1The rule in plain words
In one sentence…
COLREG: with two power-driven vessels on reciprocal or nearly reciprocal courses and risk of collision, both alter to starboard and pass port to port.
What do two power-driven vessels do on reciprocal or nearly reciprocal courses?
If there is risk of collision, each shall alter her course to starboard so each passes on the port side of the other.
How do you recognise the situation?
By night, you could see the other vessel's masthead lights in a line or nearly in a line and/or both sidelights; by day, the corresponding aspect.
What if you are in doubt?
Assume that it exists and act accordingly.
Which vessels does it concern?
Two power-driven vessels meeting on reciprocal or nearly reciprocal courses so as to involve risk of collision.
2Key points
- 1.
Pair: two power-driven vessels · reciprocal or nearly reciprocal courses.
- 2.
Risk of collision → coordinated alteration to starboard.
- 3.
The visual cue combines masthead lights and sidelights.
- 4.
Doubt about the situation = assume it exists and act.
3Frequent mistakes
Wrong: "If we meet nearly head-on, each vessel alters to port."
Right: Rule 14 requires each vessel to alter to starboard and pass on the other's port side.
Wrong: "The situation requires seeing both sidelights; masthead lights do not count."
Right: Masthead lights in a line or nearly in a line also indicate it.
Wrong: "If I am unsure, I keep my course until the view is clearer."
Right: In any doubt, assume the situation exists and act accordingly.
4Exam keys & interpretation
Exam keys
"Alter to starboard" is the manoeuvre; "pass on the port side" is the resulting passing arrangement.
"In a line or nearly in a line" refers to both masthead lights; "and/or" can add both sidelights.
If the question says "in any doubt", choose assumption and action, not waiting for clearer evidence.
How to read it
Read it as a paired action: each vessel alters to starboard, producing a port-side passing arrangement.
Doubt activates the rule; assume the situation exists and act accordingly instead of waiting for certainty.
5Official text
IMO COLREG 1972U.S. Coast Guard — International Navigation Rules (2024)
Full wording of the rule, reproduced from the official source below.
(a) When two power-driven vessels are meeting on reciprocal or nearly reciprocal courses so as to involve risk of collision each shall alter her course to starboard so that each shall pass on the port side of the other.
(b) Such a situation shall be deemed to exist when a vessel sees the other ahead or nearly ahead and by night she could see the masthead lights of the other in a line or nearly in a line and/or both sidelights and by day she observes the corresponding aspect of the other vessel.
(c) When a vessel is in any doubt as to whether such a situation exists she shall assume that it does exist and act accordingly.6Test your knowledge
Two power-driven vessels meet on reciprocal courses with risk of collision. Which way does each alter?
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