Calculate theoretical or actual propeller pitch speed, slip, RPM, pitch, or gear ratio from propeller pitch, engine RPM, gear ratio, and speed.

Gear ratio = engine revolutions per propeller revolution. Pitch speed is a geometric reference; slip is not energy efficiency.

Propeller Pitch Speed Formula

The following formula is used to calculate the theoretical (no-slip) propeller pitch speed for a given propeller pitch, engine RPM, and gear ratio.

S = (P × R) / (G × 60)

Variables:

  • S is the theoretical (no-slip) propeller pitch speed (in m/s if P is in meters)
  • P is the propeller pitch (theoretical axial distance per propeller revolution)
  • R is the engine RPM (revolutions per minute)
  • G is the gear ratio (engine revolutions per propeller revolution; propeller RPM = R / G)

To calculate theoretical propeller pitch speed, multiply the propeller pitch by the engine RPM, divide by the gear ratio to account for propeller RPM, and divide by 60 to convert minutes to seconds. If you want results in mph/knots/km/h, convert the resulting m/s (or use the calculator above). Actual speed is typically lower due to slip. Slip is a speed comparison, not energy efficiency; negative apparent slip calls for checking the inputs and speed reference.

What is Propeller Pitch Speed?

Propeller pitch speed is the theoretical forward speed a boat would achieve if the propeller advanced by exactly its pitch each revolution (i.e., with no slip). In other words, it is the propeller’s “pitch distance per revolution” multiplied by the propeller’s revolutions per unit time. The propeller pitch is the distance the propeller would move in one revolution if it were moving through a soft solid, like a screw through wood. Engine RPM and gear ratio determine propeller RPM, and slip determines how much the real-world speed falls below the theoretical value.

How to Calculate Propeller Pitch Speed?

The following steps outline how to calculate the Propeller Pitch Speed.

  1. First, determine the propeller pitch (P).
  2. Next, determine the engine RPM (R).
  3. Next, determine the gear ratio (G).
  4. Finally, calculate the theoretical (no-slip) Propeller Pitch Speed using the formula S = (P * R) / (G * 60), where the “60” converts minutes to seconds.
  5. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Propeller Pitch (P) = 15 inches

Engine RPM (R) = 5000 RPM

Gear Ratio (G) = 2.5

Using these values, the theoretical (no-slip) speed is about 12.70 m/s (about 28.41 mph, 45.72 km/h, or 24.69 knots).