The main engine locking device should only be engaged when what condition is met?

The main engine locking device is designed primarily for safety and operational stability. Engaging the locking device under the condition that the ship is dead in the water and the propeller is not turning ensures that the engine and propeller are fully inoperable. This scenario eliminates any risk of accidental movement or propulsion, which is critical when performing maintenance or inspections.

When the ship is dead in the water, all systems are stable, and there is no torque or rotational force that can create a hazardous environment. In contrast, engaging the device when the ship is at full speed or when the engine is still in operation can lead to severe mechanical failure or unexpected movements. Similarly, the condition where the engine is cold or has been shut down may not adequately guarantee that all systems are safe from unintended activation, given that other supporting systems may still remain on or energized.

Thus, to ensure maximum safety and operational integrity, the locking device should only be engaged when the ship is dead in the water and the propeller is not turning.