Official rule text as published in the U.S. Coast Guard Navigation Rules
(a) Notwithstanding anything contained in Rules 4-18, any vessel overtaking any other shall keep out of the way of the vessel being overtaken.
(b) A vessel shall be deemed to be overtaking when coming up with another vessel from a direction more than 22.5 degrees abaft her beam, that is, in such a position with reference to the vessel she is overtaking, that at night she would be able to see only the sternlight of that vessel but neither of her sidelights.
(c) When a vessel is in any doubt as to whether she is overtaking another, she shall assume that this is the case and act accordingly.
(d) Any subsequent alteration of the bearing between the two vessels shall not make the overtaking vessel a crossing vessel within the meaning of these Rules or relieve her of the duty of keeping clear of the overtaken vessel until she is finally past and clear.
MyCOLREGS explanation
Summary
Overtaking overrides the usual sailing/power-driven hierarchy in Rules 4 through 18: if you're overtaking, you keep clear, no matter what type of vessel you or the other boat is. You're considered to be overtaking when you approach from more than 22.5 degrees abaft the other vessel's beam - the zone where, at night, you'd see only her stern light and neither sidelight. If you're not sure whether you're overtaking, assume you are and act accordingly. And once you're overtaking, that duty sticks: even if the bearing between you shifts as you pass, you don't suddenly become a "crossing" vessel with different rules - you stay responsible for keeping clear until you are finally past and clear.
Real World Example
The overtaking vessel keeps clear regardless of type because she is the one who chose to close the gap with full view of the vessel ahead, while the vessel being passed may have no idea anyone is coming up from behind, let alone from which quarter. That 22.5-degree line isn't arbitrary, either - it matches the exact arc where a vessel's sidelights cut off and only her sternlight shows, so the same lighting geometry that identifies an overtaking vessel by day works identically after dark.
Practice Questions
Q1. A sailing vessel is overtaking a power-driven vessel from astern. Under Rule 13(a), who must keep clear?
A. The power-driven vessel, because power generally keeps clear of sail under Rule 18.
B. Whichever vessel is larger.
C. The sailing vessel, because she is the overtaking vessel and Rule 13 overrides the usual hierarchy.
D. Each vessel must alter course to starboard, as in a head-on situation.
Q2. At night, you can see only the white sternlight of a vessel ahead of you - neither of her sidelights is visible. What does Rule 13(b) say this indicates?
A. You are approaching from more than 22.5 degrees abaft her beam and are considered to be overtaking.
B. You are crossing from her starboard side.
C. She is restricted in her ability to maneuver.
D. You are meeting her head-on.
Practice Question Answers
Q1.
C. The sailing vessel, because she is the overtaking vessel and Rule 13 overrides the usual hierarchy. Rule 13(a) applies notwithstanding Rules 4-18, so the overtaking vessel keeps clear regardless of whether she would otherwise be a privileged sailing vessel. A applies the usual sail-over-power hierarchy from Rule 18, but Rule 13 specifically overrides that hierarchy for overtaking - whoever is overtaking keeps clear, regardless of vessel type. B substitutes size for role. D describes an unrelated situation entirely.
Q2.
A. You are approaching from more than 22.5 degrees abaft her beam and are considered to be overtaking. Rule 13(b) defines the overtaking sector by exactly this test: seeing only the sternlight, and neither sidelight, means you are more than 22.5 degrees abaft her beam. B misreads which side you're on. C reaches for an unrelated vessel status that has nothing to do with light visibility. D describes the opposite situation - head-on shows both sidelights, not just a sternlight.