In 1907, the first of what would be eight D-class submarines built for the Royal Navy was laid down at Vickers Armstrong at Barrow. The boat was basically the Dreadnought equivalent for submarines. Those built before her were smallish, typically with dangerous gasoline motors, capable of just carrying a couple of forward-launched torpedos for the use of protecting anchorages and coastal waters.
HMS/m D1 was larger, at some 500 tons and 163-feet, and was armed with three 18-inch torpedo tubes, two in the bow and one in the stern, and carried a reload for each. As a key, she also used a diesel/electric plant. She also had provision for a QF 12-pdr (76mm) deck gun, so that she could take warning shots at enemy merchant vessels in compliance with “cruiser rules” for commerce raiding. Basically, she had all of the innovations that would make the Great War-era attack subs so dangerous.
She also proved her worth in the 1910 naval exercises off Colonsay, where, as a “Red” OPFOR boat, she got close enough to mark two “Blue” cruisers hit with her torpedos, an act that should have given the Royal Navy a bellwether for the events of 22 September 1914 where the “Live Bait Squadron” of the armored cruisers HMS Cressy, Hogue, and Aboukir were dispatched in turn by SMS U-9 in the span of an hour.
D1’s performance, noted Commodore Roger Keyes, head of the navy’s submarine service between 1912 and 1915, “opened the eyes of the first sea lord, Admiral Sir Arthur Wilson, to the offensive possibilities of submarines, which he had hitherto regarded as defensive vessels.”
With that, D1 served as the prototype vessel not only for the rest of her class but also for the 58 E-class boats that served as the backbone of the British submarine fleet through WWI.
As for the boat itself, after active war service, ranging as far as the mouth of the River Ems and earning a mention in dispatches for coming into contact with the enemy during patrol operations in the Heligoland Bight, she had been relegated to training work and was scuttled about 1 nautical mile south-east of the eastern Blackstone, off Dartmouth, in 1918 for use as a known target for the trials of various submarine detection equipment.
Over time, her wreck, although first charted by the UKHO in 1920 at a depth of 50m, was forgotten to history, with most historians feeling it to be either the lost German U-boats UB-113 or UC-49.
This was washed away as explained by Historic England:
Both of these proposed identifications were disproved by the results of the 2018 investigation, as the combination of two forward torpedo tubes, single stern torpedo tube, two propellers and single rudder are not found on UBIII class and the UCII class submarines. The overall dimensions and the shape and position of the conning tower, torpedo tubes and deck fixtures are consistent with the technical plans of HMS/m D1.
Now, the Department for Digital, Culture, Media, and Sport has granted protection to the wreck.