What does a red over red light indicate for a vessel?

A red over red light indication signifies that a vessel is not under command. This means that the vessel is unable to maneuver as required under the rules of the road, often due to mechanical failure or other circumstances that prevent it from following the regulations for navigational lights. The red over red configuration is a clear visual signal to other vessels that they should exercise caution and be aware that the vessel is in a situation where it cannot take action to avoid collision.

This signaling is vital for navigation safety, especially in crowded waterways, as it alerts other vessels to the potential dangers of interacting with a vessel that is unable to maneuver. In contrast, the other options represent different conditions that vessels may be in, such as being anchored or aground, but do not share the same critical indication as a vessel that is not under command.