SM U-21 or U-XXI was a or U-boat

U-boat

U-boat is the anglicized version of the German word U-Boot , itself an abbreviation of Unterseeboot , and refers to military submarines operated by Germany, particularly in World War I and World War II...

 built for and operated by the Austro-Hungarian Navy

Austro-Hungarian Navy

The Austro-Hungarian Navy was the naval force of Austria-Hungary. Its official name in German was Kaiserliche und Königliche Kriegsmarine , abbreviated as k.u.k. Kriegsmarine....

  during the First World War. The design for U-21 was based on submarines of the Royal Danish Navy

Royal Danish Navy

The Royal Danish Navy is the sea-based branch of the Danish Defence force. The RDN is mainly responsible for maritime defence and maintaining the sovereignty of Danish, Greenlandic and Faroese territorial waters...

's Havmanden class

Havmanden class submarine (1911)

The Havmanden class was a class of six submarines built for the Royal Danish Navy from 1911 to 1914. Also later known as the A class, the boats were designed by the Austro-Hungarian firm Whitehead & Co. of Fiume...

 (three of which had been built in Austria-Hungary

Austria-Hungary

Austria-Hungary , more formally known as the Kingdoms and Lands Represented in the Imperial Council and the Lands of the Holy Hungarian Crown of Saint Stephen, was a constitutional monarchic union between the crowns of the Austrian Empire and the Kingdom of Hungary in...

), and was largely obsolete by the beginning of the war.

U-21 was just over 127 feet (38.7 m) long and was armed with two bow torpedo tubes, a deck gun

Deck gun

A deck gun is a type of artillery cannon mounted on the deck of a ship or submarine.The deck gun was used as a defensive weapon against smaller boats or ships and in certain cases where torpedo use was limited. Typically a crew of three; gunner, loader, and layer, operated the gun, while others...

, and a machine gun

Machine gun

A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rounds in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute....

. Construction on U-21 began in mid 1915 and the boat was launched in September 1916. After suffering damage during a diving trial in January 1917, U-21 underwent seven months of repairs before her commissioning in August 1917.

The U-boat conducted patrols off the Albania

Albania

Albania , officially known as the Republic of Albania , is a country in Southeastern Europe, in the Balkans region. It is bordered by Montenegro to the northwest, Kosovo to the northeast, the Republic of Macedonia to the east and Greece to the south and southeast. It has a coast on the Adriatic Sea...

n coast in October 1917, but experienced the failure of the seal on her main hatch

Trapdoor

A trapdoor is a door set into a floor or ceiling .Originally, trapdoors were sack traps in mills, and allowed the sacks to pass up through the mill while naturally falling back to a closed position....

. The repairs kept the boat out of action until June 1918. But in July a piston in her diesel engine broke, knocking the submarine out of the rest of the war. At the end of World War I, U-21 was ceded to Italy as a war reparation and scrapped in 1920. U-21 had no wartime successes.

Design and construction

When it became apparent to the Austro-Hungarian Navy

Austro-Hungarian Navy

The Austro-Hungarian Navy was the naval force of Austria-Hungary. Its official name in German was Kaiserliche und Königliche Kriegsmarine , abbreviated as k.u.k. Kriegsmarine....

 that the First World War would not be a short one, they moved to bolster their U-boat fleet by seizing the plans for the Danish Havmanden class submarines

Havmanden class submarine (1911)

The Havmanden class was a class of six submarines built for the Royal Danish Navy from 1911 to 1914. Also later known as the A class, the boats were designed by the Austro-Hungarian firm Whitehead & Co. of Fiume...

, three of which had been built at Whitehead & Co. in Fiume. Although the Austro-Hungarian Navy was not happy with the design, which was largely obsolete, it was the only design for which plans were available and which could be begun immediately in domestic shipyards. The Austro-Hungarian Navy unenthusiastically placed orders for U-21 and her three sister boats

Sister ship

A sister ship is a ship of the same class as, or of virtually identical design to, another ship. Such vessels share a near-identical hull and superstructure layout, similar displacement, and roughly comparable features and equipment...

 on 27 March 1915.

U-21 was one of two boats of the class to be built at the Pola Navy Yard. Due to demands by the Hungarian government, subcontracts for the class were divided between Hungarian and Austrian firms, and this politically expedient solution worsened technical problems with the design, resulting in numerous modifications and delays for the class in general.

U-21 was an ocean-going submarine that displaced 173 metric tons (170.3 LT) surfaced and 210 metric tons (207 LT) submerged and was designed for a complement of 18. She was 127 in 2 in (38.76 m) long with a beam

Beam (nautical)

The beam of a ship is its width at the widest point. Generally speaking, the wider the beam of a ship , the more initial stability it has, at expense of reserve stability in the event of a capsize, where more energy is required to right the vessel from its inverted position...

 of 13 feet (4 m) and a draft

Draft (hull)

The draft of a ship's hull is the vertical distance between the waterline and the bottom of the hull , with the thickness of the hull included; in the case of not being included the draft outline would be obtained...

 of 9 feet (2.7 m). For propulsion, she featured a single shaft, a single 450 bhp diesel engine

Diesel engine

A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

 for surface running, and a single 160 shp electric motor for submerged travel. She was capable of 12 knots while surfaced and 9 knots while submerged. Although there is no specific notation of a range for U-21, the Havmanden class, upon which the U-20 class was based, had a range of 1400 nautical miles (2,592.8 km) at 10 knots, surfaced, and 23 nautical miles (42.6 km) at 8 knots submerged.

