Rules 15-17 assign the roles when two power-driven vessels cross; Rule 18 then ranks priority between different types of vessel.
Rule 15 - Crossing situation
When two power-driven vessels are crossing so as to involve risk of collision, the vessel that has the other on her own starboard side shall keep out of the way and, if the circumstances admit, avoid crossing ahead of the other.
Rule 16 - Action by the give-way vessel
Every vessel directed to keep out of the way of another shall, so far as possible, take early and substantial action to keep well clear.
Rule 17 - Action by the stand-on vessel
Where one vessel is to keep out of the way, the other shall keep her course and speed. The stand-on vessel may take action by her own manoeuvre alone as soon as it becomes apparent that the give-way vessel is not taking appropriate action. If, from any cause, the stand-on vessel finds herself so close that collision cannot be avoided by the give-way vessel's action alone, she shall take whatever action best aids in avoiding collision. A power-driven vessel that takes such action in a crossing situation shall not, if circumstances admit, alter course to port for a vessel on her own port side. This rule does not relieve the give-way vessel of her obligation to keep out of the way.
Rule 18 - Responsibilities between vessels
Except where Rules 9, 10 and 13 otherwise require:
- A power-driven vessel underway shall keep out of the way of: a vessel not under command; a vessel restricted in her ability to manoeuvre; a vessel engaged in fishing; a sailing vessel.
- A sailing vessel underway shall keep out of the way of: a vessel not under command; a vessel restricted in her ability to manoeuvre.
- A vessel engaged in fishing, when underway, shall so far as possible keep out of the way of: a vessel not under command; a vessel restricted in her ability to manoeuvre.
- Any vessel other than one not under command or restricted in her ability to manoeuvre shall, if circumstances admit, avoid impeding the safe passage of a vessel constrained by her draught displaying the Rule 28 signals; such a vessel shall navigate with particular caution.
- A seaplane on the water shall in general keep well clear of all vessels, but shall comply with this Part's rules where risk of collision exists.
This hierarchy answers "who gives way to whom" by ranking vessels according to how constrained their manoeuvrability is - the most restricted vessel is protected the most.
