Specifications
8 - 5 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.
A.
Diameter
Diameter is twice the distance from the center of the
prop shaft
to
the
extreme tips
of
propeller blades.
I ncreasing or decreasing propelle r size will have a di rect
bearing on the RPM's an engine will develop. This is
due
to
the greater amount of propeller blade su rface in
contact with the water.
See
Figure 82.
8.
Pitch
Pitch is a measure of helix angle (or angle
of
attack) of
the rotating blade. Pitch is easily understood
by
imag-
ining the propeller rotating through a semi-solid such as
butter or jello.
The
distance the propeller will travel in
one revolution is called "Pitch." Increasing
or
decreas-
ing pitch will have a direct bearing on engine RPM's
because of the greater bite taken
by
the blade with each
rotation. See Figure
82.
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.
From the definition given, the deduction can be made
that a
propeller,
of
a given diameter, with a 10 inch
pitch, rotating at 3600 revolutions per minute, with a
prop slip of 30%, would move the boat at a rate of 24
miles per hour.
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
propeller(s) included
with each Carver boat provide the best general perform-
ance based on data obtained from on-the-water testing
of that model. Variations in load, operating conditions,
environment, the individual engine and hull perfor-
mance may necessitate the purchase and use
of
an-
other propeller(s).
Under your normal load conditions the engine(s) should
turn within the maximum RPM range when at full
throttle.
If the engine(s) exceeds the recommended
RPM an increase in pitch or diameter is required. A
decrease in pitch or diameter required if engine RPM is
too
low.
An engine that is not developing full power, and the load
carried in a boat, will directly effect performance of the
engine. Always be sure
the
engine is properly tuned,
and that load conditions are those normally experi-
enced, before changing propellers.
For a further explanation of conditions effecting propel-
ler RPM, see a Carver dealer.
8 - 6 ENGINE INSTRUMENTATION
Each pilot station is equipped with a complete set of
engine instruments. These instruments allow the pilot
to constantly monitor
the
operational condition of the
engine. Diesel engine boats may use visual indicators
instead
of
gauges. Close observation of these instru-
ments could save the engine from damage.
Prop makes one
revolution thru
viscous
medium -
zero
slip
condition
..
~
R=Radius • Diameter=2 x R
Prop Diameter
Prop Pitch
FIGURE
82
• PROPELLER THEORY
OMSB
B3
CARVER