The ''O 21'' class was a

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...

of seven

submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...

s, built for the

Royal Netherlands Navy The Royal Netherlands Navy ( nl, Koninklijke Marine, links=no) is the naval force of the Kingdom of the Netherlands. During the 17th century, the navy of the Dutch Republic (1581–1795) was one of the most powerful naval forces in the world an ...

. The boats were still incomplete at the start of the German invasion of the Netherlands ''O 21'', ''O 22'', ''O 23'' and ''O 24'' were hastily launched and escaped to the United Kingdom. ''O 25'', ''O 26'' and ''O 27'' were not able to escape and were captured by the German forces. The ''

Kriegsmarine The (, ) was the navy of Germany from 1935 to 1945. It superseded the Imperial German Navy of the German Empire (1871–1918) and the inter-war (1919–1935) of the Weimar Republic. The was one of three official branches, along with the a ...

'' ordered the completion of the boats and they entered German service as ''UD-3'', ''UD-4'' and ''UD-5''. The submarines diving depth was .

Design

One of the requirements the

set for the ''O 21''-class was that it had to be able to dive deeper than the previous class, which was the ''O 19''-class submarine.Gerretse and Wijn, p. 9

Construction

The ships were built by three different shipyards. ''O 21'' and ''O 22'' were built by the ''Koninklijke Maatschappij''. ''O 23'', ''O 24'', ''O 26'' and ''O 27'' by ''R.D.M, Rotterdam'' and ''O 25'' also in Rotterdam at ''Fijenoord'' shipyard.

Design

Construction

References

Bibliography

External links