What does a power-driven vessel towing astern display with its masthead lights?

A power-driven vessel towing astern displays a vertical line of masthead lights. This configuration is specified in the International Regulations for Preventing Collisions at Sea (COLREGs). The vertical arrangement of lights indicates that the vessel is engaged in towing and helps other vessels to recognize its operational status at night or in reduced visibility conditions.

The vertical line specifically helps to convey important information to other mariners about the towing vessel's size and its activity, distinguishing it from other types of vessels that may display different light arrangements. Additionally, the towing vessel typically has a second masthead light added to show that it is not just a single vessel but one that is actively engaged in towing another. This aids in ensuring safe navigation and avoidance of collisions at sea.