Collision avoidance — Rule 16 · Action by give-way vessel
Reading notes · Collision avoidance
Two lines, and both are about time. Early and substantial, or it is neither.
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What the Rule says
Every vessel which is directed to keep out of the way of another vessel shall, so far as possible, take early and substantial action to keep well clear.
COLREGs Rule 16, 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
You are the give-way vessel: she is crossing from your starboard side with her bearing steady, and Rule 15 has already settled that. She is holding her course and her speed, because that is her duty. The alteration is the same every time and it is a good one. All you are choosing is when you put the helm over — and the question is not whether you clear her. It is what your timing leaves her.
The Rule gives no figures at all — not a distance, not a time, not a number of degrees — so everything here is declared. She holds, because Rule 17 tells her to, and her track is the same straight line in every state of this screen. The alteration is fixed at forty-five degrees to starboard: bold, unmistakable, and exactly what Rule 8 asks for. It is deliberately not a control — how big to make it is Rule 8's question and Rule 8's screen, and if you could change it here this would be that screen again. The two marks on your track are the module's Rule 17 model, taken from colregs.js , which declares them as a teaching model and not as law.
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 holding
You make — 12 knots
She lies — 45° on your starboard bow, 9 M off, on a steady bearing
If neither of you acts — you meet in 26 minutes
You put the helm over — with 30 minutes to go, and you alter forty-five degrees to starboard
Rule 17 opens for her — with 12 minutes to go she may act alone, and with 4 she must — the two figures are the module's teaching model, and colregs.js says so where it declares them
Rule 8 · a safe distance — yes — you pass 2.96 M off, and half a mile is what this module declared for two ships this size
Rule 16 · well clear — yes — she held her course and her speed the whole way, and never had a decision to make
And it was substantial — the same helm at the same moment, but only ten degrees of it, would have left her 0.65 M instead of 2.96 — early and small is not what the Rule asks for
Nothing at all. You put the helm over while there was still plenty of time, and she never had grounds to do anything but hold. She sees your alteration open her bearing, she keeps her course and her speed, and the two of you pass with room to spare. That is what “keep well clear” means: not that you missed her, but that she never had to think about missing you.
Rule 16. Every vessel which is directed to keep out of the way of another vessel shall, so far as possible, take early and substantial action to keep well clear.
Rule 16. And it was substantial, which is the Rule's other word. Given at this same moment, a gentle nudge of the wheel would have left her a fraction of what a bold alteration leaves — the readings put both numbers side by side. Early and small is not what the Rule asks for; it asks for both words at once.
Nothing at all — and that is the point. You put the helm over while there was still plenty of time, and she never had grounds to do anything but hold. She sees your alteration open her bearing, she keeps her course and her speed, and the two of you pass with room to spare. That is what “keep well clear” means: not that you missed her, but that she never had to think about missing you.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.