Why pitch and not diameter

A wheel travels one circumference per turn. A propeller travels its pitch — the distance it would advance through solid water in one revolution. Diameter decides how much water the blades can push, which is thrust; it does not change how far one turn carries the boat.

Why slip matters

Water is not solid, so the propeller always advances less than its pitch. That shortfall is slip, and ignoring it overstates the speed by 10–45%. Pick the hull type to load a typical figure, then adjust it if you know your own.

The formula

speed = pitch × (motor RPM / gear ratio) × (1 − slip)

Hull-speed ceiling

A hull that does not plane cannot outrun its own bow wave, whatever the propeller delivers. That ceiling is about 2.43 × √(waterline length in metres) knots. Enter the waterline length and the calculator will warn you when the propeller figure passes it.