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What this tool does
A propeller works like a screw turning through water. If the water were solid, each turn would drive the boat forward by exactly the pitch of the propeller, the distance the screw would advance in one turn. Water is not solid, so the propeller slips backwards as it turns, and the boat goes slower than the screw theory predicts. This propeller slip calculator measures that gap. You enter the propeller pitch, the gear ratio, the engine speed, and the boat’s actual speed, and it gives you the theoretical speed, the shaft speed, and the slip percentage.
Slip is the single most useful number for judging whether a propeller suits a boat. Too little slip can mean the propeller is over-pitched or the measurements are wrong. Too much slip can mean the propeller is under-pitched, damaged, ventilating, or simply wrong for the hull. Most healthy installations fall in a known band, so a slip figure outside that band is a clear signal to investigate, rather than a mystery to live with.
How to use it
Find the propeller pitch in inches, stamped on the hub or listed in the propeller’s data. Enter the engine speed in revolutions per minute at the speed you measured, and the gear ratio of your gearbox, written as a number like 2.0, meaning the engine turns twice for each turn of the shaft. Then enter the boat’s actual speed through the water in knots, measured by a log or GPS in calm water with no current, at the same throttle setting as the engine speed. Press the button and read the three results.
Measure speed and engine revolutions at the same moment, on a steady run. Speed over ground from a GPS includes current, so a run with the tide will flatter the propeller and a run against it will make it look worse. The cleanest check is two runs in opposite directions over the same water, averaged together, which cancels most of the current. Do the same at several throttle settings, and you get a slip curve instead of a single point, which tells you far more about the installation.
The maths behind slip
First the tool divides engine speed by the gear ratio to get shaft speed. A 4,000 rpm engine with a 2.0 gearbox turns the shaft at 2,000 rpm. Then it works out the theoretical speed: pitch in feet, times shaft revolutions per minute, times 60 minutes, divided by 6,076 feet per nautical mile. A 15-inch propeller, which is 1.25 feet of pitch, at 2,000 shaft rpm gives 1.25 times 2,000 times 60 divided by 6,076, which is about 24.7 knots. That is how fast the boat would go if the propeller screwed through solid water with no slip at all.
Slip is then the difference between theoretical and actual speed, divided by theoretical speed, times 100. If the boat actually does 20 knots against a theoretical 24.7, the slip is about 19 percent. In other words, nearly a fifth of the propeller’s advance is lost to the water moving out of the way. That loss is normal and unavoidable. A propeller needs some slip to generate thrust, because thrust comes from accelerating water backwards, and a propeller with zero slip would be pushing against water that never moves.
A worked example
Take the numbers above: 15-inch pitch, 4,000 engine rpm, 2.0 gear ratio, and 20 knots actual. Shaft speed is 2,000 rpm. Theoretical speed is about 24.7 knots. Slip is about 19 percent. For a planing boat, that is on the high side of normal, and it suggests checking a few things: is the propeller the right diameter, is it damaged or fouled, is the engine reaching its rated maximum speed, and is the boat heavier than usual? For a heavy displacement boat, 19 percent is unremarkable, because displacement hulls always slip more than planing hulls.
Limits and safety
This calculator assumes a single propeller, a fixed pitch, and a direct gearbox ratio. It does not model variable-pitch propellers, surface-piercing drives, jet drives, or twin installations where the two propellers interact. It also assumes your pitch, ratio, and speed figures are correct, and a small error in any of them moves the result. Treat the slip figure as a diagnostic aid, not a verdict.
This tool is an educational estimator only. It does not replace the propeller maker’s guidance, the engine maker’s rated speed range, or a professional propeller shop’s advice. Choosing or changing a propeller affects engine loading, and an over-pitched propeller can overload and damage an engine, so any propeller change should be checked against the engine’s rated maximum speed under load. Use the calculator to understand your installation and to have an informed conversation with a specialist.
Common questions
What if my slip is negative? Check your inputs first, especially the gear ratio and the pitch. Negative slip means the boat is going faster than the screw theory allows, which only happens with measurement error or with a variable-pitch or surface drive the formula does not cover. Does a bigger pitch always mean more speed? No. A bigger pitch raises theoretical speed but also loads the engine more, and if the engine cannot reach its rated speed, the boat may actually go slower. The right pitch lets the engine reach its rated maximum under normal load.
Versión en español: Calculadora de Deslizamiento de Hélice