• The Type III U-boat was a 1934 project for a purpose-built minelayer based on the Type IA
  German Type I submarine
  The Type I U-boat was the first post-World War I attempt by the German Kriegsmarine to produce an ocean going submarine. Only two Type IAs were built, but the decision to halt production on further boats is believed to be because of political decisions and not because of major faults in the Type I...
  
   U-boat. The Type III U-Boat would have been similar to the Type IA, but with a hull lengthened by 7.5 metres, and a total displacement of 970 tons. The Type III U-Boat was planned to carry an armament of 54 to 75 mines
  Naval mine
  A naval mine is a self-contained explosive device placed in water to destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of, or contact with, an enemy vessel...
  
   (depending on the type carried), two 105mm deck guns, and one 20 mm anti-aircraft gun.
• The Type IIIA U-boat was a planned minelayer similar to the Type IA U-boat. The Type IIIA U-boat would have had a larger outer hull than the Type IA, and featured a large, watertight cylindrical hangar on the aft deck, which would have carried two small motor torpedo boats. The boat would have carried 48 mines, and used the smaller boats to help lay and recover mines. The project was dropped, as impractical.
• The Type IV U-boat was a planned re-supply and repair U-boat, intended to meet other U-boats at sea and re-supply them with torpedoes, fuel, food/water, and spare parts, and also be capable of performing light-repair work.
• The Type V U-Boat was an experimental midget submarine
  Midget submarine
  A midget submarine is any submarine under 150 tons, typically operated by a crew of one or two but sometimes up to 6 or 8, with little or no on-board living accommodation...
  
   designed by Hellmuth Walter
  Hellmuth Walter
  Hellmuth Walter was a German engineer who pioneered research into rocket engines and gas turbines...
  
   using his hydrogen peroxide
  Hydrogen peroxide
  Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
  
  -fuelled turbine. Only one vessel, the V-80
  German submarine V-80
  The V-80 was a 1939 German Navy 76-ton experimental submarine and the only representative of the German Type V design.The prototype was completed in 1940 in Germaniawerft in Kiel. The 4 man vessel was designed to test the Walter hydrogen peroxide-based turbine propulsion system...
  
  was built.
• The Type VI U-boat was a planned conversion of Type IA U-boats to run both submerged and surfaced from steam propulsion.
• The Type VIIE U-boat was planned to make use of a lightweight engine, and with the saved weight, increase the thickness of the pressure hull - thus allowing for greater diving depths. The project, however, was cancelled.
• Little information is available on the Type VIII U-boat, other than it was planned for production in the event of mobilization in 1935.
• The Type XI U-boat was planned as an artillery boat; its main armament would have been four 127 mm guns, in two twin gun turrets. It would have also carried an Arado Ar 231
  Arado Ar 231
  The Arado Ar 231 was a light-weight Floatplane, developed during World War II in Germany as a scout plane for submarines. The need to be stored inside the submarine necessitated compromises in design that made this single-seat seaplane of little practical use....
  
   collapsible floatplane. Four boats (U-112, U-113, U-114, and U-115) were laid down in 1939, but cancelled at the outbreak of World War II. Had the Type XI U-boat been constructed, it would have had a completely new hull design and a submerged displacement of 4,650 tons – she would have been by far the largest of the U-boats and the second-largest diesel submarine after the Japanese I-400-class submarine
  I-400 class submarine
  The Imperial Japanese Navy submarines were the largest submarines of World War II and remained the largest ever built until the construction of nuclear ballistic missile submarines in the 1960s. They were submarine aircraft carriers able to carry three Aichi M6A Seiran aircraft underwater to their...
  
  .
• The Type XII fleet U-boat was a design from 1938. It had 8 torpedo tubes, 6 at the bow and 2 at the stern, and was to carry 20 torpedoes. The gun armament was to be the same as for the type IX boats. No contracts were granted for these boats.
• The Type XIII was a further development of the type II coastal U-boat, with 4 torpedo tubes and one 20 mm AA gun. No contracts were granted for these boats.
• The Type XV and XVI U-boats were intended for very large transport and repair boats (5,000-ton and 3,000-ton respectively) intended to carry torpedoes, food and oil as cargo. The engine layout was to be the same as for the VIIC. No contracts were granted for these boats.
• The Type XVIII U-boat was a project for an attack boat using the Walter Propulsion System
  Air-independent propulsion
  Air-independent propulsion is a term that encompasses technologies which allow a submarine to operate without the need to surface or use a snorkel to access atmospheric oxygen. The term usually excludes the use of nuclear power, and describes augmenting or replacing the diesel-electric propulsion...
  
