Based on the recently-completed 'Parthians', the 'Porpoise' class submarines were purpose-built minelayers. German practice tended to near-vertical mine shutes located within the envelope of the pressure hull, but the British preferred external stowage, despite risk of damage from overpressures or depth-charging. '£' and 'L' class minelayers had had stowages in the saddle tanks on each side but, in the experimental conversion of the M3 in 1927, tracks were laid atop the hull over the greater part of her length and inside the free-flooding space contained within an extra-deep casing, An endless-chain mechanism fed the mines through doors right aft as the submarine moved slowly ahead. This system was basically that incorporated in the 'Porpoises'; in the name ship it extended over about threequarters of her length, but was longer in the remainder. All this gear added about 54 tons of topweight, making the boats very tender when first surfaced with a full load in a heavy sea. Extra lightening holes improved both draining and flooding time, allowing the boats also to dive more quickly. Launched between 1932 and 1938, the boats were HMS Porpoise, Grampus, Narwhale, Rorqual, Cachalot and Seal. Three other units were cancelled.
Being weight-critical the class took rather small diesel engines, resulting in a modest surface speed. To avoid detection from fuel leaks all bunkers were internal, it being found necessary to extend the pressure hull downwards like a box keelson to meet the saddle tanks. This oddly-shaped and weaker cross-section undoubtedly contributed to the designed depth being limited to 91 m (300ft) compared with the 152m (500ft) of the 'Parthians'.
The main function of the 'Porpoises' was, officially at least, superseded by the development of a mine capable of being laid through a conventional torpedo tube. But despite this the class was still to lay some 2,600 mines operationally. They proved invaluable during the height of the siege of Malta -when, in concert with the available 'O' class boats, they moved in personnel and supplies. The Seal, after damaging herself by mine in the Kattegat and unable to dive, had to surrender to two Arado floatplanes. Repaired, she was recommissioned as the UB-A but not used operationally. Only the Rorqual survived World War II.
Specification 'Porpoise' class Displacement: 1,768 tons surfaced and 2,053 tons submerged Dimensions: length 88.09 m (289.0 ft); beam 9.09 m (29.83 ft); draught 4,88 m (16,0ft) Propulsion: two diesels delivering 2460,8 kW (3,300 bhp) and two electric motors delivering 1215,5 kW ( 1,630 hp) to two shafts Speed: 15.5 kts surfaced and 9 kts submerged Endurance: 21308 km ( 13,240 miles) at 8 kts surfaced and 122 km (76 miles) at 4 kts submerged Armament: one 101.6 mm (4-in) gun, six 533-mm (21-in) torpedo tubes (all bow) for 12 torpedoes, and 50 mines Complement: 59