Prop Slip Calculator

Propeller slip is the difference between the distance a propeller should advance according to its geometric pitch and the distance the vessel or aircraft actually travels. A propeller with twelve inches of pitch would theoretically move forward twelve inches in one revolution through a solid medium. Water and air are fluids, however, so some of the blade motion accelerates fluid rather than producing vehicle advance. This prop slip calculator expresses that difference as a percentage of theoretical speed. Theoretical speed starts with propeller shaft RPM, not necessarily engine RPM. If a gearbox or lower unit reduces rotational speed, engine RPM is divided by the gear ratio. Propeller RPM is multiplied by pitch in inches and divided by 1,056 to convert inches per minute into miles per hour. Slip percentage equals theoretical speed minus actual speed, divided by theoretical speed, then multiplied by one hundred. The displayed propeller efficiency is the corresponding advance ratio expressed as one hundred minus slip percentage. It is a convenient comparison metric, not total hydrodynamic efficiency. Positive slip is normal and necessary because a propeller creates thrust by imparting momentum to the surrounding fluid. Typical values vary greatly by hull, propeller style, speed, loading, and operating point. Planing boats may show relatively high slip while accelerating and lower slip once fully on plane. Displacement boats, workboats, racing applications, and aircraft each operate under different constraints, so there is no universal ideal percentage. Unexpectedly high slip can indicate ventilation, cavitation, a damaged or poorly matched propeller, excessive vessel load, inaccurate tachometer or speed readings, an incorrect gear ratio, or measurements taken before steady speed is reached. Negative slip means measured speed exceeds the pitch-based theoretical value. It can occur because nominal pitch differs from effective pitch, blades have progressive pitch or cup, current or wind affects ground speed, or one of the inputs is inaccurate. A small negative result is not automatically impossible, but it should prompt a measurement check. For useful comparisons, record GPS speed in calm conditions over reciprocal runs, use stable wide-open or cruise RPM, confirm the actual reduction ratio, and compare runs at similar load and trim. The calculator does not estimate thrust, shaft power, cavitation, or fuel economy. Use it as a diagnostic and setup aid alongside manufacturer guidance and professional marine or aviation evaluation.