Propeller light running margin
Light and heavy running, the 4 to 10 percent light running margin recommended from 1 May 2015, and why the margin must rise as the specified MCR falls.
Light running margin is the deliberate offset between the propeller curve measured at sea trial and the engine’s layout curve, expressed as the percentage by which the trial propeller runs light of the design propeller curve. It exists so that when the hull fouls and the weather is against the ship, the propeller becomes heavier and the engine can still reach its rated power without being torque-limited.
MAN’s recommendation for new projects was revised from 3.0 to 7 percent to 4 to 10 percent with effect from 1 May 2015, applicable at all intended draughts and verified by averaging measured values across 50 to 100 percent load. A bollard-pull propeller curve is typically 15 to 20 percent heavier than the light propeller curve, and the choice of margin has a large effect on available power in that condition: taking 10 percent rather than 3 percent raises available bollard-pull power from 41 percent to 61 percent of specified MCR at 17.5 percent heavy running.
The full article will cover how light running is measured on trial, heavy running in service and its causes, and why a derated or power-limited engine needs the margin reconsidered rather than inherited. See sea margin and engine margin and slow steaming .