Enter the theoretical (no-slip) speed (based on propeller pitch/RPM) and the actual vessel speed (ideally speed through the water) into the calculator to determine the propeller slip percentage; this calculator can also evaluate any of the variables when the others are known.

Propeller Slip Calculator

Enter any 2 values to calculate the missing variable

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Propeller Slip Formula

The following formula is used to calculate the propeller slip:

\text{Slip} = \frac{N - S}{N}\times 100

Variables:

  • Slip is the propeller slip percentage
  • N is the theoretical (no-slip) vessel speed based on propeller pitch and RPM
  • S is the actual vessel speed (typically speed through the water)

To calculate the propeller slip, subtract the actual vessel speed from the theoretical (no-slip) vessel speed. Divide the result by the theoretical (no-slip) vessel speed, and then multiply by 100 to get the slip percentage.

What is a Propeller Slip?

Propeller slip refers to the difference between the theoretical distance a vessel would advance in one propeller revolution (based on the propeller’s pitch) and the actual distance it advances. This discrepancy occurs because water or air is a fluid medium: when a propeller rotates, it accelerates the fluid to generate thrust, and the resulting flow and losses mean the vessel advances less per revolution than the ideal “no-slip” case. The amount of slip is influenced by several factors, including the design and condition of the propeller, the speed and load of the vessel, and the condition of the fluid medium. It is usually expressed as a percentage, with 100% slip meaning there is no forward advance through the water, and 0% slip meaning the vessel advances exactly the theoretical distance per revolution.

How to Calculate Propeller Slip?

The following steps outline how to calculate the Propeller Slip.

  1. First, determine the engine RPM, the gear ratio (engine:prop), and the propeller pitch.
  2. Next, calculate the propeller shaft RPM: prop RPM = engine RPM ÷ gear ratio.
  3. Next, calculate the theoretical (no-slip) speed (N): N = (prop RPM ÷ 60) × pitch (with pitch converted to distance per revolution).
  4. Then, measure the actual vessel speed through the water (S) using a speed log or equivalent (speed over ground includes current and is not recommended for slip calculations).
  5. Finally, compute Slip (%) = (N − S) ÷ N × 100, and check your answer with the calculator above.

Example Problem:

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

Theoretical (no-slip) speed, N (knots) = 12

Actual speed through the water, S (knots) = 10

Slip = (12 − 10) / 12 × 100 = 16.7%