What does it mean if a commercial vessel displays a black ball over a black diamond over a black ball?

The display of a black ball over a black diamond over a black ball serves as a recognized day shape signaling that the vessel is restricted in its ability to manoeuvre. This combination of shapes is outlined in maritime navigation rules, indicating to other vessels that the represented vessel has limitations on its ability to navigate due to the nature of its work or operations.

When a commercial vessel shows this signal, it is important for other waterway users to recognize that the vessel may not be able to take the usual measures to avoid collisions, which could include altering its course or speed. Observing this configuration helps maintain safety on the water by informing others of the vessel's status, allowing them to adjust their navigation appropriately to avoid potential hazards.

The other options do not accurately represent the implications of the displayed shapes. For instance, being anchored and not moving would require a different marker, and similarly, a vessel that is preparing to fish or requires assistance would use other specific signals to ensure their intentions are clearly communicated to surrounding traffic.