What light must a sailing vessel exhibit when under sail at night?

When a sailing vessel is under sail at night, it is required to exhibit a white masthead light. This light is essential for ensuring that the vessel is visible to other boats and to indicate its presence and direction of travel. The masthead light is situated at a height, allowing it to be seen from a distance and from various angles, thus providing clear visibility to other vessels on the water.

In addition to the masthead light, sailing vessels also exhibit sidelights (red for port and green for starboard) when they are under engine power or sailing with assistance from an engine. However, the presence of the masthead light specifically indicates that the vessel is operating under sail, without the aid of its engine.

The all-round white light is typically used by vessels at anchor or for certain types of vessels like fishing boats or when operating under specific conditions as defined in various maritime rules. It does not replace the masthead light when a sailing vessel is under sail.

Understanding these rules is crucial for navigation and ensuring safety on the water, as proper lighting helps prevent collisions by informing other vessels of one's status and movements.