What does a side light indicate about a vessel?

A side light is an essential navigation light and is used to indicate the position and movement of a vessel. When a vessel displays a side light, it specifically indicates that the vessel is underway and not anchored. This means that the vessel is currently in motion, navigating through the water.

Under the COLREGs, vessels must exhibit specific lights when they are underway, including side lights that shine green on the starboard side and red on the port side. This helps other vessels understand the direction in which the vessel is heading and makes it safer for navigation and compliance with the rules of the road at sea.

The other options relate to specific scenarios that do not align with the general indication of a side light. For instance, a vessel that is anchored is not considered underway and typically shows an anchor light at its stern. Similarly, a towing vessel has specific lights that differ from those of a typical underway vessel, including additional lights to indicate its towing status. A vessel that is maneuvering might display other signals or lights depending on its specific actions, further differentiating it from simply being underway.