SMU-21 (rightmost boat in foreground)
History
German Empire
Name	U-21
Ordered	25 November 1910[1]
Builder	Kaiserliche Werft Danzig
Yard number	15[1]
Laid down	27 October 1911[1]
Launched	8 February 1913
Commissioned	22 October 1913
Fate	Sunk accidentally, 22 February 1919
Austria-Hungary
Name	U-36
Commissioned	21 September 1915
Decommissioned	1 October 1916
Fate	Returned to Imperial German Navy command
General characteristics
Class & type	Type U 19 submarine
Displacement	650 t (640long tons) surfaced 837 t (824 long tons) submerged
Length	64.15 m (210 ft 6 in)
Beam	6.10 m (20 ft)
Height	8.10 m (26 ft 7 in)
Draft	3.58 m (11 ft 9 in)
Propulsion	2 shafts 2 ×MAN 8-cylinder two strokediesel engines 2 × AEGdouble Motordynamos
Speed	15.4 knots (28.5 km/h; 17.7 mph) surfaced 9.5 knots (17.6 km/h; 10.9 mph) submerged
Complement	4 officers, 25 men
Armament	4 × 50 cm (19.7 in)torpedo tubes 1 ×8.8 cm (3.5 in) SK L/30 gun (from 1916 2 ×)
Service record
Part of:	Imperial German Navy: III Flotilla 1 August 1914 – 5 June 1915 Constantinople Flotilla 5 June 1915 – unknown end Pola Flotilla Unknown start – 4 March 1917 III Flotilla 4 March 1917 – 11 November 1918
Commanders:	Kptlt.Otto Hersing[2] 22 October 1913 – 31 August 1918 Kptlt. Friedrich Klein[3] 1 September – 11 November 1918
Operations:	11 patrols
Victories:	36 merchant ships sunk (79,005 GRT) 4 warships sunk (34,575 tons) 2 merchant ships damaged (8,918 GRT)

SMU-21 was aType U 19U-boat built for theImperial German Navy shortly beforeWorld War I. The third of four Type U-19-class submarines, these were the first U-boats in German service to be equipped with diesel engines.U-21 was built between 1911 and October 1913 at theKaiserliche Werft (Imperial Shipyard) inDanzig. She was armed with fourtorpedo tubes and a singledeck gun; a second gun was added during her career.

In September 1914,U-21 became the firstsubmarine to sink a ship with a self-propelledtorpedo when she destroyed the cruiserHMS Pathfinder off theFirth of Forth. She also sank several transports in theEnglish Channel and theIrish Sea later in the year, all in accordance with thecruiser rules then in effect. In early 1915,U-21 was transferred to theMediterranean Sea to support theOttoman Empire against the Anglo-French attacks during theGallipoli Campaign. Shortly after her arrival, she sank the BritishbattleshipsHMS Triumph andHMS Majestic while they were bombarding Ottoman positions at Gallipoli. Further successes followed in the Mediterranean in 1916, including the sinking of the Frencharmoured cruiserAmiral Charner in February.

Throughout 1916,U-21 served in theAustro-Hungarian Navy asU-36, since Germany was not yet at war with Italy and thus could not legally attack Italian warships under the German flag. She returned to Germany in March 1917 to join theunrestricted commerce war against British maritime trade. In 1918, she was withdrawn from front line service and was employed as a training submarine for new crews. She survived the war and sank while under tow by a British warship in 1919.

U-21 was 64.15 metres (210 ft 6 in)long overall with abeam of 6.10 m (20 ft) and a height of 8.10 m (26 ft 7 in). She displaced 650 tonnes (640 long tons) surfaced and 837 t (824 long tons) submerged. The boat's propulsion system consisted of a pair of 8-cylinder 2-strokediesel engines manufactured byMAN for use on the surface, and twoelectric double motor-dynamos built byAEG for use while submerged.U-21 and hersister boats were the first German submarines to be equipped with diesel engines. The electric motors were powered by a bank of two 110-cell batteries.U-21 could cruise at a top speed of 15.4knots (28.5 km/h; 17.7 mph) on the surface and 9.5 knots (17.6 km/h; 10.9 mph) submerged. Steering was controlled by a pair ofhydroplanes forward and another pair aft, and a singlerudder.^[4]

U-21 was armed with four 50-centimetre (19.7 in)torpedo tubes, which were supplied with a total of sixtorpedoes. One pair was located in the bow and the other was in the stern. She was initially fitted with a machine gun for use on the surface; by the end of 1914 this was replaced with a8.8 cm (3.5 in) SK L/30 gun. In 1916, a second 8.8 cm gun was added.U-21 had a crew of four officers and twenty-five enlisted sailors.^[5]

