USNS Mercy is the

USNS Mercy is the biggest U.S hospital ship and also the lead vessel of her class. The ship does not have any offensive weapons, in compliance with the Geneva Conventions, but defensive weapons are available.
She was constructed as an oil tanker named SS Worth in 1976 by National Steel and Shipbuilding Company, San Diego, California.

In 1984, she was converted into a hospital ship by the company and entered service in 1986. The conversion took 35 months and cost a whopping 208 million dollars.
The Mercy class ships are the 3rd biggest in the U.S. Navy, in terms of length, surpassed only by Nimitz and Gerald R. Ford Class of Aircraft Carriers.
The biggest U.S. hospital ship has a transom stern, extended deckhouse, a forward bridge, a bulbous bow and a helipad with flight control facility.
The ship provides medical and surgical services to Marine Corps, Air/Ground Task Forces, Army, Air Force units deployed ashore, naval amphibious task forces and battle forces offshore.
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