Rule 13 Overtaking (COLREGS )

(a) Notwithstanding anything contained in the Rules of part B, sections I and II, any vessel overtaking any other shall keep out of the way of the vessel being overtaken.

As per rule 13 (a), vessel “B” is obliged to keep out of the way of vessel “A”

(b) A vessel shall be deemed to be overtaking when coming up with another vessel from a direction more than 22.5° abaft the 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 stern light of that vessel but neither of her sidelights.

See the slide above first. If the vessel which is overtaking is able to see only the stern light of the vessel to be overtaken she is overtaking the other vessel.

(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.

This vessel may be in doubt as to whether she is overtaking or not but she has to consider this case as overtaking and take action accordingly.

(d) Any subsequent alteration of the hearing 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.

The vessel overtaking is obliged to keep clear of the other even the bearing between two vessel changes and situation seems like crossing.