Was filmed using an authentic WWII German Submarine, the Spanish Navy Submarine G7, it was a German Type VII-C submarine, interned at Cartagena after being damaged by a RAF plane, and later repaired and acquired from Germany.
German submarine U-47 was a VII B-type U-boat built in 1937 in Kiel, Germany, and commissioned in 1938 under Capt. Günther Prien's command. During its career U-47 sank 31 enemy vessels including the British battleship HMS Royal Oak at Scapa Flow in Scotland on 14 October 1939. In 1941 U-47 went missing and is presumed to have been sunk by British warships. It had a crew of 47 officers and men.