PE = RT · Vs (resistance × ship speed through water). Export PNG/PDF.
PE: — kW (— hp)
Inputs used: RT=— kN, Vs=— kn (— m/s)
Effective Power, written as PE, is the power required to tow a ship at a given speed in calm water. It is the cleanest “hydrodynamics-only” power number because it comes directly from total resistance and ship speed through the water.
In practice, you typically obtain resistance from a method such as Holtrop–Mennen, Savitsky, model tests, CFD, or sea-trial analysis, then convert that resistance into a power requirement using this calculator.
The definition is:
PE = RT · Vs
Where RT is the total resistance (usually in kN) and Vs is the ship speed through water (kn or m/s). The calculator handles unit conversion internally and reports power in kW and hp.
Total resistance is the sum of multiple physical components. Depending on the method you used, RT may include:
If your RT excludes some items (for example, appendages or air), your PE will also exclude them. The number is only as complete as the resistance source you feed into it.
PE is not what the engine “sees.” It is the hydrodynamic requirement at the hull. Real propulsion systems have interaction effects and losses, so you typically move along the power chain:
A common (engineering) relationship uses propulsion efficiency factors:
PB ≈ PE / (ηH · ηR · η0 · ηS)
Where: η0 is propeller open-water efficiency, ηH is hull efficiency (wake & thrust deduction), ηR is relative rotative efficiency, and ηS is shaft/gearbox mechanical efficiency (if included). This is why two ships with the same PE can require different engine powers.
Effective power rises rapidly with speed. Even without getting into detailed scaling laws, most displacement ships show a steep increase in resistance around their operating range, which means PE grows much faster than speed. This is why “+1 knot” can be a huge fuel and power penalty.
Use this calculator to compare scenarios: different speeds, different hull conditions (clean vs fouled), or different loading drafts (which influence resistance).
If you are working from sea-trial data, ensure you correct for wind/current and ideally normalize to calm-water conditions before treating the result as a “design” PE.
Effective power is the starting point for the propulsion power chain. Continue with:
Tip: If your PE looks “too low” compared to the installed engine power, remember that PE is calm-water towing power. Sea margin, engine margin, fouling, weather, and drivetrain/propulsion losses can easily move required brake power much higher.