The history of Royal Navy minelayers began when six of the E Class submarines ordered in 1914 were built to carry 20 mines in mine tubes in the saddle tanks. E24, the first Royal Navy minelayer, was completed on 9th January, 1916 and later that year it was ordered that six submarines of the L Class be fitted with mine tubes in the external tanks. In July 1920, the Naval Staff investigated the need and requirements for submarine minelayers - the main bone of contention being whether the mines should be carried internally or externally. As an experiment, M3 was converted in 1927 to carry mines externally.
The satisfactory performance of M3 led to the design of a new submarine minelayer, the famous Porpoise Class. Specifically designed for the task of minelaying, six of the class were ordered in the 1930 Programme - the first-of-class Porpoise, Narwhal and Rorqual from Vickers, Grampus and Seal from Chatham and Cachalot from Scotts. Slightly slimmer than the Overseas Patrol Submarine, but with greater displacement, the new class had a capacity to carry 50 standard Mk XVI mines in a full-length deck outside the pressure hull.
