Collisions are among the most devestating problems that can occur at sea. Collision avoidance is the fundamental rationale for keeping a watch. An aid to the human eye is radar, which has line-of-sight range to the horizon, and even can "see" through fog and the darkness of night. Other devices include the CARD (collision avoidance radar detector), which alerts you, on your small boat, that a vessel with radar is around.

Some of the U.S. Coast Guard safe-boating regulations are specifically intended to promote collision avoidance and contain yet additional methods, namely sound. …