What light configuration indicates a vessel at anchor?

A vessel at anchor is required to show a single all-round white light. This configuration is specifically mandated by the COLREGs to provide visibility and indicate the vessel’s status to other vessels. The single all-round white light is typically displayed at the stern or in a central location to be seen from all directions.

The rationale for this configuration is to ensure that other vessels can easily recognize that the anchored vessel is not under way and is effectively stationary, thus reducing the risk of collisions. This light must be visible over a certain distance and under various conditions, making it an essential part of safe navigation.

The other options do not correspond to the rules governing vessels at anchor. The two all-round white lights denote a vessel engaged in specific operations, while a red light over a white light represents a vessel constrained by its draft. An all-round green light indicates a vessel under way, particularly one that is engaged in fishing. Thus, they do not serve to indicate that a vessel is at anchor, reinforcing the correctness of the single all-round white light configuration for this situation.