The D-class submarines were designed as improved and enlarged versions of the preceding C class, with diesel engines replacing the dangerous petrol engines used earlier. D3 and subsequent boats were slightly larger than the earlier boats. They had a length of 164feet overall, a beam of 20feet and a mean draught of 11feet. They displaced 495LT on the surface and 620LT submerged.[1] The D-class submarines had a crew of 25 officers and ratings and were the first to adopt saddle tanks.[2]
For surface running, the boats were powered by two 6000NaN0 diesels, each driving one propeller shaft. When submerged each propeller was driven by a 275hp electric motor. They could reach 14kn on the surface and underwater. On the surface, the D class had a range of 2500nmi at .[2]
The boats were armed with three 18-inch (45 cm) torpedo tubes, two in the bow and one in the stern. They carried one reload for each tube, a total of six torpedoes.[2]
D8 was laid down on 14 February 1910 by Chatham Dockyard. The submarine was launched on 23 September 1911 and was commissioned on 23 March 1912.
D8 fought in the Battle of Heligoland Bight on 28 August 1914 along with sister ships and . Then on 18 October 1914, D8 shadowed the which was judged to be spying and was interned.[3]
D8 was sold on 19 December 1921 to Pounds.