Marine Engine Propulsive Efficiency Calculator
Power and operating-point data
Propulsion power result
Enter a matched operating point and calculate.
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Marine propulsive efficiency formulas
Effective power is the power required to tow the vessel at a constant speed in undisturbed water. Use total towing resistance and ship speed in consistent SI units:
Effective power, Pᴇ (W) = total resistance, Rₜ (N) × ship speed, V (m/s)
Quasi-propulsive coefficient, ηᴅ = Pᴇ ÷ delivered propeller power, Pᴅ
Overall propulsive efficiency, ηₚ = Pᴇ ÷ engine brake power, Pʙ
When brake power is selected, the optional shafting and gearing efficiency estimates delivered power as Pᴅ = Pʙ × ηₛ₊ɢ. It does not change the overall Pᴇ ÷ Pʙ result.
Choose the correct power boundary
| Selected input | Reported ratio | Included after input point |
|---|---|---|
| Delivered propeller power, Pᴅ | Quasi-propulsive coefficient, Pᴇ/Pᴅ | Propeller, relative-rotative, and hull interaction effects |
| Engine brake power, Pʙ | Overall propulsive efficiency, Pᴇ/Pʙ | Shafting/gearing plus propeller and hull interaction effects |
Do not substitute rated engine power automatically. Nameplate maximum power is not necessarily brake power at the observed speed and load. Use a measured or defensible operating-point value with the same basis as the comparison data.
How to use this calculator
- Select delivered power when power at the propeller is known, or engine brake power when power at the engine coupling is known.
- Select the unknown. Efficiency is the normal choice for an observed operating point; the reverse modes apply a target efficiency to solve one missing value.
- Enter towing resistance, speed through water, and input power from the same steady condition. Select each unit explicitly.
- For brake power, enter a documented shafting and gearing efficiency only if an estimated delivered-power breakdown is useful.
- Review the boundary label, effective power, and warnings. Confirm the measurement and correction methods before comparing trials or designs.
Worked power-balance example
For a vessel with 350 kN total resistance at 15 kn, effective power is approximately 2,700.8 kW. If 5,500 kW is delivered at the propeller, the quasi-propulsive coefficient is approximately 49.11%.
350,000 N × (15 × 1,852 ÷ 3,600) m/s = 2,700,833 W
2,700,833 W ÷ 5,500,000 W × 100 = 49.11%
This is an illustrative arithmetic example, not a representative target for every vessel type or operating condition.
Marine propulsive efficiency FAQs
What is marine propulsive efficiency?
ITTC defines it as effective power divided by engine brake power. Effective power is total vessel resistance multiplied by ship speed. The ratio includes losses and interactions downstream of the engine coupling.
What is the quasi-propulsive coefficient?
It is effective power divided by delivered power at the propeller or propulsion device. It excludes shafting and gearing losses between the engine and propeller, so its boundary differs from overall propulsive efficiency.
Should I enter propeller thrust as vessel resistance?
No. Effective power uses total towing resistance at ship speed. Propeller thrust and towing resistance are not interchangeable because propeller operation changes the pressure field around the hull. Use resistance from a suitable test or analysis.
Should I use speed through water or GPS speed?
Use speed through the water matched to the resistance and power condition. GPS speed over ground includes current. Formal speed-power trials require defined measurements and corrections beyond this calculator.
Why is my result above 100%?
First check unit choices, whether power is per shaft or total, whether all propulsors are included, and whether resistance, speed, and power describe the same point. Hydrodynamic coefficient boundaries can be subtle, but an unexpected value above 100% is a strong reason to audit the inputs and definitions.
Does the calculator include engine thermal or fuel efficiency?
No. It begins at brake power or delivered propeller power. Fuel energy, specific fuel consumption, electric generation and conversion, auxiliary loads, and engine thermal losses ahead of the selected point are outside the model.
Can I compare values from different weather or loading conditions?
Not without suitable corrections. Wind, waves, current, shallow water, displacement, trim, hull roughness, fouling, propeller condition, and measurement method can all change the speed-power relationship.
Are the entered values tracked or stored?
No. This calculator processes the values locally and does not send, store, or add them to analytics events.
Limits and engineering disclaimer
- The calculation is a steady-state scalar power balance. It does not derive hull resistance, propeller open-water efficiency, wake fraction, thrust deduction, torque, cavitation, ventilation, or engine loading.
- Resistance and effective power depend on the specified hull condition, appendages, displacement, trim, water depth, water properties, wind, waves, and correction method.
- The optional transmission percentage is user-supplied. The calculator does not estimate losses in bearings, stern tube, gearbox, couplings, generators, converters, motors, or auxiliaries.
- Reverse calculations hold resistance or efficiency constant. Real resistance and efficiency normally change with speed, RPM, loading, and sea state.
Engineering disclaimer: This educational tool is not a substitute for model testing, a resistance and propulsion study, an ITTC/ISO-compliant sea trial, machinery manufacturer data, class requirements, or analysis by a qualified naval architect or marine engineer. Do not use it alone to size engines, shafts, propellers, or safety-critical equipment.
Methodology and sources
Last reviewed: August 2, 2026. Definitions and equations were checked against the current ITTC hydromechanics dictionary; operating-condition cautions and unit factors were checked against authoritative references: