Mercury 75 OptiMax Owner's Manual page 26

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
b - Pitch ‑ Travel during one revolution
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.
GENERAL INFORMATION
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