What does three black balls in a vertical line signify?

Three black balls displayed in a vertical line signify that a vessel is aground. This specific day shape is part of the International Regulations for Preventing Collisions at Sea (COLREGs), which mandates that vessels must display certain shapes to communicate their status to others on the water.

When a vessel is aground, it is important for nearby ships to be aware of this condition to avoid collisions, as an aground vessel may not be able to move and could pose a hazard to navigation. The use of three black balls in a vertical arrangement effectively communicates this state without the need for verbal communication, enabling other vessels to take appropriate action, either by giving wide berth or by notifying the aground vessel of the situation.

The other options do not match this specific signaling requirement in the COLREGs. For instance, a vessel at anchor would display one black ball instead, while a fishing vessel would typically show a different set of shapes, such as two cones with the points facing each other. A vessel under sail similarly uses specific combinations of shapes, primarily the identification of sails rather than the display of balls.