U-21 was armed with two 45 cm (17.7 in) torpedo tubes located in the front and carried a complement of two torpedoes. She was also equipped with a 66 mm/26 (2.6 in) deck gun

Deck gun

A deck gun is a type of artillery cannon mounted on the deck of a ship or submarine.The deck gun was used as a defensive weapon against smaller boats or ships and in certain cases where torpedo use was limited. Typically a crew of three; gunner, loader, and layer, operated the gun, while others...

 and an 8 mm (0.31496062992126 in) machine gun

Machine gun

A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rounds in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute....

.

Service career

U-21 was launched on 15 August 1916, the first of the four U-20-class boats. During a diving trial in January 1917, the submarine was damaged when it sank too deep, requiring repairs that took place over the next seven months. U-21 was commissioned on 15 August under the command of Linienschiffsleutnant Hugo von Seyffertitz. A 31-year-old native of Brixen

Brixen

Brixen is the name of two cities in the Alps:*Brixen, South Tyrol, Italy*Brixen im Thale, Tyrol, AustriaBrixen may also refer to:*Bishopric of Brixen, the former north-Italian state....

, von Seyffertitz was a first-time U-boat commander.

Ten days after commissioning, U-21 safely submerged to a depth of 42 metres (137.8 ft). However, her nose was dented when she hit bottom on another test dive in September, necessitating more repairs. On 29 September, von Seyffertitz steered the boat from the submarine base at Brioni

Brioni

Brioni is an Italian fashion house founded in 1945. It specializes in the sale of hand-made suits. PPR acquired the company in 2011.-History:...

 to Cattaro, where she arrived on 1 October. On 4 October, U-21 set out for a patrol off the coast of Albania

Albania

Albania , officially known as the Republic of Albania , is a country in Southeastern Europe, in the Balkans region. It is bordered by Montenegro to the northwest, Kosovo to the northeast, the Republic of Macedonia to the east and Greece to the south and southeast. It has a coast on the Adriatic Sea...

, but had returned to Cattaro by mid October.

On 15 October, von Seyffertitz and U-21 departed from Cattaro to begin their first Mediterranean deployment. Slated to sail into the Ionian Sea

Ionian Sea

The Ionian Sea , is an arm of the Mediterranean Sea, south of the Adriatic Sea. It is bounded by southern Italy including Calabria, Sicily and the Salento peninsula to the west, southern Albania to the north, and a large number of Greek islands, including Corfu, Zante, Kephalonia, Ithaka, and...

, U-21 instead had to turn back the following day when the main hatch

Trapdoor

A trapdoor is a door set into a floor or ceiling .Originally, trapdoors were sack traps in mills, and allowed the sacks to pass up through the mill while naturally falling back to a closed position....

 seal on the conning tower

Conning tower

A conning tower is a raised platform on a ship or submarine, often armored, from which an officer can con the vessel; i.e., give directions to the helmsman. It is usually located as high on the ship as practical, to give the conning team good visibility....

 leaked and could not be repaired. After her 18 October return to Cattaro, she sailed for Pola, arriving on 24 October. There, she would undergo another lengthy stay in port for repairs. While U-21 was under repair, von Seyffertitz was transferred to .

Linenschiffleutnant Robert Dürrial was assigned the new commander of U-21 on 24 March 1918. The 26-year-old Galician had served as commander of for four months in 1917. Dürrial led U-21 out of Pola on 1 June for Cattaro, making stops en route at Arbe and Novigrad

Novigrad, Zadar County

Novigrad is a village and municipality in Croatia in the Zadar County. According to the 2001 census, there are 2,368 inhabitants, absolute majority which are Croats.- History :...

 for repairs to the gyrocompass

Gyrocompass

A gyrocompass­ is a type of non-magnetic compass which bases on a fast-spinning disc and rotation of our planet to automatically find geographical direction...

.

On 16 July, while conducting patrols off the Albania

Albania

Albania , officially known as the Republic of Albania , is a country in Southeastern Europe, in the Balkans region. It is bordered by Montenegro to the northwest, Kosovo to the northeast, the Republic of Macedonia to the east and Greece to the south and southeast. It has a coast on the Adriatic Sea...

n coast, a piston

Piston

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...

 in U-21s diesel engine broke and Dürrial put in at Djenovic. On 25 July, U-21 was towed to Pola, where she remained until the end of the war. She was ceded to Italy as a war reparation and scrapped in 1920. Like all of her sister boats, U-21 had no wartime successes.