Propeller Slip Calculator

Shaft {{ resultsReady ? `${formatNumber(computation.values.shaft_rpm)} rpm` : '—' }} No-slip speed {{ resultsReady ? formatSpeed(computation.values.theoretical_speed) : '—' }}

Propeller pitch:

Use nominal pitch stamped on the propeller, not diameter.

Engine RPM:{{ rpm }} rpm

rpm

Gear ratio:

Engine revolutions per propeller revolution; enter 2 for 2:1.

Observed boat speed:

Use a steady speed reading at the same engine RPM.

Speed measurement:

Records whether current is included in the speed reading; no automatic current correction is applied.

Speed uncertainty (±):

Optional speed tolerance. Zero adds no interval; pitch and RPM uncertainty are excluded.

Measurement	Value	Unit	Copy
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The chart renderer is unavailable. The operating point still contains the speed values.

Measurement notes

Method: Mercury Racing. Unit factors: NIST SP811.

Calculation method

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The pitch stamped on a boat propeller describes theoretical forward travel, not a distance the boat is guaranteed to cover on every turn. Propeller slip expresses the difference between that nominal advance and observed travel as a percentage of nominal advance. A 10-inch-pitch propeller associated with 8.5 inches of forward travel per revolution, for example, has 15% apparent slip.

Relating a speed reading to propeller travel requires three distinctions.

Pitch and diameter: Pitch is a forward distance per revolution. Diameter is the width of the circle swept out by the blade tips. Substituting diameter for pitch gives the wrong theoretical advance.
Engine speed and propeller-shaft speed: Revolutions per minute, or RPM, must refer to the correct shaft. A 2:1 engine-to-propeller ratio means two engine revolutions for each propeller revolution, so the propeller turns at half the engine speed.
Observed advance: This is boat travel per propeller revolution, inferred from boat speed and shaft speed. It is not another dimension stamped on the propeller.

The word apparent matters because the result depends on how boat travel is measured. Global Positioning System (GPS) speed is relative to the ground and includes movement of the water. A following current can raise ground speed and reduce the calculated slip without a corresponding improvement in propulsion. A through-water instrument measures motion relative to the water, but its calibration and the flow near its sensor still affect the reading.

Slip is not the percentage of engine power or fuel being wasted. Thrust-producing blades work with pressure differences across their surfaces, and positive slip is compatible with normal thrust production. Reducing slip alone does not establish greater efficiency.

Repeatable measurements are useful when comparing a boat before and after a setup change, or comparing steady operating conditions. Load, trim, sea conditions and the measurement method belong alongside the percentage. A larger pitch increases nominal advance at a given shaft speed, but a different propeller can also change the RPM the engine actually reaches. Propeller selection therefore requires more than a low slip number, including the engine manufacturer's operating range and the intended load.

How to Use This Tool:

Use speed and engine RPM recorded together at one steady operating point. Results update automatically.

Enter Propeller pitch with its unit, Engine RPM and the installed Gear ratio. Pitch means nominal forward travel per revolution, not diameter. Enter 2 for a 2:1 ratio; direct drive is 1.
Enter Observed boat speed and choose its unit and Speed measurement basis. The basis records over-ground, through-water or unspecified measurement without correcting for current. In Advanced, set Speed uncertainty (±) to a nonnegative tolerance or zero for no interval. Unit changes convert the existing pitch, speed and speed tolerance.
Review Apparent propeller slip and Operating point. If Check operating point appears, correct the highlighted required input. A blank or invalid speed uncertainty instead leaves the central result available, marks the interval Unavailable and needs a supported tolerance or zero.

Interpreting Results:

No-slip speed is the geometric reference calculated from the entered pitch, RPM and ratio. It is not a predicted top speed or a promise that the boat can reach it.

Observed advance / revolution translates the speed reading into distance per propeller turn. Pitch minus observed advance shows the corresponding distance difference. Both use the selected pitch unit per revolution, so a pitch entered in inches produces advance figures in inches per revolution.

A negative result is retained and triggers a verification message. Check the nominal pitch, tachometer reading, installed gear ratio, speed units and measurement conditions before treating it as a change in boat performance. Current and differences between nominal and effective pitch can affect the result; a negative percentage is not evidence of extra efficiency.

There is no universal good-slip band here and no automatic fault diagnosis. Compare steady readings under similar load, trim and sea conditions, with the same measurement method. The Speed comparison chart contains the central theoretical and observed speeds, not an uncertainty band.

Technical Details:

Pitch supplies a distance per propeller revolution, and shaft speed supplies revolutions per unit time. Their product gives nominal advance speed. Comparing measured speed with that reference is a kinematic calculation of motion, not a calculation of thrust, shaft power or propulsive efficiency.

Pitch, shaft speed and apparent slip

First divide engine RPM by the engine-to-propeller ratio, then calculate no-slip speed and the percentage difference from observed speed.

\begin{aligned} n & = \frac{N}{G} \\ v_{0} & = \frac{P \times n}{60} \\ s & = 100 \times (1 - \frac{v}{v_{0}}) \end{aligned}

Here, N is engine RPM, G is the number of engine revolutions per propeller revolution, and P is nominal pitch in metres per revolution. The derived n is propeller-shaft RPM. Both v₀, the no-slip speed, and v, the observed speed, are in m/s. The result s is apparent slip in percent. The factor 60 converts minutes to seconds.

Observed advance per revolution, A, expresses the same comparison as a distance. The two difference values are theoretical speed minus observed speed and nominal pitch minus observed advance.

\begin{aligned} A & = \frac{60 \times v}{n} \\ Δ v & = v_{0} - v \\ Δ A & = P - A \end{aligned}

Advance and its difference are in metres per revolution before conversion to the selected pitch unit; the speed difference is in m/s before conversion. At fixed pitch, RPM and ratio, a higher observed speed lowers apparent slip. At fixed observed speed, increasing pitch or engine RPM raises the no-slip reference, while increasing the engine-to-propeller ratio lowers it.

