Section II: Conduct of vessels in sight of one another
Overtaking at sea: who gives way? Rule 13
In practice
- Overtaking means giving way. Always, whatever the type of vessel (it beats the Rule 18 ladder).
- The boundary is 22.5 degrees abaft the beam: at night you know it by the lights (only the sternlight in view means you are overtaking).
- Once overtaking, always overtaking: a later change of bearing does not turn you into a crossing vessel.
Top-down view, a schematic sketch. Red = gives way, green = stands on (keeps her course and speed).
Text of Rule 13: Overtaking
(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.
(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.
Text of the International Regulations as amended, quoted from the consolidated U.S. Coast Guard Navigation Rules (public domain); spelling follows the IMO original. When in doubt, the Convention decides.
The common mistake
A sailing vessel overtaking a power-driven one, convinced she stands on. Rule 13a opens with "notwithstanding anything contained in the Rules": the overtaking vessel keeps clear, even sail overtaking power.
At night you know her by the lights
Put yourself astern in the trainer: you see only the white sternlight, no sidelight. That is exactly what the vessel you are overtaking looks like.