1907 - 1919: D Class
The D Class, approved by the Admiralty Board in 1906, was designed to overcome, as far as possible, the limitations of the earlier submarine classes. Initial design work was carried out by the Admiralty for the first time on a submarine project, but with the usual procedure for other Admiralty vessels. Between 1907 and 1912, eight of the class were built - six at Barrow and two at Chatham.
The class had far better endurance than previous boats, and can be regarded as being the first submarines to have a proper patrol capability. The introduction of twin screws meant that better manoeuvring power was obtained, and the conning tower, being much larger than previously, gave the class a profile similar to that of modern-day submarines.
D1 was the prototype for the first class of diesel-driven submarine, a development which signalled the end of the petrol engine era with its attendant dangers to crews from petrol fumes, and D4 was the first British submarine to be fitted with a gun - a 12 pounder on a mounting housed inside the superstructure.
A significant innovation was the use of saddle tanks to hold the main ballast water. The use of external tanks provided additional inboard space and improved habitability standards. However, with the introduction of a stern torpedo tube (the first in a British submarine), an increased engine size to give greater power and an increased complement in a vessel of the same hull diameter as the C Class, and with only 20 ft increase in length, it is doubtful whether the additional space was properly utilised to improve accommodation standards.
A major advance in the D Class was the incorporation of a wireless system for the first time, the aerial being rigged to the submarine's mast. Before diving, the mast had to be lowered by hand and the wireless aerial stowed away along the side of the vessel. Previous submarines had been equipped with receiving sets, but the D Class was the first to have both receiving and transmitting facilities, unfortunately, wireless signals could neither be received nor transmitted when submerged - that problem remained unsolved until the late 1920s.
D Class Submarines at War
All eight D Class submarines were in commission at the start of the First World War, and took up station on the east coast of England, where their long-range potential was exploited to the full, with overseas patrols in the Heligoland Bight.
Towards the end of the war, two more of the class were lost - D3 was accidentally sunk in the English Channel in March 1918, and three months later D6 was sunk by a U-boat off the north coast of Ireland.