Which vessel type is not under command in the collision hierarchy?

Answer: D — Restricted in her ability to maneuver

The concept of "not under command" refers to a specific category of vessels defined in the Collision Regulations. A vessel that is not under command is unable to maneuver as required by the Rules and cannot take the necessary actions to avoid a collision, often due to mechanical failure or other extraordinary circumstances. In the context of the collision hierarchy as described by the COLREGs, a vessel "restricted in her ability to maneuver" is in fact a recognized category. Such vessels may have limitations due to their work, such as those engaged in underwater construction, dredging, or other activities that impede their ability to navigate normally. While these vessels may be restricted, they are still able to take actions within the limits of their restrictions. Power vessels, sailing vessels, and fishing vessels, while they have their specific operational rules, typically have the capability to maneuver unless they're facing specific circumstances that would qualify them as not under command or restricted in their ability to maneuver. This is why a vessel "restricted in her ability to maneuver" is not categorized as not under command in the hierarchy of vessel types. It is crucial to understand these distinctions for safe navigation and collision prevention at sea.