Unit conversions

Pitch is converted to metres per revolution, and speed and speed uncertainty are converted to metres per second. These are the conversion factors used before the equations are evaluated.

Factors that convert supported pitch and speed units to the units used in the formulas
Entered unit	Multiply by	Resulting unit
in of pitch	0.0254	m/rev
mm of pitch	0.001	m/rev
cm of pitch	0.01	m/rev
km/h	1 / 3.6	m/s
kn	1,852 / 3,600	m/s
mph	1,609.344 / 3,600	m/s
m/s	1	m/s

A knot is one nautical mile per hour, using exactly 1,852 metres per nautical mile. Miles per hour use exactly 1,609.344 metres per mile. Equivalent physical inputs therefore give the same slip, apart from numerical rounding.

Sign boundaries, not performance ratings

Accepted pitch, engine RPM and gear ratio are positive, so no-slip speed is always positive. The following boundaries follow from the speed comparison alone.

Exact apparent-slip boundaries according to observed speed relative to no-slip speed
Observed speed	Apparent slip
Zero	100%
Above zero and below no-slip speed	Above 0% and below 100%
Equal to no-slip speed	0%
Above no-slip speed	Below 0%

Negative slip is not clipped to zero. Conversely, zero measured speed gives 100% apparent slip even though that stationary operating point says nothing about cruising performance. Negative boat-speed entries are not supported; they are different from a negative calculated slip.

Speed-only uncertainty

A nonnegative speed tolerance u creates two observed-speed scenarios. The lower speed stops at zero, while the upper speed is the observed speed plus the tolerance.

\begin{aligned} v_{low} & = \max (0, v - u) \\ v_{high} & = v + u \end{aligned}

Higher observed speed means lower slip, so the speed endpoints reverse when calculating the slip interval.

\begin{aligned} s_{low} & = 100 \times (1 - \frac{v_{high}}{v_{0}}) \\ s_{high} & = 100 \times (1 - \frac{v_{low}}{v_{0}}) \end{aligned}

The central result still uses the original observed speed. If the tolerance reaches or exceeds that speed, the upper slip endpoint is 100%. The upper speed scenario is not capped at the maximum permitted speed entry. Zero tolerance adds no displayed interval; a positive valid tolerance adds both speed scenarios and both slip endpoints.

This is a what-if interval, not a statistical confidence interval. It does not include uncertainty in pitch, engine RPM or gear ratio, and it does not automatically estimate or remove current. An invalid tolerance suppresses only the interval, not a valid central operating point.

Accepted ranges and numerical precision

All boundaries below are inclusive and apply after unit conversion. Required numbers must be finite and present. These are calculator input limits, not recommended vessel speeds or safe engine operating limits.

Inclusive physical input limits for the apparent propeller slip calculation
Quantity	Minimum	Maximum	Unit
Nominal pitch	0.001	5	m/rev
Engine speed	1	30,000	rpm
Engine-to-propeller ratio	0.1	20	engine rev/propeller rev
Observed speed	0	200	m/s
Speed uncertainty	0	200	m/s

The pitch limits are equivalent to 1 to 5,000 mm per revolution, and the speed and tolerance limits are equivalent to 0 to 720 km/h.

Calculations retain available numerical precision until display, which uses up to four decimal places. A very small nonzero result can therefore display as zero, while the unrounded value still determines whether the negative-slip message appears.

For observed speed and its tolerance only, a unit-conversion excess above 200 m/s is treated as exactly 200 m/s when it is no greater than the following allowance, approximately 0.000000000000178 m/s. Larger excesses and negative entries remain invalid.

4 \times 2^{- 52} \times 200 m/s

This allowance is four relative floating-point epsilons at the 200 m/s boundary and accommodates conversion rounding.

Nominal pitch is treated as a fixed advance per revolution. Effective pitch, cup (the curl at a blade's trailing edge) and hull wake are not modeled. A through-water reading does not establish the local inflow at the propeller. These equations do not predict hull performance, select a replacement propeller, diagnose damage or establish an engine RPM limit.

Worked Examples:

A steady reading with a speed tolerance

The initial sample uses 20 in of pitch, 5,000 engine rpm, a 2:1 ratio and 60.96 km/h measured over ground, with zero uncertainty. The pitch is 0.508 m per revolution and shaft speed is 2,500 rpm, giving a no-slip speed of 21.1667 m/s, or 76.2 km/h. Observed speed is 16.9333 m/s, giving 20% apparent slip using unrounded values. Observed advance is 16 in per revolution, 4 in less than nominal pitch. Adding a ±3.048 km/h tolerance leaves the central result at 20% and produces speeds of 57.912 to 64.008 km/h, corresponding to 16% to 24% slip. That range describes the entered speed tolerance only, not uncertainty from current or the other measurements.

Measured speed above the nominal reference

Keeping the sample pitch, RPM and ratio but entering 91.44 km/h produces −20% apparent slip. The no-slip reference remains 76.2 km/h, and theoretical minus observed speed becomes −15.24 km/h. The calculator keeps the negative result and asks you to verify the operating point. Confirm that speed and RPM were measured together, then check pitch, ratio, units and current conditions rather than interpreting the result as more than 100% efficiency.

References:

Prop Slip Calculator: What Is Prop Slip?, Mercury Racing.
Guide to the SI, Appendix B.8: Conversion Factors, National Institute of Standards and Technology, SP 811, 2008.