The S-class submarines of the Royal Navy

Royal Navy

The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

 were originally designed and built during the modernisation of the 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...

 force in the early 1930s to meet the need for smaller boats to patrol the restricted waters of the North Sea

North Sea

In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...

 and the Mediterranean Sea

Mediterranean Sea

The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...

 replacing the British H class submarine

British H class submarine

The British H class submarines were Holland 602 type submarines used by the Royal Navy. The submarines constructed for the British Royal Navy between 1915 and 1919 were designed and built in response to German boats which mined British waters and sank coastal shipping with ease due to their small...

s. Due to major naval construction of the Royal Navy during the Second World War, however, the S class became the largest single group of submarines ever built for the Royal Navy; a total of 62 were constructed over a period of 15 years, with fifty of the "improved" S-class launched between 1940 and 1945.
The submarines operated in the waters around the United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 and in the Mediterranean, and later in the Far East after being fitted with extra tankage.
After the war S class boats continued to serve in the Royal Navy until the 1960s.