Compute apparent slip from propeller pitch, shaft RPM, and ship speed through water. Visualise the theoretical vs actual advance and classify propulsion performance.
Input the geometric pitch in metres. This is stamped on the propeller boss or listed in the propeller certificate and ship drawings. For controllable-pitch propellers (CPP), use the actual pitch setting at the time of observation — not the design pitch.
Use the average RPM from the engine room revolution counter over the observation period. Reading the tachometer momentarily at one point is less accurate than dividing total shaft revolutions by elapsed minutes — especially in a seaway where RPM fluctuates.
Always use STW (speed through water) from the ship's electromagnetic or Doppler log — not GPS speed over ground (SOG). A favourable current inflates SOG and makes slip appear lower than it actually is, masking real performance issues.
The advance bar shows how much of each propeller revolution becomes actual ship motion (blue) vs slip (orange). The RPM sweep chart shows where your speed sits relative to the theoretical line — the gap is your slip speed in absolute knots.
Propeller slip is one of the most fundamental parameters in marine propulsion performance monitoring. It quantifies the difference between a propeller's kinematic advance — the distance it would travel if screwing through a solid medium — and the actual distance the vessel moves through the water per revolution. Understanding and tracking slip is essential for performance benchmarking, fuel efficiency management, and early detection of propulsion problems.
A propeller does not operate in a solid medium: water is a fluid that yields under thrust force. The propeller "slips" relative to the water, and this slip is precisely what generates thrust. A propeller with zero slip would produce zero thrust. What we seek in practice is the right amount of slip — enough to produce the required thrust efficiently, without excessive losses through over-loading or cavitation.
Where P = pitch (m), n = shaft RPM, Vt = theoretical speed (kn), Vs = actual ship speed through water (kn). The factor 60/1852 converts metres per minute to knots.
| Vessel / Condition | Typical Slip |
|---|---|
| Large tanker / bulker at sea speed | 5 – 12% |
| Container ship at full service speed | 8 – 15% |
| General cargo, medium speed | 10 – 20% |
| Tug or trawler (high thrust load) | 25 – 45% |
| Slow-steaming / part-load | 15 – 30% |
| Fouled hull or damaged propeller | > 30% |
| Light ship / ballast condition | 3 – 8% |
| Negative slip | Following current / data error |
Indicative values only. Always compare against your vessel's own baseline at similar draft and conditions.
The slip calculated using ship speed through water (Vs) is called apparent slip. It is "apparent" because it does not account for the wake field that surrounds the hull. The ship's hull drags water along with it, meaning the water entering the propeller disc moves more slowly than the ship itself.
Real slip (or true slip) compares propeller advance against the actual water velocity at the propeller — the speed of advance (VA):
where w is the wake fraction (typically 0.10–0.35 for conventional ships). Real slip substitutes VA for Vs:
Because VA < Vs, real slip is always larger than apparent slip. This calculator computes apparent slip, which is the practical, shipboard-measurable quantity. Real slip requires the wake fraction from model tests or CFD, which is rarely available operationally.
For trend monitoring purposes, apparent slip is entirely adequate — consistent changes over time directly reflect changes in propulsion performance regardless of the absolute wake fraction.
Tracking slip across voyage legs and comparing against reference conditions (same draft, calm weather, similar RPM) is the most reliable way to detect gradual performance degradation.
For operational performance monitoring, propeller slip is calculated at consistent intervals (typically once per watch or daily at noon) and logged against reference conditions. The key principle is comparison over time, not the absolute slip value.
The reference slip should be established shortly after dry-docking, when the hull is clean and the propeller polished. This gives the clean-ship benchmark. Subsequent deviations are attributed to fouling, damage, or changed operating conditions.
Raw slip comparisons are only valid at similar conditions. Normalise for:
Under the IMO Energy Efficiency Existing Ship Index (EEXI) and Carbon Intensity Indicator (CII) framework, propulsion efficiency directly affects a vessel's carbon rating. Increasing slip at constant power means more fuel burned per tonne-mile — a direct CII deterioration. Propeller and hull maintenance scheduling based on slip trend analysis is therefore not just good seamanship — it has regulatory and commercial consequences.
Pitch is geometrically fixed; speed is controlled by varying RPM. The design pitch is optimised for one specific RPM/speed combination. Off-design operation results in a non-optimal pitch ratio, typically producing higher slip. Slip monitoring for FPP is straightforward because pitch is constant and known.
Both pitch and RPM can be varied independently, allowing optimisation across a wide range of conditions. For CPP, slip calculations must use the actual pitch setting at the time of measurement — not the design pitch. Always check the pitch feedback indicator. CPP vessels running at reduced pitch to save fuel will show higher apparent slip if design pitch is used in the formula.
Slip in astern operation is fundamentally different from ahead running. When the propeller rotates in the astern direction, the inflow conditions are severely disrupted — blades face a reversed wake field, cavitation is more pronounced, and efficiency drops sharply. Astern slip values of 40–60% are not unusual and should not be compared with ahead-running values.
Negative slip in ahead operation (Vs > Vt) almost always indicates:
Consistently negative slip in ahead operation is never a sign of exceptional efficiency — it is almost always a data problem. Investigate the source of speed and RPM readings.
For a fixed-pitch propeller, Vt increases linearly with RPM. The actual ship speed Vs also increases with RPM, but more slowly — because hull resistance increases with approximately the square of speed, while propulsive power increases with the cube. This means propeller slip tends to increase as the vessel approaches maximum service speed.
The RPM sweep chart in this calculator plots Vt across ±50% of your entered RPM. The horizontal orange dashed line marks your actual Vs. The vertical gap between Vt and Vs at your RPM is the absolute slip speed in knots. Your operating point is marked with a dot on the Vt line.
For a well-maintained vessel running at design conditions, the operating point should sit close to the design line. As hull or propeller condition deteriorates, Vs falls below the reference line while RPM holds — the gap widens, and slip increases.