Ask two people in shipping who is responsible for a vessel’s certificates and you will often get two different answers. One will say class. The other will say flag. Both are partly right, and the confusion is understandable, because in most cases the same surveyor carries out both sets of checks during the same attendance.
For vessel superintendents, HSE officers, procurement teams and crew, the distinction is not academic. It determines which certificate expires when, who has the authority to accept a deficiency, and who must be notified when equipment fails. This article explains what each body actually does, where their responsibilities overlap, and what that means for day to day operation.
What is a ship classification society?
A classification society is a private, non-governmental technical organisation. It writes and publishes its own rules for the design, construction and ongoing condition of ships, surveys vessels against those rules, and issues a class certificate confirming compliance.
Class rules concentrate on the physical integrity of the asset. That covers hull structure and scantlings, main and auxiliary machinery, electrical installations, steering gear, cargo handling arrangements and the systems essential to the vessel’s operation. A society will approve drawings before construction begins, attend the yard during the build, and then keep the ship under a rolling survey programme for the rest of its working life.
The most established societies belong to the International Association of Classification Societies (IACS), founded in Hamburg in 1968. IACS currently has twelve members:
- ABS
- Bureau Veritas
- CCS
- Croatian Register of Shipping
- DNV
- Indian Register of Shipping
- Korean Register
- Lloyd’s Register
- ClassNK
- Polish Register of Shipping
- RINA
- Türk Loydu
Türk Loydu was admitted on 1 November 2023, restoring the membership to twelve after the Russian Maritime Register of Shipping was expelled in 2022. Between them, IACS members class more than 90 per cent of the world’s cargo carrying tonnage.
One point is often missed. A classification society holds no sovereign power of its own. It cannot legislate, it cannot detain a vessel, and it cannot grant a ship nationality. Its influence comes from contract and from commercial necessity. Hull and machinery underwriters, P&I clubs, charterers and lenders all expect a vessel to be in class, and suspension or withdrawal of class has immediate commercial consequences.
What is a flag state authority?
A flag state is the country in which a ship is registered, acting through its maritime administration. Under Article 91 of UNCLOS, each state sets the conditions for granting its nationality to ships. Article 94 then requires every state to effectively exercise jurisdiction and control in administrative, technical and social matters over the ships flying its flag.
In practice, that obligation produces a wide set of duties. The flag administration maintains the register and issues the certificate of registry. It gives legal effect to IMO instruments such as SOLAS, MARPOL, the Load Line Convention, STCW, the Ballast Water Management Convention and the Maritime Labour Convention. It sets minimum safe manning, approves exemptions and equivalents, investigates casualties involving its vessels, and issues or authorises the issue of the statutory certificates a ship must carry.
The largest registers are open registers. At the start of 2026, Liberia, Panama and the Marshall Islands hosted the world’s largest ship registers by tonnage, with Hong Kong, Singapore, Malta and the Bahamas also carrying substantial fleets. Flag choice is a commercial decision, but it carries a regulatory consequence, because a flag’s inspection and detention record follows every vessel on its register.
The core difference, in one table
| Classification society | Flag state authority | |
| Legal nature | Private technical body, non-governmental | Sovereign state acting through its maritime administration |
| Source of authority | Its own published rules and the contract with the owner | International conventions and national law |
| Main focus | Structural and mechanical condition of the ship | Compliance with conventions, manning, and the conduct of the ship |
| Documents issued | Certificate of class, survey status report, conditions of class | Certificate of registry and the statutory certificates |
| Enforcement | Suspension or withdrawal of class | Detention, deregistration, prosecution, casualty investigation |
| Who it answers to | Its members, IACS quality criteria, and the flags that authorize it | IMO, other states, and its own government |
Put simply, class asks whether the ship is technically sound against a set of engineering rules. Flag asks whether the ship, the company and the crew comply with the law of the state whose flag it flies.
Where the two meet: the recognized organization
Here is where most of the confusion originates. SOLAS regulation I/6 states that inspection and survey shall be carried out by officers of the Administration, but allows the Administration to entrust that work to nominated surveyors or to organizations it recognizes. Equivalent provisions sit in MARPOL, the Load Line Convention and the Ballast Water Management Convention.
Most flag states use that option. A classification society acting under such an authorization is called a recognized organization, or RO. The conditions are set out in the IMO Code for Recognized Organizations, adopted by resolutions MSC.349(92) and MEPC.237(65), which entered into force on 1 January 2015. The RO Code lays down the minimum criteria an organization must meet, the scope of what a flag may delegate, and the oversight the flag must maintain. Delegation must be recorded in a written agreement, and a flag may limit the scope so that an RO handles some certificates but not others.
Two consequences follow.
