X51 Stickleback

Launched: July 1954.

In 1958 it was sold to the Royal Swedish Navy, for use in harbour defence exercises. Renamed Spigger (Stickleback). In 1976 it was returned to the United Kingdom. Now at the Imperial War Museum Duxford.

X52 Shrimp

Launched: October 1954.

On completion of contractors trials, was placed in reserve in Portsmouth. It was broken up in 1966 by Metal Industries at Faslane, Scotland.

X53 Sprat

Launched: 30th December 1954.

On completion of contractors trials, was placed in reserve at Portsmouth. Employed, on occasion, to test the harbour defence systems of various ports in the United States of America. It was broken up in 1966 at Faslane, Scotland.

X54 Minnow

Launched: 5th May 1955.

On completion of contractors trials, she returned to the Barrow shipyard to carry out trials on an early version of a pump-jet propulsion system. The trial showing no appreciable improvement in performance, she was also placed in reserve. It was broken up in 1966 at Faslane, Scotland.

All of the boats were periodically taken out of reserve, to participate in exercises. They were normally based at Portland, Dorset. The X51 Class were an improvement on the earlier classes of X-craft of World War II vintage.

In the early days of the "Cold War", Britain then having no means of delivering a nuclear weapon into the U.S.S.R., it was thought that this type of boat could be used to lay nuclear mines in Warsaw Pact dockyards and harbours.

These midget submarines eventually fell victim to economic cuts, when Flag Officer submarines (F.O.S.M.) was required to reduce the number of "hulls" under his command.