Stern Drive; Propellers; Diameter; Pitch - Four winns 278 VISTA Owner's Manual

B. Stern Drive
A stern drive or inboard/outboard propulsion system
has a piston engine equipped with special marine
components mounted near the transom and coupled
to an external outdrive-type transmission unit. Shifting
is performed within the outdrive upper gear case.
This type of system is depicted in Figure E1.
Figure E1: Stern Drive
NOTICE
Always return the engine throttle lever to the
extreme low speed position before shifting.
NEVER shift the unit while engine speed is
above 00 rpm.
E - 4

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 on most 258 and 278
Vista models include stainless steel propellers.
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A. Diameter
Diameter is twice the distance from the center of
the prop shaft to the extreme tip of a propeller blade.
Increasing or decreasing propeller size will have a
direct bearing on the RPM's an engine will develop.
Engines, Drives and Instrumentation - Section E
This is due to the greater amount of propeller blade
surface in contact with the water. See Figure E2.
Figure E2: Propeller Pitch & Diameter
B. Pitch
Pitch is a measure of helix angle, or angle of attack,
of the rotating blade. Pitch is easily understood if one
imagines 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 E2.
C. 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 cruising 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
general performance based on data obtained from
on-the-water testing of that model. Variations in load,
operating conditions, environment, the individual
engine and hull performance may necessitate the
purchase and use of another propeller(s).
boat provide the best
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