How do I calculate prop slip?

Prop slip = ((Theoretical Speed − Actual Speed) / Theoretical Speed) × 100. Theoretical speed is the speed the prop would produce if water were solid: (Pitch × RPM) / (Gear Ratio × K), where K = 1056 for inches and mph. For a 21-inch pitch at 5000 RPM with a 2:1 gear ratio, theoretical speed is 49.7 mph. If you're actually running 44 mph, slip is 11.5%.

What is a good prop slip percentage?

For most planing boats, 10–15% is a healthy range. Performance boats and high-revving outboards often run 5–10%. Pontoons routinely slip 15–20% because of their low-aspect hulls, and sailboats with auxiliary engines commonly slip 20–30%. Use the boat-type selector in the calculator to compare your number against the expected band for your hull.

Why is my prop slip so high?

High slip (above ~20% on a planing boat) usually has one of four causes: (1) the prop is over-pitched and the engine isn't reaching its WOT RPM, (2) the engine is mounted too low or the anti-ventilation plate is deep in the water, (3) the prop is damaged, worn, or has a wrong blade pitch, or (4) you're measuring with a pitot-type speedometer that reads 5–15% low — always use GPS.

Does prop slip change with boat load or weather?

Yes. A fully loaded boat slips 2–5% more than an empty one. Heavy seas or head winds add a few percent. Bottom growth (barnacles, algae) increases slip noticeably. For comparable readings, measure at mid-tanks, neutral trim, calm water, and a clean hull.

How is theoretical boat speed calculated?

Theoretical speed = (Pitch × RPM) / (Gear Ratio × K). K is a unit-conversion constant: 1056 for pitch-in-inches and speed-in-mph, 1215.22 for knots, 656.17 for km/h. For metric pitch in centimetres, multiply each K by 2.54. The formula assumes the prop advances its full pitch per revolution — which nothing does in water, hence slip.

What's the difference between prop slip and prop walk?

Prop slip is the speed gap between theoretical and actual forward motion — a percentage. Prop walk is the sideways torque a spinning prop exerts on a boat's stern, which pushes the stern to one side (usually starboard with a right-hand prop in reverse). They're unrelated: you can have low slip and strong walk, or vice versa.

Can prop slip be negative or over 100%?

In clean water, no. If your actual speed exceeds theoretical speed the calculator shows a 'Check Inputs' warning — that usually means the pitch, gear ratio, or speedometer reading is wrong. The calculator caps slip at 100% (actual speed = 0), so values above 100% can't appear. In short: negative slip signals an input error, and the calculator prevents readings beyond 100%.

Can I use this as a Mercury prop slip calculator?

Yes. This prop slip calculator works with any outboard, sterndrive, or inboard — Mercury, Yamaha, Suzuki, Evinrude, Honda, MerCruiser, Volvo Penta, or any other brand. The formula is universal. Just enter your engine's RPM, lower-unit gear ratio, prop pitch, and GPS speed. It is not affiliated with or endorsed by Mercury Marine or any manufacturer.

Should I enter engine RPM or prop shaft RPM?

Enter engine (crankshaft) RPM and your lower-unit gear ratio. The calculator divides engine RPM by your gear ratio to find propshaft RPM internally. If you enter propshaft RPM directly, set the gear ratio to 1.00:1. Most outboard and sterndrive owners enter engine RPM because that's what the tachometer shows.

Does the prop slip calculator work in knots?

Yes. Use the speed-unit toggle next to the actual speed input to switch between mph, knots (kn), and km/h. The calculator uses the proper nautical-mile conversion constant (K ≈ 1215.22 for inches and knots) so slip calculations are accurate in any speed unit.

What pitch should I enter for a prop marked 14 × 21?

Enter 21. In a prop size like 14 × 21, the first number (14) is the prop diameter in inches, and the second number (21) is the pitch — how far the prop would advance in one rotation in a solid medium. Pitch is what drives the slip calculation. Diameter does not directly affect the slip formula, though it matters for overall prop selection.

Prop Slip Calculator

Engine RPM

rpm

WOT (wide-open-throttle) RPM at the engine crankshaft

Gear Ratio

Lower-unit ratio. Typical outboards: 1.7–2.3:1

Propeller Pitch

Nominal pitch stamped on the prop (the number after the × in sizes like 14 × 21)

Actual Boat Speed

GPS speed at the same RPM you entered above

Boat Type

Calibrates the interpretation bands to your hull type

Propeller Slip

How much the prop is slipping vs its theoretical pitch

Slip %

9.5%Theoretical: 49.7 mphExcellent· < 10%

Theoretical vs Actual Speed

The gap between them is the slip

Theoretical49.7 mph

Actual45.0 mph

Breakdown

The numbers behind the slip

Theoretical Speed

49.7mph

(pitch × RPM) / (gear ratio × K), K for mph

Speed Deficit

4.7mph

Theoretical minus actual (lower is better)

What This Means

Interpretation and pitch-sizing tip

Excellent — expected range for this boat type: < 10%

Slip is in the ideal range. Verify the engine reaches its rated WOT RPM — if it doesn't, the prop may be slightly over-pitched despite the low slip reading.

What Is Prop Slip?

The difference between what the prop should deliver and what it actually does

Propeller slip is the percentage gap between the theoretical speed a prop would produce if water behaved like a solid screw-thread, and the boat's actual measured speed. Water doesn't grip like metal, so a propeller always slips a little. That slip is necessary — it's how the prop generates thrust in the first place.

