Which of the following correctly indicates a vessel not under command via lights?

• A. One all-round red light
• B. Two red lights in a diagonal line
• C. Two all-round red lights in a vertical line
• D. A red flag

Answer: (C)

Not under command is signaled at night by two all-round red lights in a vertical line. This arrangement clearly marks a vessel that cannot maneuver to keep out of the way, usually due to a failure or other incapacity. Because the lights are visible in all directions and stacked one above the other, other vessels can reliably recognize the status and must take appropriate action to avoid it.

The two red lights in a vertical line are unique to this situation, so they are the best match for signaling NUC at night. Other signals—such as a single red light, two red lights in a diagonal line, or a red flag—do not represent the Not Under Command indication and would not convey the same meaning to other mariners.