About Propeller Sizing Calculator (Prop Slip, Pitch, RPM & Speed)
The propeller sizing calculator solves the standard prop slip relation four ways. At zero slip, a propeller of pitch P inches would advance P inches per revolution, so boat speed in mph is (engine RPM ÷ gear ratio) × pitch × (1 − slip) ÷ 1056 — the 1056 is not an empirical fudge but pure unit conversion, 63,360 inches per mile divided by 60 minutes per hour. Slip is the fraction by which the real boat falls short of that theoretical advance: water is a fluid, the blades must run at an angle of attack to generate thrust, and the wake never moves as fast as the blade geometry implies.
Pick which quantity to solve for and enter the other three. Solving for speed predicts WOT or cruise speed from RPM, gear ratio, pitch, and an assumed slip. Solving for slip takes a measured GPS speed and reports the actual slip — the standard prop-shop health check — and if your measured speed beats the zero-slip theoretical speed the tool flags it as a data error (usually a wrong gear ratio, or GPS knots entered as mph) instead of printing a meaningless negative slip. Solving for pitch or RPM sizes a new prop or predicts the RPM needed for a target speed. The slip badge rates the working slip against typical bands for your hull type: about 10–15% for a properly propped planing hull, and about 25–45% for displacement hulls per Dave Gerr's Propeller Handbook. The sibling hull speed calculator tells you which regime your hull is actually in.
How It Works
- Choose what to solve for: boat speed, actual slip %, required pitch, or required engine RPM. The selected quantity's input disappears and becomes the result.
- Enter the engine speed in RPM and the gear (drive) ratio — engine turns per propeller turn, e.g. 2.0:1. The propeller shaft turns at RPM ÷ gear ratio. Direct drive is 1.0; most outboards, sterndrives, and inboard reduction gears fall between about 1.1 and 3.
- Enter the propeller pitch in inches (from the prop marking, e.g. a 14¼ × 19 prop has 19 in of pitch) and either the slip percentage (when predicting speed, pitch, or RPM) or the measured boat speed (when solving for slip). Speeds can be entered in mph or knots — the toggle converts at 1.15078 mph per knot and results always show both.
- Pick your hull type. The badge rates the working slip against the typical band — planing 10–15%, displacement 25–45% — as guidance, not pass/fail: heavy planing cruisers legitimately run above the planing band, and a displacement workboat towing a load runs higher still.
- Read the result panel (mph and knots twins, plus the zero-slip theoretical speed when solving slip) and the chart, which plots boat speed against engine RPM at your pitch, gear ratio, and slip with your operating point marked.
Worked Example
A sterndrive runs 5000 RPM at wide-open throttle through a 2.0:1 drive, swinging a 19-inch-pitch prop, and the skipper assumes 12% slip. The prop shaft turns 5000 ÷ 2.0 = 2500 RPM. Zero-slip theoretical speed is 2500 × 19 ÷ 1056 = 44.981 mph, and at 12% slip the predicted boat speed is 2500 × 19 × 0.88 ÷ 1056 = 39.583 mph, or 34.397 knots. On a planing hull, 12% sits inside the 10–15% typical band. If the GPS instead showed 45 mph — faster than the 44.981 mph zero-slip ceiling — the inputs would have to be wrong: a negative slip is physically impossible, so the calculator flags the data error rather than reporting one.
Formulas
- Prop slip speed formula
V_mph = (RPM / gearRatio) * pitch_in * (1 - slip) / 1056- Actual slip from a measured speed
slip = 1 - V_actual / V_theoretical; V_theoretical = (RPM / gearRatio) * pitch_in / 1056- Pitch and RPM inversions (the relation is linear in both)
pitch_in = V_mph * 1056 * gearRatio / (RPM * (1 - slip)); RPM = V_mph * 1056 * gearRatio / (pitch_in * (1 - slip))- Knots conversion
V_kt = V_mph / 1.15078
Standards & References
- Dave Gerr, Propeller Handbook (International Marine) — apparent slip, typical slip values by boat type (about 26% for heavy powerboats at 9–15 kt up to 45% for auxiliary sailboats under 9 kt; curve fit slip ≈ 1.4/kt^0.57), and the pitch selection method
- Mercury Racing, "Prop School Part 6: Slip" — the 10–15% typical slip convention for properly propped planing hulls
- Unit derivation: 1056 = 63,360 in/mi ÷ 60 min/h; 1 knot = 1.15078 statute mph (1852 m nautical mile)
Frequently Asked Questions
What is propeller slip, and is it a measure of inefficiency?
Slip is the difference between the theoretical advance the prop's pitch implies and the distance the boat actually travels, expressed as a percentage of the theoretical speed. It is not a defect or a direct efficiency loss: a propeller blade is a foil and must run at an angle of attack to generate thrust, which is impossible at zero slip. Every working prop slips — the question is whether the number is typical for the hull type, load, and speed.
What is a normal slip percentage for my boat?
For a properly propped planing hull at wide-open throttle, about 10–15% is the widely used convention (Mercury Racing and most prop shops); light race boats can get below 10%. Displacement hulls run far higher — Dave Gerr's Propeller Handbook gives typical values from about 26% for heavy powerboats at 9–15 knots to 45% for auxiliary sailboats under 9 knots, which is why this calculator bands displacement hulls at 25–45%. Heavy planing cruisers sit between the bands, often 15–25%, so treat the badge as guidance rather than pass/fail.
The calculator says my speed is faster than theoretically possible — why?
A boat cannot outrun the zero-slip speed its prop geometry implies, so a measured speed above (RPM ÷ gear ratio) × pitch ÷ 1056 means one of the inputs is wrong. The usual culprits: the gear ratio is not what you think (check the drive model, not the engine brochure), the GPS was reading knots while you entered mph (knots are 15% bigger), the tachometer over-reads, or the prop has been re-pitched from its marking. Rather than display a physically meaningless negative slip, the tool flags the data error.
Where does the 1056 constant come from?
It is pure unit conversion, not an empirical factor. Pitch in inches times shaft RPM gives inches of theoretical advance per minute. There are 63,360 inches in a statute mile and 60 minutes in an hour, so dividing inches-per-minute by 63,360 and multiplying by 60 — equivalently, dividing by 63,360 ÷ 60 = 1056 — yields statute miles per hour. If you want knots directly, divide the mph result by 1.15078.
What gear ratio and RPM should I enter?
Enter the engine (crankshaft) RPM from the tachometer and the total gear ratio between engine and propeller shaft — for an outboard or sterndrive that is the drive's ratio (e.g. 1.86:1), for an inboard it is the reduction gear (e.g. 2.5:1), and for direct drive enter 1.0. The calculator divides engine RPM by the ratio to get prop shaft RPM. Getting the ratio wrong is the most common cause of impossible slip numbers, because it scales the theoretical speed directly.
How do I use this to pick a prop pitch?
Solve for pitch: enter the engine's rated WOT RPM, the gear ratio, your target top speed, and a typical slip for your hull type (start at 12% planing, 30–35% displacement). The result is the pitch that hits the target at rated RPM. Round to the nearest stock pitch and remember the rule of thumb that one inch of pitch shifts WOT engine speed by roughly 150–200 RPM: if the engine over-revs past its rated band, add pitch; if it cannot reach rated RPM, reduce pitch. Whether your target speed is even achievable is a hull question — check the hull speed calculator for displacement limits.