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The propeller size for your boat and engine
combination is based on the wide open throttle
operating range for your particular engine. This
can be located in your operator's manual. This
will be expressed in terms of a certain horsepower
at a certain RPM.
The goal in propeller 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.
Using your existing propeller, determine what
is the maximum RPM you are able to obtain. 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 2" will result in approximately a 300-400
RPM drop. If your testing shows however, that
you are only able to obtain an RPM somewhat lower
that the maximum rating given by your engine manufacturer,
you would need to decrease pitch. Decreasing pitch
would increase your RPM.
Switching from an uncupped to a cupped propeller
would also reduce your RPM. The cupped propeller
of the same pitch and diameter will typically
reduce your RPM by around 200.