The British U class submarines (officially "War Emergency 1940 and 1941 programmes, short hull ") were a class of 49 small submarines built just before and during the Second World War. The class is sometimes known as the Undine class, after the first submarine built.
These small submarines, of around 630 tons, were originally intended as unarmed training vessels to replace the ageing H class, to be used as practice targets in anti-submarine training exercises.

The first three boats, HMS Undine

HMS Undine (N48)

HMS Undine was a British U class submarine built by Vickers Armstrong, Barrow-in-Furness. She was laid down on 19 February 1937 and was commissioned on 21 August 1938. She was the lead ship of the first group of the U class, which is sometimes called the Undine class.-Sinking:Undine was on her...

, HMS Unity

HMS Unity (N66)

HMS Unity was a British U class submarine, of the first group of that class, built by Vickers Armstrong, Barrow-in-Furness. She was laid down on 19 February 1937 and was commissioned on 5 October 1938....

, and HMS Ursula

HMS Ursula (N59)

HMS Ursula was a British U class submarine, of the first group of that class, built by Vickers Armstrong, Barrow-in-Furness. She was laid down on 19 February 1937 and was commissioned on 20 December 1938....

 were ordered in 1936, and during construction were modified to accommodate four internal and two external bow torpedo tube

Torpedo tube

A torpedo tube is a device for launching torpedoes. There are two main types of torpedo tube: underwater tubes fitted to submarines and some surface ships, and deck-mounted units installed aboard surface vessels...

s.