Collision avoidance — Rule 14 · Head-on situation
Reading notes · Collision avoidance
The only rule that puts the same duty on both of you — and the only one where being in doubt is itself the answer.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.
What the Rule says
(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.
COLREGs Rule 14, as published in the US Coast Guard Navigation Rules Handbook. It is a work of the United States federal government and is in the public domain.
How to read it
Two power-driven vessels, and she is holding the reciprocal of your course.
The Rule gives no figures at all, so everything here is declared. Her course is the exact reciprocal of yours and stays that way, so that only one thing changes at a time. She lies 2 M off, which leaves about five minutes at a closing speed of 24 knots. The alteration is thirty degrees, which is a turn she can see, because Rule 8(b) asks for one large enough to be readily apparent and ten degrees at two miles is not. Where “nearly ahead” stops is this course's own line, not the Rule's: the Rule gives no angle, so one had to be chosen, and the band where nobody could tell is taken as three degrees either side of the line, where her masthead lights are still too nearly in line to swear to — which is to say the line is drawn and then immediately admitted to be unreadable, because that is the truth of it. Her lights are not drawn to scale and neither are the hulls: what she shows at each aspect comes from the same lights module the lights chapters use, and the separation of her masthead lights is measured, not drawn. The plate is turned a quarter turn so that your course runs across the paper: it is the same chart, north no longer up.
A worked encounter
One encounter, as the chapter sets it up. What follows is what each answer earns you.
She is — A power-driven vessel of 90 m, making 12 knots
You make — 12 knots
Her course — the reciprocal of yours, exactly — that never changes on this screen
She lies — 20° on your starboard bow, 2 M off
Her masthead lights — 27.2′ apart — 86 m of her deck, seen from two miles
“Nearly ahead” — taken as 5.95° off your bow. That figure is not the Rule's — the Rule says only ahead or nearly ahead and leaves it there. It is where this course draws the line, and the band below is why you should not lean on it
Closest approach — 0.68 M as things stand
Right. She is on your bow, but not ahead or nearly ahead. Her masthead lights are open and only one of her sidelights is showing, and one sidelight is the whole answer: you are off her centerline, so your courses are not the reciprocal pair the Rule is talking about.
Rule (b). The test is what you can see, not what you can work out. The Rule asks for the other ahead or nearly ahead, with her masthead lights in a line or nearly in a line and/or both sidelights. One sidelight, and two lights plainly open, is neither. Rule 14 does not decide this one.
No — this is not head-on. She is on your bow, but not ahead or nearly ahead. Her masthead lights are open and only one of her sidelights is showing, and one sidelight is the whole answer: you are off her centerline, so your courses are not the reciprocal pair the Rule is talking about.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.