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.
During the trials of M3 it was found that whilst the minelaying gear and compensating arrangements to maintain trim were satisfactory, the jigger-type mine-launching equipment required excessive upkeep to ensure its efficiency. Therefore, this. equipment was changed in the Porpoise Class to a chain and rack system fitted outside the hull in the superstructure casing. The mines and minelaying gear weighed approximately 54 tons. The conversion of M3 also had an adverse affect on its diving
