Tonnage is a measure of volume, not weight. Gross tonnage covers all enclosed spaces; net tonnage covers earning volume after deductions. For Indonesian-flag vessels, tonnage certificates are issued under the authority of the Directorate General of Sea Transportation — an independent surveyor measures, prepares and attends, but does not issue.
The terms, untangled
| Term | What it is | What it drives |
|---|---|---|
| Gross tonnage (GT) | Total enclosed volume of the vessel, expressed dimensionlessly | Registration category, manning scale, survey regime, port dues |
| Net tonnage (NT) | Earning volume after deductions for machinery and crew spaces | Canal and port dues, some taxation |
| Deadweight (DWT) | Weight of cargo, fuel, stores and crew the vessel can carry | Commercial capacity — not a tonnage measurement |
| Displacement | Weight of water the vessel displaces | Stability, docking, draft survey |
| Lightship | Weight of the vessel empty | Draft survey baseline, scrap valuation |
Why a rebuilt deckhouse matters
Gross tonnage counts enclosed volume. Enclose a previously open aft deck, add a deckhouse, raise a bulwark into a permanent structure, and you have added enclosed volume. That can push a vessel across a tonnage threshold, which in turn can change her registration category, her required manning, her survey regime and her port dues.
This catches out owners in Indonesia regularly, particularly on traditional vessels converted for charter. The extension is built, the vessel is presented for inspection, and the tonnage on the certificate no longer describes the vessel. Re-measurement before the work is signed off is far less costly than discovering it during an inspection.
Who issues what
- Tonnage certificates for Indonesian-flag vessels — issued under the authority of the Directorate General of Sea Transportation through its authorised offices or a recognised organisation.
- International Tonnage Certificates (1969) — issued by the flag administration or its recognised organisation for vessels in international trade.
- Independent surveyors — measure the vessel, prepare the calculation, identify discrepancies and attend the official measurement. They do not issue certificates.
- Classification societies — may issue tonnage certificates only where authorised by the flag as a recognised organisation.
Traditional vessels are frequently under-measured
Large wooden vessels built by eye on Sulawesi beaches often carry historic tonnage figures that do not reflect the vessel as she stands today, especially after conversion for charter. If you are buying a phinisi to operate commercially, treat the existing tonnage as provisional until it is verified — the manning and certification consequences of being on the wrong side of a threshold are significant.
Frequently asked questions
What does tonnage measurement mean for a vessel?
It establishes gross and net tonnage — measures of enclosed and earning volume — which determine registration category, manning, dues and applicable survey regime.
Who can issue a tonnage measurement certificate in Indonesia?
It is issued under the authority of the Directorate General of Sea Transportation, through authorised offices or a recognised organisation. Independent surveyors measure and prepare, but do not issue.
Does foreign tonnage carry over on import?
Not automatically. Re-measurement is common when a foreign-built or foreign-flag vessel is registered in Indonesia.