Conditions Affecting Operation; Weight Distribution (Passengers And Gear) Inside The Boat; Bottom Of Boat; Cavitation - Mercury TDI 4.2L Manual

Operating a boat or engine with impact damage can result in product damage, serious injury, or death. If the vessel
experiences any form of impact, have an authorized Mercury Marine dealer inspect and repair the vessel or power package.

Conditions Affecting Operation

Weight Distribution (Passengers and Gear) Inside the Boat

Shifting weight to the rear (stern):
• Generally increases speed and engine RPM
• Causes bow to bounce in choppy water
• Increases danger of following wave splashing into the boat when coming off plane
• At extremes, can cause the boat to porpoise
Shifting weight to the front (bow):
• Improves ease of planing
• Improves rough‑water ride
• At extremes, can cause the boat to veer back and forth (bow steer)

Bottom of Boat

To maintain maximum speed, ensure that the boat bottom is:
• Clean, free of barnacles and marine growth
• Free of distortion, nearly flat where it contacts 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.
90-8M0114081 DECEMBER 2015
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WARNING
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Section 2 - On The Water
Page 25