Specifications

ON THE WATER
90-860168040 JANUARY 2004 Page 43
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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
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Ventilation
Ventilation is caused by surface air or exhaust gases that are introduced around the
propeller resulting in propeller speedup and a reduction in boat speed. Excessive ventilation
is annoying and 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
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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, performance can be regained by changing to a lower pitch propeller.