What light may any size sailing vessel display at the mast in addition to her navigation lights?

Any size sailing vessel may display a red over green light at the mast in addition to her navigation lights. This combination of lights specifically conveys that the vessel is a sailing vessel under way, indicating that it is not powered by an engine. The red light signifies that the vessel is under sail, while the green light indicates that it is also engaged in forward motion.

The use of this light is a way for the vessel to communicate its status to other vessels in the vicinity, enhancing safety and awareness on the water. Sailboats can be recognized more easily by this light configuration, particularly at night or in low visibility conditions. This is a common understanding in maritime navigation rules and is part of the signals used to show a vessel's activity to avoid collisions.