The Type 201 was Germany

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

's first class of military submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

s built after World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

. They were built out of amagnetic

Ferromagnetism

Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...

 steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 to counter the threat of magnetic naval mine

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...

s. But the material had not been tested extensively enough and showed up to be problematic in service with the Bundesmarine: microscopic cracks in the pressure hull forced the cancellation of 9 of the 12 ordered submarines and the early retirement of the three built boats. They were replaced by the very similar Type 205 submarine

Type 205 submarine

The Type 205 was a class of diesel-electric German hunter-killer U-boat submarines. They were single-hull vessels optimized for the use in the shallow Baltic Sea. The Type 205 is a direct evolution of the Type 201 class with lengthened hull, new machinery and sensors...

s, that were constructed with normal steel.

List of boats

Pennant number Pennant number In the modern Royal Navy, and other navies of Europe and the Commonwealth, ships are identified by pennant numbers...	Name	Shipyard	laid down	Launched	com- missioned	decom- missioned	fate
S180	U-1	Howaldtswerke		1962	March 20, 1962	1967	rebuilt to Type 205A Type 205 submarine The Type 205 was a class of diesel-electric German hunter-killer U-boat submarines. They were single-hull vessels optimized for the use in the shallow Baltic Sea. The Type 205 is a direct evolution of the Type 201 class with lengthened hull, new machinery and sensors...  standards
S181	U-2	Howaldtswerke		1962	May 3, 1962	July 1963	rebuilt to Type 205A standards
S182	U-3	Howaldtswerke		1962	June 20, 1964
S183	U-4	Howaldtswerke		1962	not completed

U-3 was loaned to the Royal Norwegian Navy

Royal Norwegian Navy

The Royal Norwegian Navy is the branch of the Norwegian Defence Force responsible for naval operations. , the RNoN consists of approximately 3,700 personnel and 70 vessels, including 5 heavy frigates, 6 submarines, 14 patrol boats, 4 minesweepers, 4 minehunters, 1 mine detection vessel, 4 support...

and served under the name Kobben from 1962 to 1964 before being commissioned in the German Navy.