Constant Bearing, Decreasing Range (CBDR) (CBDR)

A collision-warning condition in which another object’s bearing remains nearly steady while its distance decreases.

If two vessels continue with unchanged velocity, a steady relative bearing and closing range indicate that their tracks lead to the same point at approximately the same time. Repeated visual bearings, radar plotting or reliable relative-motion data can reveal the trend.

Exact constancy is not required, and appreciable bearing change can still conceal danger at close range or with a large vessel or tow. If there is doubt, risk of collision is treated as existing and action is determined by the complete collision regulations rather than the mnemonic alone.