Final answer:
A partly submerged vessel being towed must display specific day shapes to indicate its status. A diamond shape is used if the vessel's tow exceeds 200 meters. Two black balls are used to indicate a vessel not under command.
Step-by-step explanation:
In accordance with maritime regulations, a partly submerged vessel being towed needs to display special symbols known as day shapes during daylight hours to indicate its status to other vessels. It must use a diamond shape if the vessel's tow exceeds 200 meters in length. Alternatively, if a vessel is not capable of moving under its own power (not under command), then it should display two black ball shapes, one above the other. The purpose of these day shapes is to allow other vessels to understand the state of the towed vessel and take appropriate action to avoid collisions.
Learn more about Day Shapes on Vessel