U-21 was built at theKaiserliche Werft (Imperial Shipyard) inDanzig (nowGdańsk, Poland). She waslaid down in 27 October 1911^[1] andlaunched on 8 February 1913. Afterfitting-out work was completed, she wascommissioned into the fleet on 22 October 1913.^[6]

At the outbreak ofWorld War I in August 1914,U-21 was based atHeligoland in theGerman Bight, commanded byKapitänleutnant (Captain Lieutenant) Otto Hersing. In early August, Hersing tookU-21 on a patrol into theDover Straits but he found no British vessels. On 14 AugustU-21 went on a second patrol, this time with her sister boatsU-19 andU-22, to the northernNorth Sea between Norway and Scotland. The patrol was an attempt to locate the British blockade line and gather intelligence, but they spotted only a singlecruiser and adestroyer off the Norwegian coast. Hersing attempted to enter theFirth of Forth—a majorRoyal Navy fleet base—later in the month but was unsuccessful.^[7]

On 5 September 1914,U-21 encountered the Britishscout cruiserHMS Pathfinder off theIsle of May. Hersing had surfaced his U-boat to recharge his batteries when a lookout spotted smoke fromPathfinder's funnels on the horizon.U-21 submerged to make an attack, butPathfinder turned away on her patrol line;U-21 could not hope to keep up with the cruiser while submerged, so Hersing broke off the chase and resumed recharging his batteries. Shortly thereafter,Pathfinder reversed course again and headed back towardU-21. Hersing manoeuvred into an attack position and fired a single torpedo, which hitPathfinder just aft of herconning tower. The torpedo detonated one of the cruiser'smagazines, which destroyed the ship in a large explosion.^[8] The British were able to lower only a singlelifeboat beforePathfinder sank. Other survivors were found clinging to wreckage by torpedo boats that rushed to the scene.Pathfinder was the first warship to be sunk by a modern submarine.^[9][10] A total of 261 sailors were killed in the attack.^[11]

U-21 caught the French steamerSS Malachite on 14 November; Hersing forced the ship to stop and examined her cargo manifest, ordered the crew to abandon ship, then sankMalachite with hisdeck gun.U-21's next success came three days later with the BritishcollierSS Primo, which he also sank in accordance with thecruiser rules that governedcommerce raiding.^[12] These two ships were the first vessels to be sunk in the restricted German submarine offensive against British and French merchant shipping.^[13]

On 22 January, Hersing took his U-boat through theDover Barrage in the Channel before turning into theIrish Sea. He shelled the airfield onWalney Island, though a coastal battery quickly forced him to withdraw. The next week,U-21 stopped the collierSS Ben Cruachan; after evacuating her crew, the Germans sank her with scuttling charges. Later that day, 30 January 1915,U-21 stopped and sank the steamersSS Linda Blanche andSS Kilcuan. In both cases, Hersing adhered to the prize rules, including flagging down a passingtrawler to pick up the ships' crews. After these successes,U-21 withdrew from the area to avoid the British patrols that would arrive in the aftermath of the sinkings.^[14] After passing back through the Dover Barrage,U-21 cruised back toWilhelmshaven.^[15]

In April 1915,U-21 was transferred to theMediterranean Sea to support Germany's ally,Turkey. She left Kiel on 25 April, and the first leg of the voyage, from Germany toAustria-Hungary, took eighteen days.^[16] Hersing took his submarine north around Scotland to avoid the Dover patrols, and rendezvoused with the supply shipSS Marzala offCape Finisterre to refuel. Unfortunately for the Germans,Marzala carried poor quality oil that could not be burned in the boat's diesel engines;U-21 had less than half of her fuel supply remaining, and was only halfway on the voyage to Austria-Hungary. Hersing was forced to run his U-boat on the surface to conserve fuel, which increased the risk of detection by Allied forces. While en route the Germans managed to avoid patrolling British and Frenchtorpedo boats and transport ships that might have reported their location.^[17]

U-21 finally arrived inCattaro on 13 May, with only 1.8 t (1.8 long tons) of fuel left in her tanks—she had left Germany with 56 t (55 long tons).^[17] She spent a week at the Austro-Hungarian submarine bases atPola and Cattaro in mid-May, where she was visited byGeorg von Trapp, an Austro-Hungarian U-boat commander.^[16] Several other German submarines joinedU-21 in the following months, after calls for assistance from the Ottoman ground forces on theGallipoli peninsula, who were taking heavy casualties from the bombardments from Allied warships. These U-boats includedU-33,U-34,U-35, andU-39.^[18]

