Propellers - Four winns 378 VISTA Owner's Manual

D. Propellers
Knowledge of the propeller is most easily gained through
better understanding of the terminology used to refer to
the aspects of propeller size and performance. It should
be noted that the 378 Vista™ is equipped with nibral

propellers.

1. Diameter
Diameter is twice the distance from the center of the
prop shaft to the extreme tip of a propeller blade. In-
creasing or decreasing propeller size will have a direct
bearing on the RPM's an engine will develop. This is due
to the greater amount of propeller blade surface in con-
tact with the water. See Figure E4.
Figure E4: Propeller Pitch & Diameter
2. Pitch
Pitch is a measure of helix angle, or angle of attack, of
the rotating blade. Pitch is easily understood if one imag-
ines the propeller rotating through a semisolid such as
butter or gelatin. The distance the propeller will travel in
one revolution is called "Pitch." Increasing or decreasing
pitch will also have a direct bearing on engine RPM's
because of the greater bite taken by the blade with each
rotation. See Figure E4.
378 Vista™ Owner's Manual
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378
3. Prop Slip
When traveling through water a propeller is unable to get
a complete bite because of the fluidity of water. "Prop
Slip" is usually expressed as a percent of the computed
theoretical speed. Twenty-five to thirty-five percent prop
slip is common for a cruiser-type boat operating at cruis-
ing speed.
Changing either diameter or pitch will have an effect on
engine speed and prop slip, and in turn, directly effect the
performance of a boat. The propellers included with each
Four Winns® boat provide the best general performance
based on data obtained from on-the-water testing of that
model. Variations in load, operating conditions, environ-
ment, the individual engine and hull performance may ne-
cessitate the purchase and use of another propeller(s).
Under your normal load conditions, the engines should
turn within the maximum RPM range when at full throttle.
If the engines exceed the recommended RPM, an increase
in pitch or diameter is required. If the engine RPM is too
low, a decrease in pitch or diameter is required.
The load carried in a boat will directly affect performance
of the engine. If an engine that is not developing full power,
be sure the engine is properly tuned and load conditions
are those normally experienced, before changing propel-
lers.
NOTICE
For twin engine installations, always check drive
shaft rotation before propeller installation. A right
hand rotation propeller must be installed on the
starboard propeller shaft. Similarly, a left hand
prop must be installed on the port propeller shaft.
For shipping reasons, the propellers are not factory in-
stalled. Initial installation of the propellers will be per-
formed by the dealer during pre-delivery service. See Fig-
ure E5.
Section E
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