Of the several identifiers attached to any commercial vessel, the IMO number is the one that matters most for long-term research. It is a seven-digit number issued by the International Maritime Organization (originally on behalf of Lloyd’s Register; today administered by IHS Markit) and assigned to the hull at the time of construction. Critically, an IMO number stays with the hull for the entire service life of the vessel. It does not change when the vessel is sold, renamed, re-flagged, or refurbished, and it is never reused for another vessel after the original is scrapped.
Why this matters
The fixed-for-life property is what makes the IMO number useful as a research anchor. If you are investigating a vessel’s history — its previous owners, its port-state-control inspection history, its involvement in past incidents, its insurance and class records — the IMO number is the key that joins records across decades and across multiple naming events. If you instead anchor your research on the vessel name, you will find the trail goes cold the first time the ship is renamed; if you anchor on the MMSI, the trail will go cold the first time the ship changes flag.
How to recognise an IMO number
Valid IMO numbers are seven digits long. They include a check digit that is computed from the first six digits, so it is easy to detect transcription errors. The check digit is the unit digit of the sum (7×d1 + 6×d2 + 5×d3 + 4×d4 + 3×d5 + 2×d6). When a number you have copied does not validate, the most common cause is a digit transposition or a missing leading zero rather than the source being wrong. Any reputable maritime database, including PortWatch, will reject an IMO number that fails the check-digit test.
Coverage
IMO numbers are issued to all SOLAS-class vessels of 100 gross tonnage and above. There are some excluded categories — fishing vessels of certain sizes, naval auxiliaries, pleasure craft, vessels operating only on inland waterways. PortWatch indexes commercial cargo tonnage and so virtually all the vessels in the directory carry IMO numbers. Where a small craft appears without an IMO number, the directory falls back to MMSI as the canonical identifier and clearly indicates the absence of an IMO number on the vessel’s profile page.
IMO vs MMSI vs call sign
It is worth keeping all three identifiers straight. The IMO number identifies the hull permanently. The MMSI (Maritime Mobile Service Identity) is a nine-digit number issued by the flag-state telecommunications regulator and identifies the vessel’s radio installation; it is the key for AIS reception logs and VHF radio licensing records. The call sign is a short alphanumeric string also issued by the flag-state regulator and used in voice radio communications. When a vessel is reflagged, the MMSI and call sign change but the IMO number does not. When you cite a vessel in any kind of permanent-record context, lead with the IMO number.