Five boats to a modified design were built between 1936 and 1938.

The ships were all named after marine mammals.

The naval mines were stored in a special "gallery" with a conveyor belt built into the outer casing as pioneered by the converted M-class submarine HMS M3.

Boats of this class were used extensively in the Mediterranean, particularly as part of the supply effort to the besieged island of Malta in a service nicknamed the "magic carpet".

Only one, HMS Rorqual, survived the war.