Cavitation; Ventilation; Elevation And Climate; Propeller Selection - Mercury TDI 3.0L Owner's Operation And Maintenance Manual

Section 2 - On The Water
• Straight and smooth, fore and aft
Marine vegetation may accumulate when the boat is docked. This growth must be removed before operation; it may clog water
inlets and cause the engine to overheat.

Cavitation

Cavitation occurs when water flow cannot follow the contour of a fast‑moving underwater object, such as a gear housing or a
propeller. Cavitation increases propeller speed while reducing boat speed. Cavitation can seriously erode the surface of the
gear housing or the propeller. Common causes of cavitation are:
• Weeds or other debris snagged on the propeller
• Bent propeller blade
• Raised burrs or sharp edges on the propeller

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:
• The drive unit being trimmed out too far
• A missing propeller diffuser ring
• A damaged propeller or gear housing, that allows exhaust gases to escape between the propeller and gear housing
• The drive unit installed too high on transom

Elevation and Climate

Elevation and climate changes affect the performance of your power package. Loss of performance can be caused by:
• High elevations
• High 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, the recommended RPM can be achieved by changing to a lower pitch propeller.

Propeller Selection

Operating the engine with the wrong propeller installed can limit power, increase fuel consumption, overheat the engine, or
cause internal powerhead damage. Choose a propeller that allows the engine to operate at the specified wide open throttle
RPM.
It is the responsibility of the boat manufacturer and the selling dealer to equip the power package with the correct propellers.
IMPORTANT: Be sure that the propeller being used does not allow the engine to run against the limiter, as a significant loss in
performance will result.
NOTE: Use an accurate service tachometer to verify RPM.
Select a propeller that will allow the engine power package to operate at the rated engine RPM with a full load.
If full‑throttle operation is below the rated engine RPM, the propeller must be changed to prevent loss of performance and
possible engine damage. On the other hand, operating an engine above the rated engine RPM will cause higher than normal
wear 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 an RPM loss (not as significant on these models).
• Operating in a higher elevation causes an RPM loss (not as significant on these models).
• Operating with a damaged propeller or dirty boat bottom causes an RPM loss.
• Operating with increased load (additional passengers, pulling skiers).
For better acceleration, such as is needed for waterskiing, 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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NOTICE
90-8M0145546 MAY 2018
eng