• Higher elevations
• Higher temperatures
• Low barometric pressures
• High humidity
For you to have optimum engine performance under changing weather conditions, it is
essential that the engine be propped to allow the engine to operate at or near the top end
of the specified maximum RPM range with a normal boat load during your normal boating
weather conditions.
In most cases, recommended RPM can be achieved by changing to a lower pitch propeller.
Propeller Selection
IMPORTANT: The engines covered in this manual are equipped with an RPM rev‑limiter
that is set to an upper RPM limit. This limit, which is slightly above the normal operating
range of the engine, helps prevent damage from excessive engine RPM. Once the RPM
returns to the recommended operating RPM range, normal engine operation resumes.
The boat manufacturer and the selling dealer are responsible for equipping the power
package with the correct propeller. Refer to Mercury Marine's web page
http://
www.mercurymarine.com/everything_you_need_to_know_about_propellers6
.
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.
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 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 dirty boat bottom causes a loss of RPM.
• Operating with increased load (additional passengers, pulling skiers) causes a loss of
RPM.
For better acceleration, such as is needed for water skiing, use the next lower pitch
propeller. When not pulling skiers, do not operate at full throttle when using the lower pitch
propeller.