What is the recommended action for an emergency stop of a main turbine rotor while underway at sea?

Admitting astern steam to the turbine after securing ahead steam is the recommended action for an emergency stop of a main turbine rotor while underway at sea. This procedure allows for controlled deceleration of the turbine, which helps to prevent mechanical shock and damage that could occur if the turbine were to stop abruptly. By directing astern steam to the turbine, the vessel can reverse its propulsion, effectively using the energy in the steam to slow down or stop the rotor safely. This technique promotes a gradual and controlled halt, which is critical in maintaining the integrity of the turbine and other associated machinery.

In an emergency situation, simply shutting down the fuel supply can lead to a rapid and potentially damaging stop, and reducing engine speed gradually may not provide the immediate action needed in a critical scenario. Engaging the emergency generator is unrelated to stopping the turbine and does not address the immediate propulsion concerns. Therefore, admitting astern steam to the turbine after securing ahead steam is the most effective method for managing the emergency stop safely.