Derating

Derating runs a main engine continuously at a reduced maximum continuous rating, shifting the operating point to a lower specific fuel consumption and cutting CO2 per mile, often supported by retuned injection timing and turbocharger cut-out. It overlaps with the engine power limitation used for EEXI compliance but is chosen for fuel economy on a slow-steaming profile rather than to pass an index. The trade-off is reduced reserve power, so the derated MCR must still cover heavy-weather and maneuvering demand.