What do two yellow lights in a vertical line indicate for a vessel?

Two yellow lights in a vertical line indicate that the vessel is proceeding under sail while also using its machinery, which occurs typically when a sailing vessel has its sails up but is benefiting from the power of an engine. This signal serves to alert other vessels that the ship is not solely relying on wind power but is also operating under mechanical propulsion.

This configuration is particularly important for other vessels in order to determine the status and intentions of the sailing vessel. It helps prevent collisions by providing clear visibility of the vessel's operational status, ensuring that other mariners understand that the ship is not just a sailing craft but is actively utilizing its engine.

The other options represent different navigational situations: A vessel at anchor would show a single white light; a vessel towing would display a specific combination of lights to indicate the nature of the tow; and a vessel engaged in fishing has its own unique light signals. Therefore, the standard for the configuration of lights as per COLREGs provides navigational safety by clearly defining the vessel's operational mode.