MerCruiser Black Scorpion, User Manual

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Section 3 - On The Water
Page 38 90-865253070 JUNE 2007
• 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, 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 permits the propeller to speed up,
but the boat speed to reduce. 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
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.