Submarines are small compared to most oceangoing vessels, and generally do not have the ability to carry large amounts of food, fuel, torpedoes, and other supplies, nor to carry a full array of maintenance equipment and personnel. The tender carries all these, and either meets up with the submarines at sea to replenish them or provides these services while docked at a port near the area where the submarines are operating. In some navies, the tenders were equipped with workshops for maintenance, and as floating dormitories with relief crews.
In the Royal Navy, the term used for a submarine tender is "submarine depot ship", for example HMS Medway and HMS Maidstone, and the term used in the Chilean Navy is "submarine mother ship", as for example the BMS (buque madre de submarinos) Almirante Merino.
With the increased size and automation of modern submarines, plus in some navies the introduction of nuclear power, tenders are no longer as necessary for fuel as they once were.
In the United States Navy, submarine tenders are considered auxiliaries, with hull classification symbol "AS". As of 2015, the Navy only maintains two such tenders, USS Emory S. Land (AS-39) and USS Frank Cable (AS-40).
The Russian Navy decommissioned all its Don and Ugra class tenders inherited from the Soviet Navy by 2001. The last remaining ship of this class was the INS Amba (A54), initially sold to the Indian Navy in 1968 for use with their fleet of Foxtrot-class submarines. She was reportedly decommissioned from service in July 2006.
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