The first is that when a surveyor boards your vessel, you should know which hat is being worn. The same person may endorse a class annual survey and, minutes later, sign a Cargo Ship Safety Equipment Certificate on behalf of the flag. These are separate acts under separate authority, even though they appear on the same attendance report.
The second is that delegation does not transfer responsibility. The flag state remains accountable for the performance of the ships on its register. If an RO issues a certificate to a vessel that is subsequently detained, the flag carries the consequence alongside the society.
Who checks the checkers?
Both layers are themselves subject to external scrutiny, which is worth understanding when you are assessing risk across a fleet.
Flag states are audited under the IMO Member State Audit Scheme, which became mandatory on 1 January 2016. The audit standard is the IMO Instruments Implementation Code, adopted by resolution A.1070(28), and every member state must be audited within the audit cycle. A revised framework, adopted through resolution A.1211(34), introduced a continuous monitoring mechanism to supplement periodic audits.
Port state control provides the second check. Regional regimes such as the Paris MoU and the Tokyo MoU inspect foreign flagged vessels calling at their ports and publish performance data on both flags and recognized organizations. The Paris MoU White, Grey and Black list is built on inspections and detentions over a three year rolling period, covering flags with at least thirty inspections in that window. The lists derived from the 2025 inspection results apply from 1 July 2026. Flag and RO performance feed directly into ship risk profiling, which in turn drives how often a vessel is selected for inspection and how deeply it is examined.
This is why the flag and class combination on a vessel has a practical operating cost. A poorer performing pairing means more frequent and more detailed port state control attention.
What this means in practice on board
The clearest way to keep the two apart is to look at the certificate folder. Class documents relate to the condition of the structure and machinery. Statutory documents relate to convention compliance and are issued in the name of the flag, whether signed by the administration itself or by an RO on its behalf.
The class survey cycle runs on a five year period. Annual surveys fall on the anniversary date with a window of three months either side. An intermediate survey falls between the second and third anniversary. The class renewal survey, still widely called the special survey, closes the five year period and is the most extensive examination the ship undergoes, typically involving thickness measurement and dry docking. Bottom surveys are required twice in each five year period, with no more than thirty six months between them. Bulk carriers and oil tankers carry the additional requirements of the Enhanced Survey Programme.
Statutory certificates broadly follow the same harmonised five year pattern for cargo ships, with annual and intermediate endorsements, though passenger ships and certain certificates follow shorter validity periods.
Where crews and superintendents feel the difference most is in the response to a defect. If equipment covered by class fails, the society must be informed and the item may be raised as a condition of class with a due date attached. If the failure affects a convention requirement, the flag or the RO acting for it must be notified, and the master is also required to report to the port state authorities if the vessel is in a foreign port.
Common questions
Is classification legally required?
Classification itself is a commercial and technical arrangement rather than a treaty obligation. In practice it is unavoidable, because most flag administrations require registered vessels to be classed with an approved society, and insurers and charterers require it independently.
Does a class certificate replace statutory certificates?
No. A vessel in class with expired statutory certificates is not compliant, and a vessel with valid statutory certificates whose class has been suspended will still face serious commercial and insurance problems.
Who issues the ISM and ISPS certificates?
The Document of Compliance, Safety Management Certificate and International Ship Security Certificate are issued under the authority of the flag state. Most flags delegate the audits and issue to an RO.
Can a vessel change class or change flag?
Both are possible, and both are scrutinised. Transfer of class between IACS members follows a defined procedure requiring the outgoing and incoming societies to exchange survey history. Frequent flag changes are treated by port state control as a risk indicator.
Does a good flag guarantee a good inspection outcome?
It helps, but the ship risk profile also reflects the class society, the company performance, the ship type and age and the vessel’s own deficiency history.
Where Mr. Marine fits in
Whichever body attends, the questions on board are the same. Is the equipment functioning, is it calibrated, and can you evidence it.
That is the layer Mr. Marine supports. We are a partner to owners, managers and superintendents on the equipment and competence items that surveyors, whether attending for class, for flag or for port state control, will ask to see:
- Ballast water treatment system support, including BWTS operator training and the BWTS annual calibration and certification, so the installation and the crew operating it both stand up to examination under the Ballast Water Management Convention.
- Gas detection readiness, covering the supply of calibration gas alongside portable gas detectors. Being able to source both from a single supplier removes a recurring procurement problem and keeps bump testing and calibration records current for enclosed space entry.
- MIC and monitoring equipment calibration, keeping oil content meters and related instruments within their certification intervals ahead of survey.
- Marine elevator annual safety inspections, an item that is easy to overlook until it appears on a deficiency list.
If you are preparing for a renewal survey, a flag inspection or a port state control visit, talk to our team about consolidating this equipment and training scope into a single supply and service plan.
