BOTH INTERNATIONAL & INLAND You see a vessel displaying three lights in a vertical line. The highest and lowest lights are red and the middle light is white. She is also showing a white light at the stern, which is lower than the forward light. Which vessel could you be observing?

Correct Answer: Option D — a survey vessel

Rule 27(b) describes the lights for a vessel restricted in her ability to maneuver. A survey vessel, when conducting operations that restrict her maneuverability, displays these lights. The additional white stern light indicates the vessel is underway, and the configuration matches the requirements for a vessel engaged in survey work.