IMO COLREG Rule 19

Rule 19 applies to vessels not in sight of one another when navigating in or near an area of restricted visibility. There is no stand-on vessel; every vessel proceeds at a safe speed adapted to the conditions and has her engines ready for immediate maneuver. A vessel detecting another by radar alone must determine whether a close-quarters situation is developing, and if so take avoiding action in ample time, avoiding an alteration to port for a vessel forward of the beam other than one being overtaken, and an alteration toward a vessel abeam or abaft the beam.