The best propeller size for your boat and engine
combination is based on the recommended operating
range at wide open throttle (w.o.t.) for your
engine, which you will find in your operator's
manual. This will be expressed in terms of a certain
horsepower at a certain RPM (revolutions per minute).
The goal in prop selection is to determine what
propeller style and size will maximize performance
for your boat, while allowing your engine to operate
in the recommended RPM range. The correct propeller
will prevent the engine from over-revving, yet
allow it to reach the minimum RPM where maximum
horsepower is produced.
Run the boat/motor at w.o.t. under normal operating
load to determine the maximum RPM you are able
to obtain. A tachometer is necessary for this
test. Adjust the motor trim angle for the optimum
performance. If during this test, you begin to
exceed the maximum rated RPM of the engine, reduce
throttle setting to a position where maximum RPM
is not exceeded.
If your test results in your being able to over-rev
the engine, you need to increase the pitch of
the propeller. Increasing the pitch increment
by 1" will result in approximately 200 RPM
drop. If your testing shows, however, that you
are only able to obtain a RPM somewhat lower than
the maximum rating given by your engine manufacturer,
you would need to decrease pitch. Decreasing pitch
would increase your RPM.
Example:
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Top End of Operating Range =
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For every 1" of pitch size, the effect
will be approximately 200 RPM. Knowing this, take
the difference in the above example at 800 and
divide it by 200. The result is 4. The prop to
use will be 4" in pitch less than the prop
that was used.
Switching from an uncupped to a cupped propeller
will also reduce your RPM. The cupped propeller
of the same pitch and diameter will typically
reduce your RPM by approximately 200.
Once your wide open throttle RPM falls within
the recommended range of the engine manufacturer,
you have a propeller that is suited correctly
for your boat with respect to RPM. If you use
your boat for fishing, cruising and skiing, one
prop probably won't do all three things equally
well. It is best in circumstances like this to
have two propellers. One to accommodate one set
of circumstances and the other to perform best
under the different load. It is imperative that
the wide open throttle RPM fall within the range
specified by your engine manufacturer.
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