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Head-on (Rule 14) vs Crossing (Rule 15) — telling them apart
Two of the three core in-sight situations in COLREG Part B Section II look almost identical from the helm, especially at night: another vessel approaching from somewhere ahead of the beam. Misclassify the geometry and you'll apply the wrong rule.
This page reduces both situations to the angle of approach and the lights you see, then gives you the decision rule that survives panic.
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Rule 14 vs Rule 15 — at a glance
| Aspect | Rule 14 — Head-on | Rule 15 — Crossing |
|---|---|---|
| Trigger angle | Reciprocal or nearly reciprocal courses, both vessels ahead of one another | Courses that cross at any angle other than head-on or overtaking |
| What you see at night | BOTH sidelights (red AND green) of the other vessel | ONE sidelight only — red OR green |
| What you see by day | Other vessel bow-on | Other vessel on a converging bearing, beam-on-ish |
| Who gives way | BOTH alter course to starboard | The vessel that has the OTHER on her starboard side gives way |
| Required action | Each turns starboard so each passes on the other's PORT side | Give-way vessel alters early and substantially; stand-on vessel maintains course (Rule 17) |
| Sound signal on action (whistle) | 1 short blast ("I am altering to starboard") | Give-way: 1 short on turn to starboard / 2 short on turn to port |
| If in doubt which it is | Treat as Rule 14 — symmetric starboard turn is the safe default | If neither vessel can safely alter to starboard, the give-way vessel must avoid by slowing or turning to port (Rule 8) |
The decision rule examiners love
Look at the other vessel's coloured sidelights. The number you can count is the rule that applies.
- Two sidelights (red AND green) = Rule 14 head-on. Both turn starboard.
- One sidelight = Rule 15 crossing. The vessel that sees the other's RED light has the other on her starboard side and gives way.
- No sidelights, only a sternlight = you are OVERTAKING (Rule 13) — give way regardless of the rule that would otherwise apply.
- If you can see her stern AND her red AND her green, you're not in a Rule 14 anymore — you've passed the turning point. Switch to Rule 17 stand-on if applicable, or Rule 13 if you've fallen behind her beam.
- Always assume head-on if you're not sure: the symmetric starboard turn (Rule 14) keeps you clear in almost every ambiguous geometry. The crossing rule's asymmetric "one turns, one doesn't" only works if both vessels agree on who's the give-way.
Next: read the rules in full
Frequently asked questions
When is a situation "head-on" under COLREG Rule 14?
When two power-driven vessels meet on reciprocal or nearly reciprocal courses such that risk of collision exists. Rule 14(b) lists the cues: both vessels see the other ahead, you see the other's masthead lights in a line or nearly in a line, AND both sidelights. Each vessel then alters course to starboard so each passes on the other's PORT side.
When is a situation "crossing" under COLREG Rule 15?
When two power-driven vessels are crossing so as to involve risk of collision. The vessel which has the other on her starboard side gives way and shall, if the circumstances allow, avoid crossing ahead of the other vessel. You typically see only ONE of the other vessel's sidelights — the red one if she is on your port bow and you must give way, or the green one if you are stand-on.
What's the difference between Rule 14 and Rule 15?
Rule 14 is symmetric — both vessels turn starboard. Rule 15 is asymmetric — one vessel gives way, the other stands on. The visual cue: in Rule 14 you see BOTH sidelights of the other vessel; in Rule 15 you see only ONE sidelight.
Who gives way in a crossing situation?
The vessel that has the other on her starboard side. Memory aid: "if she's on your right, you give way". The give-way vessel must take early and substantial action (Rule 16), avoiding crossing ahead of the other vessel.
What does the stand-on vessel do?
Under Rule 17(a), the stand-on vessel maintains her course and speed. But Rule 17(a)(ii) permits her to take action as soon as it becomes apparent the give-way vessel is not taking appropriate action. Rule 17(b) requires her to take whatever action best avoids collision when she finds herself so close that the give-way vessel's action alone cannot avoid it — and Rule 17(c) bans her from altering to PORT for a vessel on her own port side.
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