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The Registration of a vessel is required under UN Law of the Sea convention and is governed by the Ship Registration Act. 

It is an International Proclamation of the ownership and the purpose of vessel. The vessel must have a Genuine Link to Flag State.

The Ship Registration Act allows for South Africans to register in South Africa and thus fly the South African Flag on their vessels.

Registration is required if the vessel is operating outside the maritime zones of South Africa. (On the high seas and inside another states maritime zones.) 

Foreign States can obtain information regarding SA registered vessels from Registrar of Ships.