Mercury Optimax User Manual page 24

Propellers are designated by the diameter, pitch, number of
blades, and material. The diameter and pitch are stamped (cast)
into the side or the end of the propeller hub. The first number
represents the diameter of the propeller and the second number
represents the pitch. For example, 14x19 represents a propeller
with a 14 inch diameter and 19 inches of pitch.
a - Diameter
The following are some propeller basics that will help you
determine the correct propeller for your boating application.
Diameter ‑ The diameter is the distance across the imaginary
circle that is made when the propeller rotates. The correct diameter
for each propeller has been predetermined for the design of your
outboard. However, when more than one diameter is available for
the same pitch, use a larger diameter for heavy boat applications
and a smaller diameter for lighter applications.
Pitch ‑ The pitch is the theoretical distance, in inches, that a
propeller travels forward during one revolution. Pitch can be
thought of similar to gears in a car. The lower the gear, the faster
the car will accelerate, but with lower overall top speed. Likewise,
a lower pitch propeller will accelerate quickly, but top‑end speed
will be reduced. The higher the propeller pitch the faster the boat
will usually go; though typically slowing acceleration.
Determining the Correct Pitch size ‑ First, check the full throttle
RPM under normal load condition. If the full throttle RPM is within
the recommended range, select a replacement or upgrade
propeller with the same pitch as the current propeller.
GENERAL INFORMATION
b
b - Pitch ‑ Travel during one
revolution
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