Answer
IMO number is a permanent international identification number of the ship and remains unchanged throughout the vessel’s life, irrespective of change of name, owner or flag. Official Number is a national registration number allotted by the Flag State registry when the ship is registered, and it can change when the vessel changes registry or Flag.
| Point | IMO Number | Official Number |
|---|---|---|
| Purpose | Permanent international identity of the ship | National registration identity given by the Flag State |
| Issued/allocated by | IMO Ship Identification Number Scheme; number is assigned through IHS Markit/S&P Global Maritime on behalf of IMO | Flag State / ship registry |
| Changes with change of flag? | No | Yes, may change when ship changes registry/flag |
| Changes with change of name/owner? | No | Normally remains linked to that national registration; may change on re-registration |
| Format | Usually IMO + 7 digits, e.g. IMO 9876543 | Format depends on the country/registry |
| When assigned? | Generally when the ship becomes eligible and is entered into the identification scheme, commonly from construction/newbuilding stage | When the ship is registered under a particular Flag State |
| Main use | Global identification, safety, security, PSC, certificates, databases | National legal registration and registry records |