Collision avoidance — Rule 18 · Responsibilities between vessels
Reading notes · Collision avoidance
Not a situation but a table: what each of you is, and nothing else.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.
What the Rule says
Except where Rules 9, 10 and 13 otherwise require: (a) A power-driven vessel underway shall keep out of the way of: (i) a vessel not under command; (ii) a vessel restricted in her ability to maneuver; (iii) a vessel engaged in fishing; (iv) a sailing vessel. (b) A sailing vessel underway shall keep out of the way of: (i) a vessel not under command; (ii) a vessel restricted in her ability to maneuver; (iii) a vessel engaged in fishing. (c) A vessel engaged in fishing when underway shall, so far as possible, keep out of the way of: (i) a vessel not under command; (ii) a vessel restricted in her ability to maneuver. (d) (i) Any vessel other than a vessel not under command or a vessel restricted in her ability to maneuver shall, if the circumstances of the case admit, avoid impeding the safe passage of a vessel constrained by her draft, exhibiting the signals in Rule 28. (ii) A vessel constrained by her draft shall navigate with particular caution having full regard to her special condition. (e) A seaplane on the water shall, in general, keep well clear of all vessels and avoid impeding their navigation. In circumstances, however, where risk of collision exists, she shall comply with the Rules of this Part. (f) (i) A WIG craft shall, when taking off, landing and in flight near the surface, keep well clear of all other vessels and avoid impeding their navigation; (ii) A WIG craft operating on the water surface shall comply with the Rules of this Part as a power-driven vessel.
COLREGs Rule 18, 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
A vessel constrained by her draft is not on it — she has a strip of her own, underneath.
That is not a saying: it is a count of this table.
And the switch was measured before it was written. Put these two in an encounter and sweep the other vessel right round you — 360 bearings, all of them on a steady bearing — and Rule 13 takes 135 of them away from Rule 18 outright. In those 135, changing what she is does not change the answer once. The whole ladder, all 7 rungs of it, is silent. And in 44 more she is abaft your beam and still not overtaking you, because the line is 112.5° and not the beam. None of that is drawn here, because none of it is what Rule 18 is about.
Everything here is declared. This is a table and not a situation: there is no chart, no bearing and no track, because Rule 18 describes no encounter — it says what each of you is. The shapes are day shapes, which is how you tell one from another by daylight; they belong to the lights-and-shapes Part, which this module has not reached, so they are drawn and named and not given a Rule number. Four of the eight carry no mark at all, and that is worth as much as the ones that do. And the seaplane is shown where the first half of 18(e) puts her — keeping well clear of everybody in general; the second half sends her back into these Rules where risk of collision exists, and the Rule does not say what she counts as when it does. And a sailing vessel here is one under sail alone: put her engine on and she is a power-driven vessel, which moves her from one column to the other. A sailing vessel under power is no longer a sailing vessel: Rule 18 treats her as a power-driven vessel, and by day she shows a cone, point down. And by definition, Rule 3(h), a vessel constrained by her draft is a power-driven vessel. Rule 28 lets her show a cylinder; it does not oblige her to. An unmarked vessel may still be constrained by her draft.
A worked encounter
One encounter, as the chapter sets it up. What follows is what each answer earns you.
You are — a power-driven vessel — no mark: she is what the others are measured against
The ladder from here — 1 of them keep out of your way, 4 you keep out of the way of, and 1 the Rule does not rank against you at all
She is not on it — a vessel constrained by her draft has one line of her own, and it is not part of the list
Which makes it — 18(d)(i) on you, and 18(d)(ii) on her
No — and this is the one everybody gets wrong. You have put her on the ladder, and she is not on it. Rule 18 nowhere says that anyone keeps out of the way of a vessel constrained by her draft. What it says is that you avoid impeding her safe passage, and that is a different duty: keeping out of the way is what you do once an encounter has arrived, and not impeding is not taking her water away in the first place, so that the encounter never arrives at all. It begins far earlier than the steering rules do.
Rule 18. The Rule does not open with the ladder. It opens by standing aside. Except where Rules 9, 10 and 13 otherwise require — a narrow channel, a traffic separation scheme, and an overtaking. Whatever the ladder says, it says it after those three have spoken.
Rule 18. And she has a duty back, which is the half that is never taught. A vessel constrained by her draft shall navigate with particular caution having full regard to her special condition. Not impeding her does not give her right of way — nothing in these Rules gives anybody right of way, and the words do not appear anywhere in them.
Rule 3. And this is where it all comes from. The term vessel constrained by her draft means a power-driven vessel which, because of her draft in relation to the available depth and width of navigable water is severely restricted in her ability to deviate from the course she is following. Read the first six words of that and the ladder sorts her for you.
No — the Rule does name you. You have said you owe her nothing, and the Rule gives that to two vessels only. Any vessel other than a vessel not under command or a vessel restricted in her ability to maneuver — you are neither of them, so the not-impeding duty is yours. It is not the same thing as keeping out of the way, and it starts much earlier.
Right — you do not impede her. And notice that it is not the only thing going on. A vessel constrained by her draft is a power-driven vessel — that is the definition, not an inference — so the ladder reaches her like it reaches anyone else. The ladder does not rank the two of you — two power-driven vessels are Rule 15's business, not Rule 18's — and the not-impeding duty runs anyway, before any of that.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.