The 1st U-boat flotilla also known as the Weddigen flotilla, was the first operational U-boat unit in Nazi Germany's Kriegsmarine (navy). Founded on 27 September 1935 under the command of Fregattenkapitän Karl Dönitz, it was named in honor of Kapitänleutnant Otto Weddigen. Weddigen, a U-boat commander during World War I, died on 18 March 1915 after his submarine U-29 was rammed by the British battleship HMS Dreadnought in the North Sea.

German submarine U-18 was a Type IIB U-boat of Nazi Germany's Kriegsmarine during World War II. It was laid down 10 July 1935 and commissioned on 4 January 1936. It served in many U-boat flotillas during its service.