Section II: Conduct of vessels in sight of one another
Crossing courses: who gives way? Rule 15
In practice
- She is on your starboard side: you give way. You are on hers: she gives way.
- The night shortcut: you see her red sidelight, so you are on her port side and she has you on her starboard side. You give way.
- When giving way, avoid crossing ahead of the other vessel. Depending on the situation that may take a bold alteration of course, a reduction of speed, or both.
Top-down view, a schematic sketch. Red = gives way, green = stands on (keeps her course and speed).
Text of Rule 15: 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.
Text of the International Regulations as amended, quoted from the consolidated U.S. Coast Guard Navigation Rules (public domain); spelling follows the IMO original. When in doubt, the Convention decides.
The common mistake
Giving way "across the bow": speeding up to make it ahead of the other ship. Rule 15 says it straight: avoid crossing ahead. Altering to starboard and passing astern is the typical solution, but the rule does not prescribe one manoeuvre for every situation.
At night you know her by the lights
Place yourself on her port bow in the trainer. You see the red sidelight, which means you have the other vessel on your starboard side: in a crossing of two power-driven vessels, you are the give-way vessel.