Collision avoidance — Rule 15 · Crossing situation
Reading notes · Collision avoidance
The rule everybody remembers, and the one people reach for when it does not apply.
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What the Rule says
When two power-driven vessels are crossing so as to involve risk of collision, the vessel which has the other on her own starboard side shall keep out of the way and shall, if the circumstances of the case admit, avoid crossing ahead of the other vessel.
COLREGs Rule 15, 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 her bearing from you is not changing — which is the whole of what Rule 7 asks you to watch.
Of the 271 bearings on this dial, “alter to port” is the right answer on none of them. Holding is right on 155 and starboard on 116. Where you keep out of the way, Rule 15's own second half sends you the other way; where you hold, Rule 17 names port as the very thing a stand-on vessel does not do for a vessel on her own port side. That is not a saying — it is a count of this dial.
The Rule gives no figures at all, so everything here is declared. Her bearing does not change — her course is worked back from that, so this is a risk of collision by construction, and the Rule says nothing whatever without one: if her bearing were opening, Rule 15 would have nobody to point at and neither of you would be the give-way vessel. She lies 2.5 M off, and that distance is not a taste: a ninety-meter vessel's sidelight is good for three miles, so beyond that there is no color to look at. Passing clear is taken as half a mile, the same figure Rule 8 already had for two ships this size. The helm is searched for, not chosen: five degrees at a time to starboard until you pass under her stern with that much water.
A worked encounter
One encounter, worked through, and taken early. She bears 45° on your starboard side, 4.5 miles off, and that bearing is not changing. You are both power-driven and both under way. At this range you have the best part of a quarter of an hour before you would meet, which is what Rule 16 means by acting in ample time.
She is — A power-driven vessel of 90 m, making 15 knots
You make — 12 knots
Her bearing — 45° on your starboard side, 4.5 M off
If neither of you acts — you meet in 13 minutes, at nothing at all between you
Her course — 259°, which is the course that keeps that bearing steady
What she sees of you — your starboard sidelight, green
To pass astern of her — 35° of starboard helm, and you pass 1.15 M under her stern
Starboard, and astern of her. You cannot make out either of her sidelights from here. You are the vessel which has the other on her own starboard side, so you keep out of the way. And the Rule does not stop there: it tells you which way round her to go. You avoid crossing ahead, which means you put the helm to starboard and go under her stern. From here it can be done, and the readings say with how much helm.
Rule 15. When two power-driven vessels are crossing so as to involve risk of collision, the vessel which has the other on her own starboard side shall keep out of the way and shall, if the circumstances of the case admit, avoid crossing ahead of the other vessel.
Rule 8. And do it so she can see it. Rule 8 asks for an alteration large enough to be readily apparent to another vessel observing visually or by radar, and warns against a succession of small ones. A helm order she cannot read is not a maneuver, it is a hope.
To starboard. Never to port.
Rule 15. Port is the one thing the sentence rules out. Going to port takes you across her bow — which is the second half of Rule 15, word for word — and it takes you towards the water she is about to be in.
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