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.
