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IMO numbers are unique 7-digit identifiers assigned by IHS Maritime (on behalf of the IMO) to ships of 100 GT and above engaged in international voyages, and to all motorised inshore craft of 12 m and above. The number is permanent — it stays with the vessel regardless of flag, name, or owner change.
An IMO number is six significant digits followed by one check digit. To validate: multiply each of the first six digits by the weights 7, 6, 5, 4, 3, 2 respectively; sum the products; the units digit of the sum (i.e. sum modulo 10) must equal the check digit. This makes it easy to detect single-digit transcription errors.
Example: IMO 9074729 — digits 9,0,7,4,7,2 × 7,6,5,4,3,2 = 63+0+35+16+21+4 = 139 → last digit = 9. ✓
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