1The rule in plain words
In one sentence…
When two power-driven vessels cross with risk of collision, the one with the other on starboard keeps out of her way and avoids crossing ahead if possible.
Who gives way?
If two power-driven vessels are crossing with risk of collision, the one with the other on her starboard side keeps out of the way.
Which manoeuvre do you avoid?
If the circumstances admit, the vessel keeping out of the way avoids crossing ahead of the other.
How can you remember it?
When you see her red (port) light, remember: you give way and do not cross ahead.
2Key points
- 1.
Two power-driven vessels crossing with risk of collision.
- 2.
Other vessel on starboard → you keep out of the way.
- 3.
If circumstances admit → avoid crossing ahead.
- 4.
Other vessel's red (port) light → quick memory: you give way.
3Frequent mistakes
Wrong: "If the other vessel is on my starboard side, I keep my course."
Right: If you have the other vessel on your starboard side, you keep out of her way.
Wrong: "I may cross ahead if I get there first."
Right: Avoid crossing ahead when the circumstances of the case admit it.
Wrong: "Rule 15 applies to every crossing between vessels."
Right: It requires two power-driven vessels crossing so as to involve risk of collision.
4Exam keys & interpretation
Exam keys
"On her own starboard side" identifies the vessel that must keep out of the way: the one seeing the other there.
When the question says "if the circumstances of the case admit", that condition qualifies avoiding crossing ahead.
Do not merge "avoid crossing ahead" with "keep out of the way": the former is an added, conditional instruction.
How to read it
Read the sentence from the observing vessel: where is the other? If it is on starboard, that vessel keeps out of the way.
Treat "if the circumstances of the case admit" as a condition on avoiding crossing ahead, not permission to ignore the Rule.
5Official text
IMO COLREG 1972U.S. Coast Guard — International Navigation Rules (2024)
Full wording of the rule, reproduced from the official source below.
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.6Test your knowledge
Two power-driven vessels are crossing with risk of collision; you see the other vessel's red (port) light. What do you do?
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