Brake Power (P_B)

Brake Power (P_B) in Marine Propulsion

Brake Power, denoted as P_B, represents the mechanical power produced at the engine crankshaft. It is the highest power level in the classical ship propulsion power chain and defines the actual load imposed on the main engine.

Unlike effective and delivered power, brake power includes all hydrodynamic, propulsive, and mechanical losses between the hull and the engine.

Position in the propulsion power chain

Marine propulsion power is commonly described as a sequence of power levels:

• P_E — Effective Power required to overcome hull resistance.
• P_D — Delivered Power at the propeller shaft.
• P_B — Brake Power produced by the engine.

Brake Power is always greater than Delivered Power due to propeller inefficiency and drivetrain losses.

Definition and fundamental equation

When Delivered Power and propeller efficiency are known, Brake Power is defined as:

P_B = P_D / η₀

Here, η₀ is the open-water propeller efficiency, which accounts for losses associated with converting shaft power into useful thrust.

Full propulsion efficiency chain

When starting from hull resistance, Brake Power can be computed using the full propulsion efficiency chain:

P_B = P_E / (η_H · η_R · η₀)

This formulation highlights the cumulative effect of hull–propeller interaction (η_H), rotational interaction (η_R), and propeller open-water efficiency (η₀).

Torque- and RPM-based formulation

Brake Power may also be obtained directly from engine torque and rotational speed:

P_B = 2π · n · Q

where n is the engine rotational speed (RPM/60) and Q is engine torque. This approach is commonly used with engine shop test data or onboard measurements.

Typical values and interpretation

• P_B > P_D — always true due to propeller losses.
• P_B > P_E — reflects total system losses.
• Modern merchant ships typically operate with η₀ ≈ 0.55–0.70.
• Brake Power determines engine rating, MCR, and fuel consumption.

Engineering use cases

• Main engine selection and rating.
• Fuel consumption and SFOC estimation.
• Power margin and sea margin assessment.
• Comparison of trial results with design predictions.

Related propulsion & power calculators

Brake Power is the final link in the propulsion power chain:

• Effective Power (P_E) – hull resistance power.
• Delivered Power (P_D) – shaft power at the propeller.
• Propeller Efficiency (η₀) – open-water propeller performance.
• Relative Rotative Efficiency (η_R) – hull–propeller rotational interaction.

Tip: Brake Power should always be evaluated with realistic efficiency values and appropriate sea margins to avoid engine under- or over-rating.