Tonnage Certificate and IMO Number
The Tonnage Certificate records GT and NT; the IMO number permanently identifies the ship. Tonnage and identity are distinct data that must remain consistent across documents.
Operational Explanation
GT and NT derive from volumes and Convention formulas: they are not ship weight or deadweight in tonnes. The tonnage certificate does not follow five-year HSSC renewal; alterations and flag transfers instead require checking specific provisions.
Regulatory Reference
Tonnage 1969, articles 3–4, covers ships on international voyages within its scope, excluding warships, ships under 24 m in convention length and ships navigating solely in the waters listed in article 4.2. Domestic measurement requirements may differ. Tonnage 1969, articles 2–4.
Within its scope, SOLAS XI-1/3 requires permanent IMO-number marking both outside in a visible position and inside in an easily accessible position, at locations permitted by paragraph 4. Check both markings as well as consistency with documents. SOLAS XI-1/3.4.
Scope of Application
SOLAS XI-1/3 requires the number for passenger ships from 100 GT and cargo ships from 300 GT within its scope. A.1215 also includes voluntary allocation: the scheme is not identical to those thresholds.
Procedure / How to Complete It
- Compare GT, NT and principal characteristics against the certificate.
- Have tonnage-affecting alterations assessed.
- On flag transfer check certificate continuity and replacement.
- Retain the ship IMO number and distinguish company and owner numbers.
- Align certificates, records and required markings.
Practical Example
On transfer to another Contracting State, Article 10(3) retains the old tonnage certificate for up to three months or until replacement if earlier. Do not extend this rule to other certificates; the ship IMO number remains unchanged.
What Typically Goes Wrong
Confusing GT with tonnes of weight, assuming unlimited validity after alterations or requesting a new ship IMO number for flag transfer.
Common Mistakes Mistake Library
| Mistake | How to avoid it |
|---|---|
| Confusing GT with tonnes of weight, assuming unlimited validity after alterations or requesting a new ship IMO number for flag transfer. | Check the individual ship’s requirements and documents before closing the work. |
What the PSCO Checks
Operational Tips
- Keep documents and reports in the same file.
- Record the owner, action and deadline for each finding.
- Check consistency after each document change.
Preparation checklist
- Applicable regime checked
- Documents consistent
- Changes assessed
- Surveys and authorisations documented
- Required actions tracked
FAQ
Related Topics
Last substantive revision of this page: 16 September 2026 · page fingerprint 6d6c48b348b1