Crossing Situation.
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.
You may have noticed, that many OOW’s like to stay on the Starboard side of the bridge with their lookout (if they have one) positioned on the Port side. This enables a good watch to be kept in the sector represented by the arc of the starboard navigation light. Another vessel approaching within this area is deemed to be a ‘stand-on vessel’ and your own vessel is the ‘give-way vessel’. The precise definitions of these terms follow in the next two rules, but the general idea is portrayed by the names.
If you have difficulty sorting out your ‘stand on’ from your ‘give way’, an aide memoire would be your view of the approaching vessel’s sidelights to be the red and green of road traffic lights (red, take appropriate action; green go ahead). Alternatively, remember the rhyme “If to starboard a red does appear, ’tis your duty to keep clear”. Although the lights will not actually be seen in daylight, it is relatively easy to remember Rule 15 in this way.
The requirement to avoid crossing ahead, only applies in a crossing situations where there is a risk of collision. It does not apply to any action you may take at long distance, before a risk of collision can be deemed to exist.
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