Result

The theoretical speed here treats the propeller as a screw advancing one full pitch per revolution through a solid medium, which water is not — so theoretical speed is always higher than reality and slip is the normal, expected difference rather than a fault. Because everything is derived from that idealisation, this is a selection and diagnosis aid, not a performance prediction: it takes no account of hull resistance, displacement, trim, blade count, blade area or cup, engine torque curve, transom height, ventilation or the state of the bottom, all of which move real speed substantially. Slip figures are only as good as the speed measurement behind them, so use GPS speed over ground taken on a calm day and, where current is significant, average reciprocal runs in both directions — a paddlewheel log or a single downstream run will produce misleading numbers, and a negative slip result almost always means a measurement problem rather than a remarkable propeller. Pitch-to-diameter reference bands are broad generalisations across hull families and a well-matched propeller can sit outside them. The pitch-change rule of thumb assumes everything else stays constant and gets less reliable the further you move from the current propeller. Confirm any final propeller selection against the engine and gearbox manufacturer's recommendations and, where possible, by testing on the water.