Propeller Slip Calculator: Prop Slip %, Theoretical Speed and Pitch Choice
Calculate boat propeller slip from engine RPM, gear ratio, prop pitch and GPS speed in mph, knots or km/h. Or predict top speed from an assumed slip, or find the pitch for a target speed, with a WOT rpm check and a pitch-change table.
How it works
prop rpm = engine rpm ÷ gear ratio · theoretical speed (mph) = prop rpm × pitch (in) ÷ 1,056 (knots: ÷ 1,215.2; km/h: ÷ 656.17) · slip % = (1 − actual speed ÷ theoretical speed) × 100 · speed = theoretical × (1 − slip) · pitch = speed × 1,056 × gear ratio ÷ (engine rpm × (1 − slip))
Pitch is how far a propeller would move forward in one turn if it were screwing through a solid. Multiplying the propeller's rpm by its pitch gives inches per minute, and dividing by 1,056 turns that into miles per hour (63,360 inches in a mile ÷ 60 minutes). Water gives way, so the boat always travels less than that theoretical figure, and the shortfall as a percentage is slip. Some slip is needed: a blade only makes thrust when it meets the water at an angle of attack. A well-set-up planing runabout usually runs about 8–15% at wide-open throttle, pontoons and heavier boats 15–25% or more, and displacement hulls far more. Very low slip usually points to a wrong gear ratio, tach error or a speedometer that reads high. High slip points to too much pitch for the load, ventilation, a damaged prop, a spun hub, a fouled bottom or an overloaded boat. Slip on its own does not choose a propeller. The engine must also reach its rated wide-open-throttle (WOT) rpm range. Each inch of pitch moves WOT rpm by roughly 150–200 rpm, so the table shows what one or two inches up or down would do, holding slip constant. Switch the mode to predict the speed of a prop from an assumed slip, or to find the pitch needed for a target speed.
Worked example
An outboard turns 5,500 rpm through a 1.86:1 gearcase with a 19 in prop. The prop shaft turns 5,500 ÷ 1.86 = 2,957 rpm, so the theoretical speed is 2,957 × 19 ÷ 1,056 = 53.2 mph (46.2 kn). The GPS shows 44 mph, so slip = (1 − 44 ÷ 53.2) × 100 = 17.3%. That is high for a planing runabout (8–15%), so look at load, trim, ventilation or prop condition. The 5,500 rpm sits inside the 5,000–6,000 WOT range, so the pitch suits the engine; a 17 in prop would turn about 5,850 rpm but, at the same 17.3% slip, top out near 41.9 mph, while a 20 in prop would drop to about 5,325 rpm for about 44.8 mph.
Sources: Omni Calculator: Prop slip · Best Calculators: Prop slip calculator (mph, km/h and knot constants)
Frequently asked questions
What is a normal propeller slip?
About 8–15% for a well-set-up planing runabout or bass boat at wide-open throttle, 15–25% for pontoons and deck boats, 15–30% for heavy cruisers and 25–45% for displacement hulls and sailboat auxiliaries. Slip is higher at lower speeds and with heavy loads.
Is lower slip always better?
Not below the normal band. Very low slip usually means an input error, such as the wrong gear ratio, tach error or a speedometer that reads high. A prop that really ran at 3% slip would be so heavily loaded that the engine could not reach its rpm range.
How much does an inch of pitch change speed and rpm?
Each inch of pitch changes wide-open-throttle rpm by roughly 150–200 rpm. Top speed changes less than you might expect, often 1–2 mph, and keeping the engine in its rated rpm range matters more than the last mph.
What is the constant 1,056?
It converts inches per minute to miles per hour: a mile is 63,360 inches and an hour is 60 minutes, and 63,360 ÷ 60 = 1,056. For knots the constant is 1,215.2 and for km/h it is about 656.2.
Why is my slip negative?
Negative slip is not physically possible. It means one of the inputs is off: the gear ratio, the pitch (props are sometimes re-pitched), the tachometer reading, or a speed taken with the current or wind behind you. Run both directions and average the GPS speed.
What causes high slip?
Too much pitch for the load, ventilation (engine mounted too high or a damaged anti-ventilation plate), a damaged or bent prop, a spun hub, a fouled hull, poor trim or an overloaded, stern-heavy boat.
Related calculators
Boating rules of thumb from common practice and propeller-maker guidance. Measure at wide-open throttle with a verified tachometer and GPS, and follow your engine maker's rated WOT range when choosing a propeller.