If you see only a green light over a white light and sidelights, what type of vessel is this?

The presence of a green light over a white light and sidelights indicates a vessel that is underway and possibly fishing. According to the COLREGs (International Regulations for Preventing Collisions at Sea), a fishing vessel under 50 meters in length that is actively trawling while making way will display this specific configuration of lights.

The green light signifies the starboard side of the vessel, while the white light indicates the stern. Sidelights are also present, helping to identify the vessel from the port and starboard sides. This light configuration allows other vessels to discern that the vessel is engaged in fishing while also navigating.

In contrast, the other options do not align with the described lighting configuration. A tugboat typically has additional lights indicating its role and the type of tow. A fishing vessel at anchor would show different lights, usually a single white light at the stern, and a sailing vessel typically has its own unique light arrangement, differing from what is described here. Thus, the specific arrangement of lights correctly identifies it as a vessel under 50 meters trawling and making way.