What is indicated by three balls displayed in a vertical line?

Three balls displayed in a vertical line indicate that a vessel is aground. This signaling is specified in the COLREGs (International Regulations for Preventing Collisions at Sea), where a vessel that is unable to move properly due to grounding must use these visual signals to inform other vessels of its situation.

This flagging serves as an important navigational warning, alerting nearby vessels that the aground ship is currently not making way and may pose a hazard. Understanding this visual cue is crucial for safe navigation, as it helps prevent collisions and informs other vessels of the limitations and status of the aground vessel.

The other choices might represent different situations but do not apply to the vertical alignment of three balls. A vessel at anchor, for example, would display a different signal by using one ball, while a vessel restricted in its ability to maneuver would show a different configuration of lights or shapes, such as a ball and diamond arrangement.