Constant Bearing, Decreasing Range (CBDR)

A navigational situation where two vessels maintain the same relative bearing to each other while the distance between them diminishes, often indicating a potential risk of collision.

Constant bearing, decreasing range (CBDR) is a critical concept in maritime navigation, used to assess collision risks between vessels at sea. When two vessels are on a CBDR trajectory, their relative bearing remains unchanged over time, meaning that one vessel appears stationary from the perspective of the other, even as the distance between them decreases. This situation often signals that the vessels are on a collision course unless corrective action is taken. 

The principle of CBDR is fundamental to collision avoidance and is closely tied to the International Regulations for Preventing Collisions at Sea (COLREGs). Mariners monitor CBDR by observing the bearing of another vessel using a compass or radar. If the bearing remains constant and the range (distance) decreases, it is a clear indication of a potential hazard. In such cases, the “give-way” vessel, as defined by the COLREGs, is required to take early and substantial action to avoid a collision, while the “stand-on” vessel must maintain its course and speed unless it becomes apparent that the give-way vessel is not taking appropriate action.

CBDR situations are particularly critical in restricted waters, during low visibility, or in areas with heavy traffic. Proper lookout, situational awareness, and adherence to navigational rules are essential to managing these scenarios safely. Failure to address a CBDR situation can result in dangerous close-quarters encounters or collisions, emphasising the importance of vigilance and proactive decision-making in maritime operations.