A well-matched prop on a planing boat typically slips 10%–15%. Less than that and you may be under-pitched (engine over-revs). Much more than that and you're likely over-pitched, dragging hardware, or the prop is damaged or mounted at the wrong height.

Quick reference

Low slip = efficient thrust, high top speed. High slip = engine working harder for less boat speed. But "good" slip depends on the hull— bass and performance boats expect single digits; pontoons and sailboats routinely run 20%+.

How to Use This Prop Slip Calculator

Five inputs, one result — here's what to enter and how to read it

1
Enter Engine RPM. Use the wide-open-throttle (WOT) RPM from your tachometer at the engine crankshaft — not the prop shaft. This is the RPM the engine is actually turning at full throttle.
2
Enter Gear Ratio. Your lower-unit gear ratio from the engine spec sheet. Typical outboards run 1.7:1 to 2.3:1. Stern drives and inboards vary. Don't use 1:1 unless you're measuring at the propshaft and entering propshaft RPM instead of engine RPM.
3
Enter Propeller Pitch. The nominal pitch stamped on your prop — for example, 21 is the pitch in a prop marked “14 × 21.” Switch between inches and centimetres with the toggle next to the input.
4
Enter Actual Boat Speed. GPS-measured speed at the same RPM you entered above. Choose mph, knots, or km/h with the toggle. Always use GPS — pitot-style speedometers can be 5–15% off and will skew your slip reading.
5
Read Your Result. The calculator displays your prop slip percentage, theoretical boat speed, speed deficit, and a pitch-sizing recommendation calibrated to your selected boat type. A slip in the green or yellow band is normal for most hulls.

How Prop Slip Is Calculated

The formula, the conversion constant, and a worked example

Two formulas do all the work. First, theoretical speed is how far the prop would advance per minute if it were threading through a solid — converted to your speed unit via a constant K. Then slip is simply the shortfall between theoretical and actual, as a percentage.

Theoretical Speed = (Pitch × RPM) / (Gear Ratio × K)

Slip % = ((Theoretical − Actual) / Theoretical) × 100

Conversion constant K

Pitch unit	Speed unit	K
inches	mph	1,056
inches	knots	1,215.22
inches	km/h	656.17
cm	mph	2,682.24
cm	knots	3,086.67
cm	km/h	1,666.67

Worked example

Outboard with a 6000 RPM WOT, 2.00:1 gear ratio, 24″ pitch, running 60 mph on GPS:

Theoretical = (24 × 6000) / (2 × 1056) = 68.18 mph
Slip = (68.18 − 60) / 68.18 × 100 = 12.0%
Interpretation: Good for a general planing boat

What's a Good Prop Slip Percentage?

Expected ranges by hull type — hulls slip very differently

Hull / Boat Type	Excellent	Typical	Investigate Above
Performance / Racing	< 5%	5–15%	20%
Bass / Walleye (planing)	< 8%	8–16%	20%
Offshore / Center Console	< 8%	8–16%	20%
General Runabout / Cruiser	< 10%	10–20%	25%
Pontoon	< 12%	12–20%	25%
Sailboat (auxiliary)	< 20%	20–30%	35%

These bands are guidance, not gospel. Load, trim, weather, and bottom condition all shift slip by a few percent. Use the calculator's boat type selector to re-band your result appropriately.

Using Slip to Choose a Pitch

A two-minute framework for dialling-in a prop

Prop selection is a balance between RPM and slip. Both numbers tell you something different, and they're easy to confuse.

RPM below target + slip low

Prop is over-pitched. Go −1 pitch to let the engine reach its WOT range.

RPM above red-line + slip low

Prop is under-pitched. Go +1 pitch to protect the engine and gain top-end.

RPM at target + slip high

Prop may be damaged, worn, or the engine is mounted too low. Inspect the prop and measure anti-ventilation plate height relative to the pad.

RPM at target + slip normal

You're dialled in. Don't chase a lower slip number — for many hulls it's physically unavailable.

Common Mistakes and Assumptions

Where prop-slip numbers go wrong

Speedometer, not GPS

Boat speedometers (pitot or paddlewheel) are often 5–15% off. Always use GPS speed for slip calculations — a 6 mph error turns a 12% slip into a 22%.

Advertised vs true pitch

Nominal pitch stamped on a prop can vary from its true pitch by 1–2 inches, especially on worn or repaired props. For diagnostic work, have the prop measured by a shop.

Wrong gear ratio

Some owners input engine RPM but use the propeller shaft ratio (always 1:1). Confirm the lower-unit ratio from your engine model's spec sheet.

Ignoring load and trim

Slip changes with boat load, trim, and water conditions. Measure at mid-tanks and neutral trim, in calm water, for a comparable number.

Frequently Asked Questions

Common questions about propeller slip, pitch selection, and typical ranges

Check two numbers together: WOT RPM and slip. If RPM is below your engine's rated WOT and slip is low, go down one pitch size to let the engine rev higher. If RPM is over red-line and slip is low, go up one pitch size. If RPM is right and slip is high (>20% on a planing boat), inspect the prop — the fix may be hardware, not pitch. The calculator gives a pitch suggestion based on your boat type and current slip.

Enter five values: (1) engine WOT RPM, (2) lower-unit gear ratio, (3) prop pitch from the stamp on the prop, (4) GPS-measured boat speed at that RPM, and (5) your boat type. The calculator instantly shows your slip percentage, theoretical boat speed, speed deficit, and a pitch-sizing suggestion. Choose inches or centimetres for pitch and mph, knots, or km/h for speed with the toggles next to each input.

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Last updated Jun 13, 2026