Merchant ships are commonly classed by a classification society. A vessel belongs to a certain Country and this is known as the nationality of the vessel, or flag of the vessel. Vessels are generally registered at a prominent port of that country. This nationality allows a ship to travel internationally as it is proof of ownership of the vessel. 

Vessels are said to be "in class" when their hull, structures, machinery, and equipment conform to International Maritime Organization and MARPOL standards. Vessels out of class may be un-insurable and/or not permitted to sail by other agencies.

A vessel's class may include particular endorsements for the cargo type carried; such as "oil carrier", "bulk carrier", "mixed carrier" etc. In some occasions such vessels may also include certain class notations detailing special abilities of the vessel and it's operation. Examples of this include an ice class, fire fighting capability, oil recovery capability, automated machinery space capability, or other special ability.

Sydney and Melbourne are among the ports of registry of Australia. Monrovia is the Port of Registry for a vessel registered in Liberia etc.

Vessels registered in a particular country by the Owner of the vessel are obliged to follow the shipping law or the relevant Marine Act of that country. However, generally speaking the governing safety management systems inboard relevant to the marine law of any particular country is in keeping with the minimum requirements set out within the SOLAS Convention.

The SOLAS convention ensures that ships flagged by signatory States comply with minimum safety standards in construction, equipment and operation. The SOLAS Convention in its successive forms is generally regarded as the most important of all international treaties concerning the safety of merchant ships.

The decision of a shipowner to register his vessel in a particular country is commonly commercially driven. Taxes, tariffs and crewing play a major part in the registration process of a vessel.

The Flag of Convenience describes the business practice of registering a merchant ship in a sovereign state different from that of the ship's owners, and flying that state's civil ensign on the ship. Ships are registered under flags of convenience to reduce operating costs or avoid the regulations of the owner's country. Panama is currently the world's largest flag state, with almost a quarter of the world's ocean-going tonnage registered there.

The Administrating Countries as they are called, have their vessels inspected from time to time at regular intervals and verify that the vessels conform with the Maritime Laws of that country and within the safety standards of that country in accordance with the SOLAS convention. At regular intervals, when a vessel is in a reasonably busy port, the Administrating Country appoints a surveyor to carry out the flag state control on that vessel.

AmSpec CR Cox are the official flag state surveyors for some of the prominent Flag States. These include:

  • Bureau of Liberian Maritime Affairs.

  • Panama Bureau of Shipping.

  • Cyprus Shipping Register.

  • The Bahamas Maritime Authority.

Similarly, when a vessel visits a port, the maritime authority of that country has a right to inspect the vessel and ensure that it is maintained properly and is seaworthy in all respects. An un-seaworthy vessel on the coast of any country would pose enormous danger to the coastal environment of that country. In Australia these inspections are carried out by A.M.S.A. This stands for Australian Maritime Safety Authority. This type of an inspection is called Port State Control inspection.