Evinrude 4906B 4HP 1969 Owner's Manual page 9

1-4 SAFETY
with average loads, therefore,
necessary to make a change in size or pitch,
in order to obtain the desired results for the
hull design or load condition.
A wide range of pitch is available
each of the larger horsepower engines.
choice available for the smaller engines, up
to about 25 hp, is restricted
sizes. Remember,
smaller bite of the water than the high pitch
propeller. This means the low pitch propel-
ler will travel less distance
water per revolution.
require less horsepower and will allow the
engine to run faster and more efficiently.
It stands to reason, and it's true, that the
high pitch propeller will require more horse-
power, but will give faster boat speed if the
engine is allowed to tu rn to its rated rpm.
If a higher-pitched
on a boat, in an effort to get more speed,
extra horsepower will be required.
extra power is not available, the rpms will
be reduced to a less efficient
actual boat speed will be less than if the
lower-pitched propeller
stalled.
All engine manufacturers
units to operate with full throttle
slightly above, the rated rpm.
your engine at the
increase spark plug life, receive better fuel
economy, and obtain the best performance
CAVITATION
o
Cavitation (air bubbles)
Manufacturers are constantly
explained in the text.
it may be
for
The
to one or two
a low pitch takes
through the
The low pitch will
propeller is installed
If the
level and the
had been left in-
design their
at, or
If you run
rated rpm, you will
BURN
formed at the propeller.
fighting this problem,
from your boat and engine. Therefore, take
time to make the proper propeller selection
for the rated rpm of your engine at full
throttle with what you consider to be an
average load.
rectly balanced
peller throughout the entire speed range.
A reliable tachom eter must be used to
a measure engine speed at full throttle
ensure the engine will achieve full horse-
power and operate
To test for the correct propeller, make your
run in a body of smooth water with the
lower unit in forward gear at full throttle.
Observe the tachometer
NEVER run the engine at a high rpm when a
flush attachment
is above the manufacturer's
operating range, you must try propellers of
greater pitch, until you find the one that
allows the engine to operate
within the recommended full throttle range.
If the engine is unable to deliver top
performance
tuned, then the propeller
blame. Operating conditions have a marked
effect on performance.
engine will lose rpm when run in very cold
water. It will also lose rpm when run in salt
water as compared with fresh water.
low-barometer
gine to lose power.
of a
Example
have been replaced
as
was sustained.
Your boat will then be cor-
between engine and pro-
efficiently and safely.
at full
is installed. If the reading
recommended
continually
and you feel it is properly
may not be to
For instance,
day will also cause your en-
damaged propeller. This unit should
long before this amount of damage
to
throttle.
an
A hot,