Rule 15 — Crossing situation
If she is on your starboard side, she is the stand-on vessel and you keep out of the way — and you avoid passing ahead of her if you can. The rule in one sentence, how it fits with overtaking and head-on, and what each vessel does next under Rules 16 and 17.
Between two power-driven vessels crossing with risk of collision, the one that has the other 'on her own starboard side' keeps out of the way. 'If the circumstances of the case admit', she also avoids passing ahead of the other vessel.
The one with the other vessel on her starboard side. The other — the vessel that sees you on her port side — is the stand-on vessel under Rule 17(a)(i) and keeps her course and speed.
In practice, at night, a power-driven vessel crossing from your starboard side towards your port side shows you her port sidelight: between two power-driven vessels, red on your starboard side means you keep out of the way. If her compass bearing is not appreciably changing as she closes, risk of collision is deemed to exist (Rule 7(d)(i)).
Only two power-driven vessels (Rule 3(b): any vessel propelled by machinery, including a sailing yacht with her engine in use), in sight of one another (Rule 11), crossing with risk of collision.
It gives way to the other rules on its edges. A vessel coming up from more than 22.5 degrees abaft the other's beam is overtaking, not crossing (Rule 13(b)), and keeps out of the way whichever side she is on. Two power-driven vessels ahead or nearly ahead on reciprocal or nearly reciprocal courses are head-on (Rule 14). Two sailing vessels: Rule 12. Power-driven and sailing, fishing, not under command or restricted in ability to manoeuvre: Rule 18(a).
Yes, between two power-driven vessels. Rule 10(a) says Rule 10 does not relieve any vessel of her obligation under any other rule, so a power-driven vessel in a traffic lane with another power-driven vessel crossing from her starboard side keeps out of the way. Some vessels also carry a 'not to impede' duty: vessels under 20 metres and sailing vessels must not impede the safe passage of a power-driven vessel following a lane (Rule 10(j)), and in a narrow channel a vessel must not cross if that impedes a vessel which can safely navigate only within it (Rule 9(d)). That duty means acting early, before risk of collision develops (Rule 8(f)(i)). If risk of collision does develop, Rule 15 still applies: the vessel that must not impede still must not impede (Rule 8(f)(ii)), and the vessel in the lane or channel remains fully bound by the steering rules, Rule 15 included (Rule 8(f)(iii)).
Rule 16 says how: 'early and substantial action to keep well clear', so far as possible. Rule 8 adds that the action must be positive and in ample time, large enough to be readily apparent, and result in passing at a safe distance.
In practice the usual action is a bold alteration to starboard to pass astern of her, a substantial reduction of speed, or both. An alteration to port for a vessel on your starboard side is rarely the right answer, in our view: it turns you towards the side she is heading for, so the two tracks can keep converging, and it can put you across her bow.
That the give-way vessel should aim to pass astern of the stand-on vessel, not in front of her. It is qualified — 'if the circumstances of the case admit' — so it is not an absolute ban, but in our view crossing ahead should be the exception you can justify, not the plan.
Rule 17 sets it out in stages. (a)(i): keep her course and speed. (a)(ii): she may act to avoid collision by her manoeuvre alone as soon as it becomes apparent the give-way vessel is not taking appropriate action. (b): when so close that collision cannot be avoided by the give-way vessel alone, she must take 'such action as will best aid to avoid collision'. (d): none of this relieves the give-way vessel of her obligation to keep out of the way.
Rule 17(c): when a power-driven stand-on vessel in a crossing takes action under 17(a)(ii) to avoid another power-driven vessel, she should not turn to port for a vessel on her own port side, 'if the circumstances of the case admit'. As we read the reason: the give-way vessel is on her port side, and her most likely late action is an alteration to starboard — towards a stand-on vessel turning to port.
If she fails to understand the give-way vessel's intentions, or doubts that sufficient action is being taken, Rule 34(d) requires her to indicate it immediately with at least five short and rapid blasts. If she then manoeuvres, Rule 34(a) applies: one short blast for an alteration to starboard, two short blasts for an alteration to port, three short blasts for operating astern propulsion.
It may come up as a collision scenario, by night or by day. Examples of the kind of question to be ready for:
“At night you see a masthead light and a red sidelight on your starboard bow, on a steady bearing. What is it, and what do you do?”
“Same vessel, but on your port bow showing green. What now?”
“You're the stand-on vessel and the give-way vessel isn't altering. Talk me through what you do, and when.”
“Why not alter to port for a vessel on your starboard bow?”
“A vessel on your starboard quarter, about 30 degrees abaft the beam, is closing. Is she crossing?”
The common slips: applying Rule 15 to a sailing vessel under sail; crossing ahead of the stand-on vessel without needing to; making small alterations she cannot see; as stand-on, acting too early or altering to port for a vessel on the port side; and forgetting that a vessel more than 22.5 degrees abaft the beam is overtaking, not crossing.
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