United States Coast Guard Captains License Practice Exam

A vessel displaying three all-round red lights at night is indicating that it is constrained by its draft, which is a specific condition that affects its ability to navigate safely due to the depth of water. When constrained by its draft, the vessel is required to display a specific day shape to communicate its status to other vessels during daylight hours.

The correct day shape that corresponds to this nighttime signal of three all-round red lights is a cylinder. This shape clearly conveys to other vessels that the vessel is restricted in its ability to maneuver due to its draft. The use of the cylinder as a day shape provides essential information to nearby ships, helping to ensure safe navigation and avoid potential collisions.

In contrast, the other options represent different statuses or types of vessels. For example, three balls in a vertical line signal a different condition, while the two diamonds in a vertical line and two cones with apexes together denote other types of restrictions or equipment on vessels. These day shapes are critical for the safe operation and communication among vessels on the water.