IMO COLREG Rule 13

Rule 13 covers overtaking and overrides the crossing rules in Section II. A vessel is overtaking when it approaches another from more than 22.5 degrees abaft the beam, that is, in a position where at night it would see only the sternlight and neither sidelight. The overtaking vessel must keep out of the way of the vessel being overtaken until finally past and clear, and any subsequent change of bearing does not relieve it of that duty or make it a crossing vessel. When in doubt whether she is overtaking, a vessel must assume she is and act accordingly.