1914 - 1921: G Class
In December 1913, after discussing the German submarine programme, the Admiralty decided that they should prepare a design for an overseas patrol boat of about E Class surface displacement, of partial double-hull construction and with single 21-inch torpedo tubes forward and aft and two 18-inch beam tubes. This would be the G Class.
Vickers engines of the E Class type were fitted in all seven boats, although it was originally intended to fit G6 and G7 with Nuremberg (MAN) and Sultzer engines. (Difficulties obtaining the Sultzer engine and the impracticability of a MAN design prevented this.)
Costing an estimated £125 000, the G Class were twin-shafted vessels, with two eight-cylinder diesels that together generated 1600 bhp, giving a surface speed of 14 knots. Two single armature motors, each of 420 bhp, gave a submerged speed of 9 knots. The class carried 200 cells in two battery tanks, which gave a submerged endurance of 95 miles at 3 knots.
Early war experience gained by other classes led to the proposed G Class armament being changed to two 18-inch bow tubes, two 18-inch beam tubes and one 21-inch stern tube. This signalled the beginning of the 21-inch torpedo in Royal Navy submarines, although the experimental submarine Swordfish ordered from Scotts a year earlier but launched after G1, was also fitted with 21-inch tubes. The class also carried one 3-inch Quick Fire High Angle (OF HA) gun which was fitted just forward of the bridge and a portable 2-pounder which could be fixed to a pedestal at the after end of the bridge.
Living conditions on board were considered by crew members to be good because the G boats boasted such luxuries as an electric oven!
Although the designed diving depth of the G Class was given as 200 feet, the operational depth was probably 100 feet. However, it was noted that in 'an exceptional circumstance one G boat dived to 170 feet when chased by mistake by British destroyers'.
During the First World War G7, G8 and G11 were lost on active service through unknown causes and in September 1917. G9 was sunk in error by HMS Petard off the Norwegian coast. Of those that survived, four were taken out of service at the end of the war and the remaining six were withdrawn from service in January 1921.