  . Two boats (U-796 and U-797) were laid down in 1943, but construction was cancelled in March 1944.
• The Type XIX U-boat was a project for an unarmed transport U-boat, based on the Type XB mine layer.
• The Type XX U-boat was another project for a transport U-boat based on the Type XB, it would have had a shorter hull than the Type XB, but had a greater beam and draft. 30 Type XX U-boats were laid down in 1943, but construction stopped in 1944. In August 1944, construction on three Type XX U-boats (U-1701, U-1702, and 1703) resumed, but again stopped in early 1945.
• The Type XXII U-boat was intended for coastal and Mediterranean use. They used the Walter Propulsion System
  Air-independent propulsion
  Air-independent propulsion is a term that encompasses technologies which allow a submarine to operate without the need to surface or use a snorkel to access atmospheric oxygen. The term usually excludes the use of nuclear power, and describes augmenting or replacing the diesel-electric propulsion...
  
   and would have had a crew of two officers and 10 men. They were to have three torpedo tubes, two at the bow (below the CWL) and one aft of the bridge (above the CWL). Initially 72 contracts were awarded to Howaldtswerke (36 to the yard in Hamburg and 36 in Kiel), but of those 72 only two had actually been laid down and had received U-boat numbers (U-1153 and U-1154) before they were all cancelled in the autumn of 1943.
• The Type XXIV was a 1943 design for an ocean-going U-boat using the Walter Propulsion System
  Air-independent propulsion
  Air-independent propulsion is a term that encompasses technologies which allow a submarine to operate without the need to surface or use a snorkel to access atmospheric oxygen. The term usually excludes the use of nuclear power, and describes augmenting or replacing the diesel-electric propulsion...
  
  . It was to have 14 torpedo tubes, six at the bow, and 4 each side aft. Never constructed.
• The Type XXV U-boats were intended to be electric propulsion-only boats for coastal use. The design was 160 tons with a crew of about 58 men and would have had two torpedo tubes fitted at the bow.
• The Type XXVI was a high-seas U-boat propelled by the Walter Propulsion System
  Air-independent propulsion
  Air-independent propulsion is a term that encompasses technologies which allow a submarine to operate without the need to surface or use a snorkel to access atmospheric oxygen. The term usually excludes the use of nuclear power, and describes augmenting or replacing the diesel-electric propulsion...
  
  . They would have had a crew of three officers and 30 men, with ten torpedo tubes, 4 at the bow and 6 in a so-called Schnee organ, and no deck guns. 100 contracts were initially awarded to the Blohm + Voss
  Blohm + Voss
  Blohm + Voss , is a German shipbuilding and engineering works. It is a subsidiary of ThyssenKrupp Marine Systems; there were plans to sell 80% of Blohm + Voss to Abu Dhabi Mar Group, but talks collapsed in July 2011.-History:It was founded on April 5, 1877, by Hermann Blohm and Ernst Voss as a...
  
   yard in Hamburg (U-4501 through U-4600) and sections were under construction for U-4501 through U-4504 when the war ended. The other contracts had been cancelled.

See also

• Type I
  German Type I submarine
  The Type I U-boat was the first post-World War I attempt by the German Kriegsmarine to produce an ocean going submarine. Only two Type IAs were built, but the decision to halt production on further boats is believed to be because of political decisions and not because of major faults in the Type I...
  
  
• Type II
  German Type II submarine
  The Type II U-boat was designed by Germany as a coastal U-boat, modeled after the CV-707 submarine, which was designed by the Dutch dummy company NV Ingenieurskantoor voor Scheepsbouw den Haag and built in 1933 by the...
  
  
• Type VII
  German Type VII submarine
  Type VII U-boats were the most common type of German World War II U-boat. The Type VII was based on earlier German submarine designs going back to the World War I Type UB III, designed through the Dutch dummy company Ingenieurskantoor voor Scheepsbouw den Haag which was set up by Germany after...
  
  
• Type IX
  German Type IX submarine
  The Type IX U-boat was designed by Germany in 1935 and 1936 as a large ocean-going submarine for sustained operations far from the home support facilities. Type IX boats were briefly used for patrols off the eastern United States in an attempt to disrupt the stream of troops and supplies bound for...
  
  
• Type X
  German Type X submarine
  Type X U-boats were a special type of German submarine . Although intended as long-range mine-layers, they were later used as long-range cargo transports, a task they shared with the Type IXD and Italian Romolo-class submarines.-History:...
  
  
• Type XIV
  German Type XIV submarine
  The Type XIV U-boat was a modification of the Type IXD, designed to resupply other U-boats. They were nicknamed "Milchkuh/Milchkühe " . They had no torpedo tubes or deck guns, only anti-aircraft guns. Due to its large size, the Type XIV could resupply other boats with 400 tons of fuel, four...
  
   - used to resupply other U-boats; nicknamed the Milk Cow
• Type XVII
• Type XXI
  German Type XXI submarine
  Type XXI U-boats, also known as "Elektroboote", were the first submarines designed to operate primarily submerged, rather than as surface ships that could submerge as a means to escape detection or launch an attack.-Description:...
  
  
• Type XXIII
  German Type XXIII submarine
  German Type XXIII submarines were the first so-called elektroboats to become operational. They were small coastal submarines designed to operate in the shallow waters of the North Sea, Black Sea and Mediterranean Sea, where larger Type XXI Elektro boats were at risk in World War II. They were so...
  
  
• Midget submarines, including Biber, Hai
  Hai (midget submarine)
  The Hai was an advanced model of the Marder-class midget submarines created in Germany during World War II and operated by the K-Verband. Its prototype performed poorly during test runs and therefore no other boats were produced....
  
  , Molch
  Molch
  The Molch was an unsuccessful, one-man series of German midget submarines created during World War II...
  
  , Seehund
  Seehund
  The Seehund , also known as Type XXVII, was a successful series of German midget submarines created during World War II...
  
  .