Which vessel is required to show two masthead lights in a vertical line?

The correct answer is that a vessel of more than 50 meters in length is required to show two masthead lights in a vertical line. This is a specific requirement outlined in the International Regulations for Preventing Collisions at Sea (COLREGs), which governs the lighting requirements for vessels to ensure safe navigation and collision prevention.

For vessels that exceed 50 meters in length, the display of two masthead lights helps indicate their size and operational capabilities to other vessels. The upper light must be placed at least 1 meter higher than the lower light, ensuring visibility and aiding in identifying the type of vessel from a distance, particularly during the night or in poor visibility conditions.

Other vessels, like power-driven vessels less than 50 meters in length or tows that are less than 20 meters, have different lighting requirements that do not necessitate the display of two vertical masthead lights. Similarly, sailing vessels have their own specific lighting regulations, focusing on sails and rigging. Thus, for the question asked, the requirement for two vertically aligned masthead lights is specifically applicable to vessels over 50 meters in length.