Under what condition do the Rules require both vessels to change course?

The requirement for both vessels to change course arises when two power-driven vessels are meeting head-on. In this situation, the navigational rules specify that each vessel should alter its course to pass safely on the port side of the other. This mutual course change is essential to prevent collisions, as both vessels are equipped to maneuver and can adjust their paths effectively.

In broader terms, this situation is covered under the International Regulations for Preventing Collisions at Sea (COLREGs). The rule that applies states that when two vessels are approaching each other head-on, both must take action to avoid a collision, thus ensuring safer navigation.

The other scenarios do not create a requirement for both vessels to change course. For instance, when only one vessel is power-driven or when two sailing vessels are meeting, the rules dictate different responsibilities depending on the status of the vessels involved. The situation where either vessel signals to turn does not inherently require a course change from the other vessel without considering the relative positions or danger presented.