SM U-20 was a German Type U 19 U-boat built for service in the Imperial German Navy. She was launched on 18 December 1912, and commissioned on 5 August 1913. During World War I, she took part in operations around the British Isles. U-20 became infamous following her sinking of the British ocean liner RMS Lusitania on 7 May 1915, an act that dramatically reshaped the course of the First World War.

On 7 May 1915, U-20 was patrolling off the southern coast of Ireland under the command of Kapitänleutnant Walther Schwieger. Three months earlier, on 4 February, the Germans had established a U-boat blockade around the British Isles and had declared any vessel in it a legitimate target.

At about 13:40 Schwieger was at the periscope and saw a vessel approaching. From a distance of about 700 metres (770 yd), Schwieger noted she had four funnels and two masts, making her a passenger liner of some sort. He fired a single torpedo. It hit on the starboard side, almost directly below the bridge. Schwieger wrote that he was surprised by the size of the explosion, reasoning that a second explosion must have happened, possibly caused by coal dust, a boiler explosion, or powder. According to his logs, only then did he recognise her as the Lusitania, a vessel in the British Fleet Reserve. In 18 minutes, Lusitania sank with 1,197 casualties. The wreck lies in 300 feet (91 m) of water.

Fifteen minutes after he had fired his torpedo, Schwieger noted in his war diary:

"It looks as if the ship will stay afloat only for a very short time. [I gave order to] dive to 25 metres (82 ft) and leave the area seawards. I couldn't have fired another torpedo into this mass of humans desperately trying to save themselves."

There was at the time a great controversy about the sinking, over whether Lusitania was armed, carrying troops or illegal explosives to England and over Schwieger's method of attack. The Allies and the United States originally thought the U-20 fired two torpedoes. Postwar investigations showed only one was fired.

Before Schwieger got back to the docks at Wilhelmshaven for refuelling and resupply, the United States had formally protested to Berlin against the brutality of his action.

Kaiser Wilhelm II wrote in the margins of the American note, "Utterly impertinent", "outrageous", and "this is the most insolent thing in tone and bearing that I have had to read since the Japanese note last August." Nevertheless, to keep America out of the war, in June the Kaiser was compelled to rescind unrestricted submarine warfare and require all passenger liners be left unmolested.

On 4 September 1915 Schwieger was back at sea with U-20, 85 nautical miles (157 km; 98 mi) off the Fastnet Rock in the south Irish Sea. This rock held one of the key navigational markers in the western ocean, the Fastnet Lighthouse, and any ships passing in and out of the Irish Sea would be within visual contact of it.

RMS Hesperian was beginning a run outward bound from Liverpool to Quebec and Montreal, with a general cargo, also doubling as a hospital ship, and carrying about 800 passengers when she was attacked and sunk by U-20 off the Fastnet. The History of the Great War: The Merchant Navy, Vol. II, by Hurd, reads:

"Only a few days before, Count Bernsdorff, the German Ambassador, had assured the United States government that passenger liners will not be sunk without warning and without ensuring the safety of the non-combatants aboard providing that the liners do not try to escape or offer resistance."

Schwieger was reprimanded by the Admiralty but was unrepentant. The Germans decided to report that the ship was hit by a mine.

On 4 November 1916, U-20 grounded on the Danish coast south of Vrist, a little north of Thorsminde after suffering damage to its engines. Her crew attempted to destroy her with explosives the following day, succeeding, however, only in damaging the boat's bow (see picture) but making it effectively inoperative as a warship.

The U-20 remained on the beach until 1925 when the Danish government blew it up in a "spectacular explosion". The Danish navy removed the deck gun and made it unserviceable by cutting holes in vital parts. The gun was kept in the naval stores at Holmen in Copenhagen for almost 80 years. The conning tower was removed and placed on the front lawn of the local museum Strandingsmuseum St. George Thorsminde, where it still is today.

Novelist Clive Cussler claimed his National Underwater and Marine Agency (NUMA) located the remains of U-20 in 1984, about 400 yards from shore.

