De-rating a propulsion engine means operating it at a lower maximum continuous rating (MCR) than its design rating, i.e. limiting the maximum power/speed to a value below the engine's full capability. The principle is to select a lower "de-rated" MCR (e.g. 80% of the design MCR) and to match the propeller to that lower power, so the engine runs at a lower load and speed. The benefits:

1. Lower specific fuel consumption (SFOC): an engine running at a lower percentage of its (de-rated) MCR often operates in a more efficient region, giving lower fuel consumption per unit power.
2. Lower thermal and mechanical loading: the engine components (bearings, pistons, liners, exhaust valves) are less stressed, giving longer component life and lower maintenance.
3. Lower emissions: lower load gives lower NOx, SOx, and CO2 emissions.
4. Increased reliability and reduced maintenance costs.
5. The engine can be optimised (e.g. injection timing, turbocharger) for the de-rated operating point.

Can a derated engine be run at full power? Yes, a derated engine can be run at its full (design) power, but only under certain conditions:

1. The engine must be capable of delivering the full power (it is the same engine, just rated lower).
2. Running at full power would exceed the de-rated MCR, so it would be running at a higher load than the de-rated rating, which would increase the thermal/mechanical loading and the fuel consumption, and could reduce the component life.
3. It is usually only done for a limited time (e.g. to make up lost time or in an emergency), and the engine must be monitored closely.
4. The propeller may not be matched to the full power (if the propeller is designed for the de-rated power, running at full power would overload the engine or the propeller).

So a derated engine can be run at full power only if the engine and propeller can handle it and it is done for a limited time with monitoring.

The de-rating of an existing propulsion engine must be done in accordance with approved procedures:

1. The de-rating is proposed and documented, specifying the new (de-rated) MCR, the new speed, and the operating parameters.
2. The engine manufacturer's approval is obtained, and the de-rating is carried out to the maker's instructions (e.g. adjusting the fuel injection, the turbocharger, the governor, and the load limits).
3. The propeller is re-matched (or a new propeller fitted) to the de-rated power, so the engine operates at the correct load/speed.
4. The engine is tested (on the test bed or in service) to verify the de-rated performance (power, speed, fuel consumption, emissions) and that it operates within the limits.
5. The engine's documentation (the engine log, the NOx Technical File, the EIAPP certificate) is updated to reflect the de-rated rating.
6. The de-rating is approved by the classification society and the flag state, and the ship's documentation is updated.
7. The operating procedures and the watchkeeping are updated for the de-rated operation.

The de-rating is thus carried out in a controlled, documented, and approved manner.