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
