Which shape indicates a vessel is restricted in its ability to maneuver?

The shape indicating a vessel is restricted in its ability to maneuver is two black balls displayed in a vertical line. This specific signal is part of the navigation rules set out by the Convention on the International Regulations for Preventing Collisions at Sea (COLREGs). The display of two black balls signifies that the vessel may be unable to change its course or position easily due to specific operational limitations, such as being involved in dredging, underwater work, or other activities that restrict its maneuverability.

Understanding this signal is crucial for safe navigation as it alerts other vessels to the fact that they need to exercise caution when approaching or passing by the vessel displaying this shape. Other options, such as a single red ball, a diamond shape, or three balls in a vertical line, represent different conditions or types of vessels that do not imply restrictions on maneuvering in the same manner. The clarity of these signaling shapes plays a vital role in maritime safety by conveying critical information about a vessel's status to other mariners.