Constrained by Draught (CBD)

A vessel whose ability to deviate from its current course is significantly restricted due to the depth of water relative to its draught.

A vessel “Constrained by Draught” refers to a specific situation under the International Regulations for Preventing Collisions at Sea (COLREGs), where a ship’s manoeuvrability is severely limited because of the relationship between its draught (the vertical distance between the waterline and the bottom of the hull) and the available depth of water. This designation is particularly relevant for large, heavily laden ships navigating in shallow or narrow waterways, where the keel (the bottom-most part of the vessel) is close to the seabed. Such vessels cannot easily alter their course or speed without risking grounding or collision with underwater hazards. 

Under Rule 3: General Definitions, part (h) of the COLREGs, a vessel constrained by its draught is given special consideration, as its restricted manoeuvrability makes it less capable of avoiding other vessels. To signal this status, the vessel may display specific day shapes (a cylinder) or lights (three all-round red lights in a vertical line) to alert other mariners. It’s important to note that this designation is distinct from “restricted in ability to manoeuvre,” which refers to operational limitations rather than draught-related constraints. Mariners encountering such vessels are expected to exercise caution and provide ample room to ensure safe passage. This concept underscores the importance of understanding a vessel’s draught and the surrounding water depth when planning navigation, particularly in congested or shallow areas.