In which situation do the Rules require both vessels to change course?

The situation where both vessels are required to change course occurs when two power-driven vessels are meeting head-on. Under the International Regulations for Preventing Collisions at Sea (COLREGs), specifically Rule 14, when two power-driven vessels approach each other in such a manner that there is a risk of collision, they must each alter their course to avoid the other. This scenario is characterized by both vessels heading directly towards one another, necessitating a mutual change of course in order to prevent an accident.

In contrast, the other scenarios do not require both vessels to change course. When a sailing vessel and a power-driven vessel meet, the sailing vessel typically has the right of way and the power-driven vessel must give way, which does not necessitate a mutual course change. Additionally, when two sailing vessels have the wind behind them, they must take care to avoid collisions, but they do not necessarily both have to change course—it's based more on their positions. In the case of a power-driven vessel overtaking a sailing vessel, the overtaking vessel is responsible for ensuring that it passes safely, again without the requirement for both to change course.