U-21 arrived in her operational area off Gallipoli on 25 May; that day, she encountered the Britishpre-dreadnought battleshipHMS Triumph. Hersing brought his U-boat to within 300 yards (270 m) of his target and fired a single torpedo, which hitTriumph.U-21 then dived under the sinking battleship to escape the destroyers hunting her.^[19] Hersing took his boat to the sea floor to wait for the Allied forces to abandon the chase. After twenty-eight hours on the bottom,U-21 surfaced to recharge her batteries and bring in fresh air. On 27 May, Hersing attacked and sank his second battleship,HMS Majestic. This time, the British had attempted to protect her withtorpedo nets and several small ships, but Hersing was able to aim a torpedo through the defences.Majestic sank in four minutes.^[20] These two successes brought significant dividends: all Allied capital ships were withdrawn to protected anchorages and were thus unable to bombard Ottoman positions on the peninsula.^[21] For these two successes, the crew ofU-21 was awarded theIron Cross byKaiser Wilhelm II, while Hersing himself received thePour le Mérite, Germany's highest award for valour.^[22]

After sinkingMajestic, Hersing took his submarine to refuel at a Turkish port before attempting the dangerous route through theDardanelles to Constantinople. While transiting the straits,U-21 was nearly pulled into a whirlpool but the Germans managed to escape. After arriving in the Ottoman capital, the crew were given a large welcoming ceremony attended byEnver Pasha.U-21 required significant maintenance, and so the crew was given a month of shore leave while the repairs were carried out. Once the repair work was finished,U-21 sortied through the Dardanelles for another patrol. Hersing spotted the Allied munitions shipCarthage, which he sank with a single torpedo. Later on the patrol, a lookout on an Alliedtrawler spottedU-21's periscope; the Germans had to crash-dive to escape being rammed, but doing so brought them into a minefield. One mine exploded off the U-boat's stern but it caused no significant damage, andU-21 was able to withdraw to Constantinople.^[23]

U-21 thereafter moved to theBlack Sea where she andUB-14 served as the nucleus of the newly formedBlack Sea Flotilla.^[24] In September,U-21 undertook another patrol in the eastern Mediterranean. In the meantime, the Allies had established a completeblockade of the Dardanelles with mines and nets to prevent submarines from operating out ofConstantinople. Unable to return to Constantinople, Hersing instead took his U-boat back to Cattaro.^[25] Germany would not be in a formal state of war with Italy until August 1916. As a result, German U-boats could not legally attack Italian ships, despite the fact that Italy was at war with Austria-Hungary. To circumvent this restriction, German submarines operating in the Mediterranean were commissioned into theAustro-Hungarian Navy, though their German crews remained aboard. Following her arrival in Cattaro,U-21 wascommissioned as the Austro-HungarianU-36. She served under this name until Italy declared war on Germany on 27 August 1916.^[26]

In the meantime,U-36 began to have further successes against Allied maritime trade. On 1 February 1916, she sank the British steamerSS Belle of France.^[1] A week later,U-36 torpedoed and sank the Frencharmoured cruiserAmiral Charner off the Syrian coast. The cruiser sank quickly with heavy loss of life; 427 men went down with their ship.^[27] In early 1916, while patrolling off Sicily,U-36 encountered an AlliedQ-ship, anauxiliary cruiser disguised as an unarmed merchant ship.U-36 fired a shot across the Q-ship's bow, but it refused to stop and returned fire with a small deck gun. Hersing decided to close and sink the ship, which then revealed her heavy armament. Wounded by shell splinters, Hersing withdrew his submarine under cover of a smoke screen before submerging.^[28]

On 30 April, Hersing sank the British steamerCity of Lucknow. He sank three small Italian sailing vessels offCorsica between 26 and 28 October, and on 31 OctoberU-21 sent the 5,838grt steamshipGlenlogan to the bottom. Over the next three days, another four Italian ships—the steamshipsBernardo Canale andTorero and two small sailing vessels—were sunk offSicily. On 23 December,U-21 torpedoed the British steamerSS Benalder east ofCrete, but the ship managed to reachAlexandria.^[1]

In early 1917,U-21 was recalled to Germany to join theunrestricted submarine warfare campaign being waged against Britain. While en route, she stopped and sank a pair of British sailing vessels offPorto on 16 February and another pair of Portuguese sailing ships the next day. On 20 February,U-21 sank the French steamerCacique in theBay of Biscay. Two days later in theWestern Approaches, she finished off the Dutch steamerBandoeng, which had been damaged by the submarineUC-5 on 15 February. Seven more ships followedBandoeng that day. They included six more Dutch steamers—Eemland,Gaasterland,Jacatra,Noorderdijk,Zaandijk, andMenado—and the Norwegian steamerNormanna. On another patrol in late April, Hersing caught four more ships: the NorwegianGiskö andTheodore William on 22 April andAskepot on 29 April, along with the RussianBorrowdale on 30 April. Another Russian vessel,Lindisfarne, followed on 3 May. The British steamersAdansi andKillarney were sunk on 6 and 8 May, respectively. The SwedishBaltic, which proved to be Hersing's last victory, was sunk on 27 June.^[1][29]

