U-20

The U-20 was a German U-boat in service in the 1910s. During World War I, it was under the command of Kapitänleutnant Walther Schwieger. On 7 May 1915, the U-20 torpedoed the RMS Lusitania.

In an alternate timeline, the U-20 was hijacked by the Fifth Doctor so he could get back to his TARDIS. (BFA: The Sirens of Time)