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

The tonnage certificate provides two Convention measures of the ship: GT describes its overall size and NT its useful capacity under the applicable definitions and formulas. The IMO number instead allows the same ship’s identity to be followed through changes of name, ownership or flag. Document review must therefore verify both the dimensional data and the subject identified.

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.

A.1215(34) integrates the ship and Company/Registered Owner number schemes while retaining distinct subjects. The ship number’s permanence through name, flag or ownership changes is confirmed by the IMO scheme page.

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

1. Compare GT, NT and principal characteristics against the certificate.
2. Have tonnage-affecting alterations assessed.
3. On flag transfer check certificate continuity and replacement.
4. Retain the ship IMO number and distinguish company and owner numbers.
5. Align certificates, records and required markings.

For an alteration retain a description of the affected spaces or characteristics, drawings and competent assessment with the resulting certificate. On flag transfer record the actual transfer date and document replacement; compare ship number, entity numbers and markings separately.

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.

The file retains the actual flag-transfer date, applicable continuity limit and replacement-document date. Identity review uses the same ship IMO number, while flag and entity particulars are reconciled in the pertinent documents.

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

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

Certified substantive revision: 13 October 2026 · page fingerprint 30b73561a825