What number should be displayed on a ship at all times?

• A. IMO number
• B. Vessel's name
• C. Official number
• D. Call sign

Answer: (C)

The number that should be displayed on a ship at all times is the Official Number. This is the unique identifier assigned by the U.S. Coast Guard when a vessel is documented or registered, and it stays with the vessel regardless of changes in name or ownership. Displaying the Official Number on the hull (often near the vessel name and port of registry) allows authorities and registries to positively identify the vessel at a glance during inspections, searches, or paperwork checks.

The IMO number is an international identifier used primarily in global shipping and for tracking on international voyages, but it is not the standard identifier that must be displayed on every U.S.-registered vessel at all times. The vessel’s name is important for identification but is not a number. The call sign is a radio identifier used for communications, not a persistent hull identifier.