How the Prop Slip Calculator Works
A boat propeller is a screw moving through a fluid, not a solid. In one revolution, a prop with a given pitch would theoretically pull the boat forward by exactly that pitch distance — if the water offered zero resistance. In reality, the blades push water backward to generate thrust, so the boat always advances less than the theoretical pitch distance per revolution. That shortfall, expressed as a percentage, is propeller slip. This calculator converts your prop pitch, engine RPM, and gear ratio into a theoretical "no-slip" boat speed, compares it against your actual GPS speed, and reports the slip percentage.
The theoretical pitch speed formula
Pitch is the distance (in inches) a propeller would advance through a solid medium in one full revolution. Multiplying pitch by the propeller shaft's revolutions per minute gives inches traveled per minute; converting inches/min to miles/hour (÷12 for feet, ×60 for hours, ÷5280 for miles) collapses to a single constant: ST (mph) = Pitch (in) × Prop RPM ÷ 1056. Because most tachometers read engine RPM rather than prop-shaft RPM, the calculator first divides engine RPM by the gear (reduction) ratio to get prop shaft RPM: Prop RPM = Engine RPM ÷ Gear Ratio. Use a gear ratio of 1 for a direct-drive setup with no reduction.
Calculating slip percentage
Once the theoretical speed (ST) is known, compare it to the boat's actual speed over the ground (SA), ideally read from a GPS chartplotter, GPS watch, or phone app rather than a paddlewheel log: Slip % = (ST − SA) ÷ ST × 100. If your actual speed is entered in knots or km/h, the calculator converts everything to a common unit before computing the percentage, then converts the theoretical speed and speed-lost-to-slip figures back to your chosen display unit. Some slip is unavoidable and even useful — a propeller with zero slip would be generating zero thrust — so the goal is not to eliminate slip but to keep it in the normal range for your boat type.
What affects propeller slip
Slip climbs with hull loading (fuel, gear, passengers), a fouled or damaged hull or prop, incorrect trim, cavitation or ventilation (air being drawn into the blades), and a pitch that is too aggressive for the boat and engine combination. Slip drops with a lighter, cleaner hull, a well-matched or cupped propeller, and proper trim that keeps the prop fully loaded in clean water. Comparing slip at different RPM and trim settings on the same boat is a practical way to diagnose whether a prop change, cleaning, or trim adjustment is worth pursuing.