Which indication shows that a vessel is constrained by its draft?

A vessel that is "constrained by its draft" refers to a situation where the vessel's ability to navigate is restricted due to the depth of water in which it operates. This is a specific condition recognized in maritime navigation rules. The correct indication for a vessel constrained by its draft is the display of two vertical black shapes or two black balls, but it is also recognized in the context of navigation lights and signals.

When a vessel is not under command, it is incapable of maneuvering as required by the navigation rules and may exhibit special signals. The appropriate indication for a vessel not under command includes displaying two all-round red lights. These signals are critical for other vessels in the area to recognize that the vessel is unable to comply with typical navigation rules, which makes it important for safety.

The rationale for "not under command" being the correct choice stems from the recognition that vessels constrained by their draft and those not under command share the characteristic of being limited in their ability to navigate in the standard manner. Hence, even though the specific indications for each scenario differ, highlighting 'not under command' in the context of navigation rules emphasizes the importance of recognizing the limitations of any vessel, which can also apply to those constrained by their draft.