Mercury 225 Manual page 36

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.
• Adding 1 inch of pitch will reduce the full throttle RPM by
150 to 200
• Subtracting 1 inch of pitch will increase full throttle RPM by
150 to 200
• Upgrading from a 3‑blade propeller to a 4‑blade propeller
will generally decrease full throttle RPM by 50 to 100
IMPORTANT: Avoid damage to the engine. Never use a
propeller which allows the engine to exceed the recommended
full throttle RPM range when under normal full throttle operation.
GENERAL INFORMATION
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