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