What additional light is required on a 30-meter vessel that is not towing?

A 30-meter vessel that is not towing is required to display specific navigation lights to ensure its visibility and to communicate its presence to other vessels. The correct configuration involves showing one masthead light and one stern light.

The masthead light is positioned at the centerline of the vessel and should be visible from a distance of 225 degrees, providing indicators of the vessel's heading when seen from the front. The stern light, located at the opposite end of the vessel, is designed to be visible from behind the vessel, covering 135 degrees of visibility. Together, these lights allow other vessels to ascertain the vessel's size, direction, and activity.

Other lighting configurations mentioned in the other choices do not meet the requirements set forth in the International Regulations for Preventing Collisions at Sea (COLREGs) for vessels of this size and operational status. For example, having just a masthead light or sidelights would not provide adequate information about the vessel's position and direction, which is crucial for safe navigation and avoiding collisions. Additionally, two stern lights would not be appropriate because they do not meet the regulatory requirements for vessels of this category.