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conditions, the engine RPM is the controlling factor in determining the
correct propeller blade pitch. To obtain peak performance, the engine RPM
at full throttle should be in the full throttle operating range specified by the
engine manufacturer. If engine RPM is below that range, install a prop of
reduced pitch to increase engine RPM. If engine RPM is above the
recommended range, install a prop of increased pitch.
VENTILATION: Ventilation occurs when air from the water's surface or
exhaust gases from the exhaust outlet are drawn into propeller blades. The
normal water load is reduced and the propeller over-revs, losing much of its
thrust. However, as the propeller momentarily over-revs, this brings on
massive cavitation (see below), which can further "unload" the propeller
and stop all forward thrust. It continues until the propeller is slowed down
enough to allow the bubbles to surface. This action most often occurs in
turns, particularly when trying to plane in a sharp turn or with an excessively
trimmed-out engine or drive unit. With Pontoon boats ventilation can be
caused by an improperly loaded boat. If all passengers are in the front bow
benches ventilation can happen. Passengers should be situated evenly
throughout the boat. Passengers should not sit in front recliner chairs or
front fishing chairs while the boat is underway. Not only can this cause
ventilation, but the lower walls cause this activity to be unsafe. Boats that
have rear fishing chairs are particularly susceptible to ventilation if passen-
gers are not situated evenly throughout the boat. Ventilation can also occur
if the propeller's pitch
causes the engine to
operate above or below its
suggested RPM range.
CAVITATION: Depending
upon water temperature,
when the pressure on the
leading edge of a propeller
blade drops enough, water vapor bubbles are created by "boiling." As the
water vapor moves downstream into higher pressure, it collapses back into
liquid releasing energy that chips away at the blades, causing a "cavitation
burn" or erosion of the metal. Initial cause of the low pressure may be nicks
in the leading edge, too much cup, improper polishing, etc. If cavitation is
caused by a damaged propeller, have it reconditioned or replaced.
ENGINE OVERSPEED: This can occur when a propeller of too Iowa pitch is
used or engine is set at wrong tilt angle or height on transom. Over-
speeding can cause damage to the powerplant in the form of broken
connecting rods, crank shafts and valve train components (in four-cycle
engines). Broken parts can be thrown from the engine at high velocity and
cause injury or damage. A tachometer should be used to monitor engine
speed and to avoid "over-revving." If you're using a low-pitch propeller for
better load carrying or water skiing performance, be careful when operating
under light load conditions to avoid over-speeding the engine.
