Mercury MERCRUISER - Operating with Low Water Inlets in Shallow Water; Conditions Affecting Operation; Weight Distribution (Passengers and Gear) Inside the Boat; The Bottom of the Boat

Section 3 - On the Water
90-864197076 NOVEMBER 2008 Page 49
Operating with Low Water Inlets in Shallow Water
a - Low Water Inlets
NOTICE
Operating in shallow water can cause severe engine damage due to clogged water inlets. Ensure that the water inlets on the
gearcase do not ingest sand, silt, or other debris, which can restrict or stop cooling water supply to the engine.
Extreme care should be exercised when operating a boat equipped with low water inlets while maneuvering in shallow water. Also,
avoid beaching a boat with the engine operating.
Conditions Affecting Operation
Weight Distribution (Passengers and Gear) Inside the Boat
Shifting weight to 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 front (bow):
• Improves ease of planing
• Improves rough water ride
• At extremes, can cause the boat to veer back and forth (bow steer)
The Bottom of the Boat
To maintain maximum speed, the boat bottom should be:
• Clean, free of barnacles and marine growth
• Free of distortion; nearly flat where it contacts 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 the 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:
• Drive unit trimmed out too far
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