Date	Name	Nationality	Tonnage	Fate
30 January 1915	Ikaria	United Kingdom	4,335	Sunk
30 January 1915	Oriole	United Kingdom	1,489	Sunk
30 January 1915	Tokomaru	United Kingdom	6,084	Sunk
7 March 1915	Bengrove	United Kingdom	3,840	Sunk
9 March 1915	Princess Victoria	United Kingdom	1,108	Sunk
11 March 1915	Florazan	United Kingdom	4,658	Sunk
5 May 1915	Earl of Lathom	United Kingdom	132	Sunk
6 May 1915	Candidate	United Kingdom	5,858	Sunk
6 May 1915	Centurion	United Kingdom	5,495	Sunk
7 May 1915	Lusitania	United Kingdom	30,396	Sunk
8 July 1915	Marion Lightbody	Russia	2,176	Sunk
9 July 1915	Ellesmere	United Kingdom	1,170	Sunk
9 July 1915	Leo	Russia	2,224	Sunk
9 July 1915	Meadowfield	United Kingdom	2,750	Sunk
13 July 1915	Lennok	Russia	1,142	Sunk
2 September 1915	Roumanie	United Kingdom	2,599	Sunk
3 September 1915	Frode	Denmark	1,875	Sunk
4 September 1915	Hesperian	United Kingdom	10,920	Sunk
5 September 1915	Dictator	United Kingdom	4,116	Sunk
5 September 1915	Douro	United Kingdom	1,604	Sunk
5 September 1915	Rhea	Russia	1,145	Sunk
6 September 1915	Guatemala	France	5,913	Sunk
7 September 1915	Bordeaux	France	4,604	Sunk
7 September 1915	Caroni	United Kingdom	2,652	Sunk
8 September 1915	Mora	United Kingdom	3,047	Sunk
30 April 1916	Bakio	Spain	1,906	Sunk
1 May 1916	Bernadette	France	486	Sunk
2 May 1916	Ruabon	United Kingdom	2,004	Sunk
3 May 1916	Marie Molinos	France	1,946	Sunk
6 May 1916	Galgate	United Kingdom	2,356	Sunk
8 May 1916	Cymric	United Kingdom	13,370	Sunk
1 August 1916	Aaro	United Kingdom	2,603	Sunk
29 August 1916	Ibo	Portuguese Navy	397	Damaged
26 September 1916	Thelma	United Kingdom	1,002	Sunk
18 October 1916	Ethel Duncan	United Kingdom	2,510	Sunk
23 October 1916	Arromanches	France	1,640	Sunk
23 October 1916	Chieri	Italy	4,400	Sunk
23 October 1916	Felix Louis	France	275	Sunk
26 October 1916	Fabian	United Kingdom	2,246	Damaged

• Room 40

• Bailey, Thomas A.; Ryan, Paul B. (1975). The Lusitania Disaster: An Episode in Modern Warfare and Diplomacy. New York/London: Free Press/Collier Macmillan.
• Gröner, Erich; Jung, Dieter; Maass, Martin (1991). German Warships 1815–1945, U-boats and Mine Warfare Vessels. Vol. 2. Translated by Thomas, Keith; Magowan, Rachel. London: Conway Maritime Press. ISBN 0-85177-593-4.
• Spindler, Arno (1966) [1932]. Der Handelskrieg mit U-Booten. 5 Vols. Berlin: Mittler & Sohn. Vols. 4+5, dealing with 1917+18, are very hard to find: Guildhall Library, London, has them all, also Vol. 1-3 in an English translation: The submarine war against commerce.
• Beesly, Patrick (1982). Room 40: British Naval Intelligence 1914–1918. London: H Hamilton. ISBN 978-0241108642.
• Halpern, Paul G. (1920). A Naval History of World War I. New York: Routledge. ISBN 978-1857284980.
• Roessler, Eberhard (1997). Die Unterseeboote der Kaiserlichen Marine. Bonn: Bernard & Graefe. ISBN 978-3763759637.
• Schroeder, Joachim (2002). Die U-Boote des Kaisers. Bonn: Bernard & Graefe. ISBN 978-3763762354.
• Koerver, Hans Joachim (2008). Room 40: German Naval Warfare 1914–1918. Vol I., The Fleet in Action. Steinbach: LIS Reinisch. ISBN 978-3-902433-76-3.
• Koerver, Hans Joachim (2009). Room 40: German Naval Warfare 1914–1918. Vol II., The Fleet in Being. Steinbach: LIS Reinisch. ISBN 978-3-902433-77-0.

• U20 at Strandingsmuseum St. George, Thorsminde
• Royal Danish Naval Museum Archived 23 July 2012 at the Wayback Machine
• Photos of cruises of German submarine U-54 in 1916–1918. Great photo quality, comments in German.
• A 44 min. film from 1917 about a cruise of the German submarine U-35. A German propaganda film without dead or wounded; many details about submarine warfare in World War I.
• Helgason, Guðmundur. "WWI U-boats: U 20". German and Austrian U-boats of World War I - Kaiserliche Marine - Uboat.net.
• Room 40: Archived 15 October 2018 at the Wayback Machine original documents, photos and maps about World War I German submarine warfare and British Room 40 Intelligence from The National Archives, Kew, Richmond, UK.

56°35′00″N 08°07′50″E / 56.58333°N 8.13056°E / 56.58333; 8.13056