Hersing attacked a convoy of fifteen merchant ships escorted by fourteen destroyers in August south-west of Ireland. He tookU-21 between two of the escorting destroyers and briefly used his periscope to gauge the speed and course of the transports before firing two torpedoes and diving. Hersing reported both torpedoes hit and the destroyers immediately rushed to begin theirdepth charge attacks. After a five-hour hunt, the destroyers withdrew to rejoin the convoy.^[30] The experience led Hersing to change tactics in future attacks on escorted convoys; instead of attacking the ships from as far away as possible, he chose to fire his torpedoes at closer range and then dive under the transport ships, where the destroyers would be unable to launch their depth charges for fear of damaging the transports.^[31] As of 1918, she was assigned to the III U-boat Flotilla.^[32] Later in 1918, the submarine was used as a training boat for new crews. She survived the war, but on 22 February 1919, she accidentally sank in the North Sea while under tow to Britain, where she was to be formally surrendered.^[33]

In the course of her commerce raiding,U-21 sank forty ships for a combined 113,580 gross register tons (GRT), and damaged two more for a total of 8,918 gross register tons (GRT). The ships sunk included two battleships and two cruisers.^[1]

Date	Name	Nationality	Tonnage[Note 1]	Fate[34]
5 September 1914	HMS Pathfinder	Royal Navy	2,940	Sunk
23 November 1914	Malachite	United Kingdom	718	Sunk
26 November 1914	Primo	United Kingdom	1,366	Sunk
30 January 1915	Ben Cruachan	United Kingdom	3,092	Sunk
30 January 1915	Kilcoan	United Kingdom	456	Sunk
30 January 1915	Linda Blanche	United Kingdom	369	Sunk
25 May 1915	HMS Triumph	Royal Navy	11,985	Sunk
27 May 1915	HMS Majestic	Royal Navy	14,900	Sunk
4 July 1915	Carthage	France	5,601	Sunk
1 February 1916	Belle of France	United Kingdom	3,876	Sunk
8 February 1916	Amiral Charner	French Navy	4,750	Sunk
30 April 1916	City of Lucknow	United Kingdom	3,669	Sunk
26 October 1916	Marina G.	Italy	154	Sunk
28 October 1916	Gilda R.	Italy	37	Sunk
28 October 1916	Tre Fratelli D.	Italy	190	Sunk
31 October 1916	Glenlogan	United Kingdom	5,838	Sunk
1 November 1916	Bernardo Canale	Italy	1,346	Sunk
1 November 1916	Torero	Italy	767	Sunk
2 November 1916	San Antonio O.	Italy	113	Sunk
3 November 1916	San Giorgio	Italy	258	Sunk
23 December 1916	Benalder	United Kingdom	3,044	Damaged
16 February 1917	Mayola	United Kingdom	146	Sunk
16 February 1917	Rose Dorothea	United Kingdom	147	Sunk
17 February 1917	Emilia I	Portugal	25	Sunk
17 February 1917	Lima	Portugal	108	Sunk
20 February 1917	Cacique	France	2,917	Sunk
22 February 1917	Bandoeng	Netherlands	5,851	Sunk
22 February 1917	Eemland	Netherlands	3,770	Sunk
22 February 1917	Gaasterland	Netherlands	3,917	Sunk
22 February 1917	Jacatra	Netherlands	5,373	Sunk
22 February 1917	Noorderdijk	Netherlands	7,166	Sunk
22 February 1917	Normanna	Norway	2,900	Sunk
22 February 1917	Zaandijk	Netherlands	4,189	Sunk
22 February 1917	Menado	Netherlands	5,874	Damaged
22 April 1917	Giskö	Norway	1,643	Sunk
22 April 1917	Theodore William	Norway	3,057	Sunk
29 April 1917	Askepot	Norway	1,793	Sunk
30 April 1917	Borrowdale	Russia	1,268	Sunk
3 May 1917	Lindisfarne	Russia	1,703	Sunk
6 May 1917	Adansi	United Kingdom	2,644	Sunk
8 May 1917	Killarney	United Kingdom	1,413	Sunk
27 June 1917	Baltic	Sweden	1,125	Sunk

• Gardiner, Robert; Gray, Randal, eds. (1985).Conway's All the World's Fighting Ships 1906–1921. Annapolis: Naval Institute Press.ISBN 0-87021-907-3.
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• Gröner, Erich (1991). Jung, Dieter; Maass, Martin (eds.).German Warships 1815–1945, U-boats and Mine Warfare Vessels. Vol. 2. London: Conway Maritime Press.ISBN 0-85177-593-4.
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• Helgason, Guðmundur."WWI U-boats: U-21".German and Austrian U-boats of World War I - Kaiserliche Marine - Uboat.net.

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