Collision avoidance — Rule 17 · Action by stand-on vessel
Reading notes · Collision avoidance
Holding your course is a duty and not a right, and it stops being lawful well before the collision would.
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What the Rule says
(a) (i) Where one of two vessels is to keep out of the way the other shall keep her course and speed. (ii) The latter vessel may, however, take action to avoid collision by her maneuver alone, as soon as it becomes apparent to her that the vessel required to keep out of the way is not taking appropriate action in compliance with these Rules. (b) When, from any cause, the vessel required to keep her course and speed finds herself so close that collision cannot be avoided by the action of the give-way vessel alone, she shall take such action as will best aid to avoid collision. (c) A power-driven vessel which takes action in a crossing situation in accordance with subparagraph (a)(ii) of this Rule to avoid collision with another power-driven vessel shall, if the circumstances of the case admit, not alter course to port for a vessel on her own port side. (d) This Rule does not relieve the give-way vessel of her obligation to keep out of the way.
COLREGs Rule 17, 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
She is crossing from your port side with her bearing steady, so she is the give-way vessel and you are the stand-on vessel — Rule 15 settled that. And she is doing nothing at all: her course and her speed have not changed since you first saw her. Rule 17 opens by telling you to keep your course and your speed, and the two marks on your track are the two places where that changes.
Of the 26 moments on this dial, keeping your course and your speed is a lawful answer in 22. Altering is lawful in 12 of them, and altering to port in none. Rule 17 is not a rule about when to move: it is a rule about holding on, with two places where that changes. That is not a saying — it is a count of this dial.
The Rule gives no figures at all, so everything here is declared. She is doing nothing — that is the premise of the screen, and her track is the same straight line in every state of it. The helm is fixed at forty-five degrees either way, and hers at thirty degrees to starboard, the same figure Rule 16's screen declares for a give-way vessel: how much helm is Rule 8's question, not this one. The two marks are the module's Rule 17 model, drawn as bands because the line is soft. Passing clear is half a mile, as everywhere else in this module.
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 15 knots, and doing nothing
You make — 12 knots
She lies — 45° on your port bow, 9 M off when this began, on a steady bearing
If neither of you acts — you meet in 30 minutes
Rule 17 gives you — the power to act alone with 12 minutes to go, and the duty to act with 4 — the two figures are the module's teaching model, and colregs.js says so where it declares them
What each helm leaves you — if she goes on doing nothing — holding 0.00 M, starboard 2.46 M, port 2.96 M
Which is why — port measures better than starboard against a ship that does nothing, and worse than anything else against a ship that obeys. That is the whole reason (c) is written down instead of worked out
Hold. There is nothing else to do yet. She has time in hand and she is the one who has to use it. Your job is to be predictable: she is working out her maneuver on the assumption that you will still be here, on this course, at this speed. Take that away from her and you have taken away the thing she is planning against.
Rule 17. Where one of two vessels is to keep out of the way the other shall keep her course and speed. That is the whole of (a)(i), and it is a duty, not a courtesy.
Rule 17. And the reason is not that acting would be dangerous. On this plot it would not be: try the alteration and compare the two readings. It is the line above — she is counting on you.
Not to port. Never to port for her. She is on your own port side, and that is the one maneuver the Rule names and forbids. And look at why it has to be written down rather than worked out: against a ship that goes on doing nothing, turning to port actually opens the distance more than turning to starboard does — the readings put the two numbers side by side. The plot rewards it. What the plot does not know is that she is the give-way vessel, and that when she finally does what the Rules tell her, she alters to starboard — straight into the water you just turned towards.
Rule 17. A power-driven vessel which takes action in a crossing situation in accordance with subparagraph (a)(ii) of this Rule to avoid collision with another power-driven vessel shall, if the circumstances of the case admit, not alter course to port for a vessel on her own port side.
Rule 8. And the numbers are worth reading twice. A maneuver that measures well against one assumption about the other ship, and measures like a collision against another, is not a maneuver: it is a bet. Rule 8 wants action whose effect is carefully checked until the other vessel is finally past and clear — and this one stops working the moment she behaves.
Hold — it is not your turn yet. She has time in hand and she is the one who has to use it. Your job is to be predictable: she is working out her maneuver on the assumption that you will still be here, on this course, at this speed. Take that away from her and you have taken away the thing she is planning against.
Rule 17. And acting now is not permitted yet, not merely unnecessary. The power to act alone arrives with (a)(ii) — as soon as it becomes apparent to her that the vessel required to keep out of the way is not taking appropriate action. From this far out, nothing is apparent yet: she has not failed to do anything, because there is still nothing she is late for.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.