As per International regulations for preventing collisions at sea, when two vessels in sight of each other are approaching with no change of compass bearing, so that when there is risk of collision one of them is required to keep out of the way by a Rule from Section II, there may be four stages relating to the permitted or required action for each vessel:
- At long range, before risk of collision exists, both vessels are free to take any action.
- When risk of collision first begins to apply the give-way vessel is required to take early and substantial action to achieve a safe passing distance and the other vessel must keep her course and speed.
- When it becomes apparent that the give-way vessel is not taking appropriate action in compliance with the Rules the stand-on vessel is required to give the whistle signal prescribed in Rule 34(d) and is permitted to take action to avoid collision by her manoeuvre alone, but a power-driven vessel must not alter course to port to avoid another power-driven vessel crossing from her own port side. The give-way vessel is not relieved of her obligation to keep out of the way.
- When collision cannot be avoided by the give-way vessel alone the stand-on vessel is required to take such action as will best aid to avoid collision.
The distances at which the various stages begin to apply will vary considerably. They will be much greater for high speed vessels involved in a fine crossing situation. For a crossing situation involving two power-driven vessels in the open sea it is suggested that the outer limit of the second stage might be of the order of 5 to 8 miles and that the outer limit for the third stage would be about 2 to 3 miles