Propeller Slip Tool: Pitch, RPM, Speed & Propeller Efficiency

Propeller Slip • Pitch Speed • Shaft RPM • STW • Current Correction • Advance Ratio • Wake • Open-Water Efficiency • Propulsive Efficiency

Propeller Slip Calculator: Pitch, RPM, Speed & Propeller Efficiency

Calculate propeller theoretical speed, shaft RPM, advance per revolution and geometric slip from pitch and vessel speed. Simple mode answers the core slip question quickly. Advanced adds current-corrected speed through water, propeller diameter, wake fraction, advance ratio, thrust deduction, open-water coefficients or direct efficiency, relative-rotative efficiency, drivetrain losses, thrust/torque estimates and a full propulsive-efficiency chain.

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Need much more depth? Click Advanced above.Advanced adds current correction, propeller diameter, wake fraction, advance ratio, thrust deduction, K_T/K_Q or direct open-water efficiency, relative-rotative efficiency, shaft/gear efficiency, thrust and torque estimates, and RPM/pitch sensitivity.

Fast6 primary controls

Core answerPropeller slip %

ClearSlip ≠ propeller efficiency

Fast Propeller Slip Screen

Enter the effective pitch actually being used. For controllable-pitch propellers, do not assume pitch-setting percentage is linearly equal to geometric pitch unless the maker’s data says so.

Effective propeller pitchmetres per revolution.

Engine RPMrpm.

Gear ratioEngine RPM ÷ shaft RPM. Use 1.00 for direct drive.

Speed basis

Vessel speed through waterknots.

Along-track currentknots. Positive = following current, negative = head current. Used only with SOG basis.

Propeller, Shaft & Speed Basis

Propeller type

Effective pitchm/rev at the operating point.

Propeller diameterm.

Engine RPM

Gear ratioEngine RPM ÷ shaft RPM.

Speed basis

Vessel speed through water

Along-track currentknots. Positive following, negative head current.

Hull–Propeller Interaction

Use vessel/model-test/CFD or approved performance data where available. The tool does not infer these coefficients from vessel type.

Effective wake fraction (w)Propeller advance speed Vₐ = Vₛ × (1 − w).

Thrust deduction factor (t)

Relative rotative efficiency (ηR)

Water densitykg/m³.

Open-Water Propeller Basis

Open-water input method

Open-water efficiency η₀Used only in direct η₀ mode.

Thrust coefficient K_T

Torque coefficient K_Q

Drivetrain & Operating Check

Shafting efficiency ηS

Gearing efficiency ηGUse 1.000 for direct drive.

Measured delivered / shaft poweroptional kW. Compared with KQ-derived delivered power when coefficients are used.

Measurement Readiness

Effective pitch verifiedPitch is the actual geometric/effective operating pitch. Shaft RPM / gear ratio verifiedTachometer and gearbox basis checked. Speed through water basis verifiedCurrent correction applied if only SOG is available. Weather / manoeuvring effects limitedSteady straight-line operation is reasonably represented. Hull / propeller condition consideredFouling, damage and roughness may change slip and performance. Wake / thrust-deduction source verifiedInteraction coefficients come from an appropriate vessel basis. Open-water data matchedK_T, K_Q or η₀ corresponds to the propeller and operating advance ratio. CPP / nozzle configuration matchedAny pitch setting, duct or propulsor configuration matches the reference data.

Engineering limitation: geometric propeller slip is a useful operational indicator, but it is not the same as propeller or propulsive efficiency. Slip can move with current, wake, sea state, draft/trim, hull fouling, propeller condition, manoeuvring and measurement error. Open-water efficiency and full propulsive efficiency require hydrodynamic coefficients or validated vessel-specific data. Use approved propeller curves, model-test/CFD results and sea-trial/performance data for engineering decisions.

Methodology context: Wärtsilä defines propeller slip as the difference between actual ship travel and the theoretical travel from propeller pitch × revolutions, and notes that apparent slip can be negative under following-current or wind conditions. ITTC’s 2024 Hydromechanics Dictionary defines open-water propeller efficiency η₀ = TVₐ/(2πQn), hull efficiency ηH = (1−t)/(1−w), quasi-propulsive efficiency ηD = η₀ηHηR, and overall propulsive efficiency ηP = η₀ηHηRηSηG. ITTC performance procedures use advance speed, advance coefficient, wake fraction, thrust deduction and open-water propeller data as separate hydrodynamic quantities. The tool follows those distinctions and never treats 100 − slip as propeller efficiency.

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