Determine the efficiency of your boat's propeller by calculating the percentage of slip.
Last updated: March 2026 | By ForgeCalc Engineering
Propeller slip is the difference between the theoretical distance a propeller should move in one revolution and the actual distance it moves through the water. Because water is a fluid, it doesn't provide a solid surface for the prop to 'screw' into, resulting in some loss of efficiency.
Slip is not necessarily a bad thing; it is a natural part of propeller operation. However, excessive slip can indicate that the propeller is poorly matched to the boat or that the propeller is damaged or cavitating.
In theory, yes, but in practice, no. A boat requires some slip to generate thrust. If slip were zero, the propeller would be moving through the water without pushing against it.
Common causes include a propeller with too much pitch, a damaged propeller, a hull with excessive drag, or 'ventilation' where air is sucked into the prop blades.
Mathematically, yes, if your actual speed exceeds theoretical speed. This usually happens due to following seas, currents, or inaccurate speed/RPM readings.
Ensuring the propeller is clean and undamaged, optimizing the engine mounting height, and choosing a propeller with more blade area (e.g., 4-blade vs 3-blade) can help.
Related Tools