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 qualities - the time taken to flood the mine casing meant that it took about 5 minutes to dive in calm weather and 13 minutes or more in rough weather. Only by careful design arrangements was this reduced in Porpoise - she could dive, with mines on board, to periscope depth from full buoyancy in 1 minute 32 seconds and, using her Q tank, in 1 minute 14 seconds. The hull form of Porpoise was very similar to that of the Overseas Patrol Submarine Parthian, but the effect of carrying 50 mines meant that the stern torpedo tubes were deleted and the main engine horsepower was reduced by 25 per cent - resulting in a loss in surface speed. The design diving depth was also reduced to 300 feet.
Although the external tanks were of 'non-leaking' welded construction (as in the Thames Class) it was thought that, if exposed to depth charge attacks, leaks would occur and give away the submarine's position. Therefore, the hull form for the Chatham-built Grampus was radically changed (saddle tanks were extended, the pressure hull was altered, etc.) so that oil fuel, which was carried externally in Porpoise, could be carried in internal tanks. This change also increased the main ballast water carried by Grampus by about 100 tons, and at the same time improved the stability and reserve buoyancy of the boat.
All vessels of the class had balloon tanks, ie pressure-tight tanks, in the forward superstructure to balance the buoyancy of the mines aft and prevent the submarine from diving stern first.
Built between 1930 and 1938, the six boats of the Porpoise Class were fitted with two Admiralty-designed vertical four-stroke blast-injection six-cylinder diesels that together generated 3300 bhp, giving a surface speed, in service, of 16 knots. Tandem sets of motors on each shaft developed a total of 1630 bhp, giving a submerged service speed of 8.9 knots. Carrying 336 cells in three battery tanks weighing a total of 139 tons, the class had a submerged endurance of 66 miles at 4 knots.
In addition to the 50 mines, the armament of the class consisted of six 21-inch bow torpedo tubes (with 12 torpedoes carried) and a 4-inch gun (a 4.7-inch gun was originally fitted in Porpoise) with 120 rounds of ammunition - this being chosen as the standard gun to be fitted in Royal Navy submarines. When the gun of Porpoise was changed to the standard calibre, a weight-saving of about seven tons was achieved.
