Prop Slip Calculator (theoretical vs actual boat speed)

Understanding the propeller slip calculator — concept and principle

Propeller slip is the difference between how far a propeller would travel through the water if it acted like a screw in solid wood and how far the boat actually goes. This calculator compares the theoretical speed from pitch, rpm and gear ratio with your GPS speed and gives slip in percent, plus the actual advance per propeller turn.

The prop would screw forward at 49.5 mph; the boat makes 45, so slip is 9.1 %.

How it is calculated

Theoretical speed (mph) = pitch (in) × rpm ÷ (gear ratio × 1,056)

Slip (%) = (1 − actual speed ÷ theoretical speed) × 100

Actual advance per turn = actual speed × 1,056 × gear ratio ÷ rpm

1,056 inches per minute equals 1 mph.
Theoretical speed is also shown in km/h and knots.

The principle

A propeller pushes water backward to create thrust. To do that it must move through the water a little faster than the boat moves forward, so it always "slips" somewhat. The pitch and the propeller's revolutions per minute give the theoretical speed; comparing it with the measured speed shows what fraction was lost. The same idea in reverse lets you choose a pitch for a target speed with the propeller pitch calculator.

Worked example

Default inputs: pitch 19 in, 5,500 rpm, gear ratio 2.0, measured 45 mph.

Propeller rpm = 5,500 ÷ 2.0 = 2,750.
Theoretical speed = 19 × 5,500 ÷ (2.0 × 1,056) = 49.5 mph = 79.6 km/h = 43 knots.
Slip = (1 − 45 ÷ 49.48) × 100 = 9.1%.
Actual speed = 45 mph.
Actual advance per propeller turn = 45 × 1,056 × 2.0 ÷ 5,500 = 17.3 in.

Each point is the calculator re-run with only “Propeller pitch (in)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Propeller pitch (in)	Propeller slip (%)
4.75	-263.8
9.5	-81.9
14.2	-21.3
19	9.1
23.8	27.2
28.5	39.4
38	54.5

How to read the result

Slip around 10% is typical for many fast, light boats, while heavier or slower boats often show more. A sudden increase in slip can point to a damaged propeller, ventilation, the engine mounted at the wrong height or a heavier load. A negative slip usually means a measurement error, for example a speed reading taken with a current or a tachometer that reads low.

Common mistakes

Measuring speed with the current or wind behind you.
Reading rpm and speed at different moments.
Using the propeller's label pitch when it has been reworked or damaged.
Forgetting the gear ratio.

Keep in mind: Slip is a diagnostic guide. For performance problems, consult a qualified marine technician and the engine maker's guidance.

Frequently asked questions

Is propeller slip bad?

No. A propeller needs some slip to produce thrust; only unusually high slip suggests a problem.

How do I measure speed accurately?

Use GPS on calm water, run in both directions and average to cancel current.

What does negative slip mean?

Usually a measurement error in speed, rpm or pitch.

Can slip change with load?

Yes, heavier loads usually increase slip.

Sources: Geometric pitch definition; slip = 1 − actual advance ÷ geometric advance; 1 mile = 63,360 in, 1 knot = 1.852 km/h

Checked 9 Nov 2026. Geometric pitch: forward travel per revolution (definition); slip = 1 − actual advance ÷ geometric advance; 1 mile = 63,360 in, 1 knot = 1.852 km/h (standard values).

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