Rule 15 governs crossing situations between power-driven vessels.
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.
The give-way vessel should preferably alter course to starboard to pass astern of the stand-on vessel. The key principle is: if the other vessel is on your starboard side, you give way.
IMO COLREG 1972Official text
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.Reproduced verbatim from the IMO COLREG 1972 Convention (as amended).
STCW Bridge Watch Lens
Decide applicability before manoeuvring: Rules 4-10 apply in any visibility, Rules 11-18 only when vessels are in sight, and Rule 19 governs radar-only encounters in restricted visibility.
Build the traffic picture with sight, hearing, radar/ARPA and chart context.
Do not let AIS or one isolated bearing replace systematic observation.
After manoeuvring, keep monitoring bearing, range, CPA/TCPA and passing distance until the other vessel is finally past and clear.
Exam Focus
Identify the vessel types first, then the relative bearing, then whether one vessel is overtaking.
Misclassifying the encounter is the usual exam failure.
If two rules seem to conflict, check the order carefully: overtaking duties still apply, and Rule 2 still requires ordinary seamanship.
Key Takeaways
Give way to vessels on YOUR starboard side
Avoid crossing ahead of the stand-on vessel
Preferably alter to starboard to pass astern
Only applies to power-driven vessels in sight of each other
Common Mistakes
Crossing ahead of the stand-on vessel when it can be avoided
Confusing which vessel has the other on starboard side
Test Your Knowledge
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More rules in this Part
- Sailing VesselsWhen two sailing vessels approach, the one with wind on port side keeps clear. When both have wind on same side, the windward vessel keeps clear.
- OvertakingAny vessel overtaking another shall keep out of the way of the vessel being overtaken.
- Action by Stand-on VesselThe stand-on vessel shall keep her course and speed, but may take action when it becomes apparent the give-way vessel is not acting.
- Responsibilities Between VesselsRule 18 sets the basic responsibilities between vessel types. Constrained-by-draught vessels receive special consideration but are not simply another rung in a fixed hierarchy.
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