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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)
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:
•
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
NOTE: Engines equipped with an Engine Control Module (ECM) reduce the effects of changes in elevation and climate by
automatically adjusting fuel flow for weather conditions and elevation. ECM controlled engines, however, do not compensate for
increased loading or hull conditions.
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 optimum engine performance under changing weather conditions and high elevation, use a propeller that allows the engine
to operate at rated RPM at wide open throttle (WOT) with a maximum boat load during your normal boating.
In most cases, the rated RPM at WOT can be achieved by changing to a lower pitch propeller.
90-8M0113987
DECEMBER 2015
eng
Section 2 - On the Water
Page 27
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