HMS D7 was a Chatham-built D Class submarine. The D Class represented a major advance in submarine design for the Royal Navy and featured many improvements over the previous C Class.
The first improvement was their size; they were twice the size of the previous C Class boats. This enabled them to have much better accommodation for their crews. They also featured diesel engines for the first time in a Royal Navy submarine, avoiding all the problems associated with the petrol engines in previous submarines. They also featured twin engines, rather than just a single one giving them more power and better handling. They featured external saddle tanks, an enlarged conning tower and were built from the start with a wireless set installed. With the ballast tanks outside the pressure hull, much more space was available inside the boat. They were the first British submarines capable of overseas operations. They were also the first British submarines to feature a deck gun and a stern torpedo tube. Such were the number of improvements that the first boat, HMS D1 was built under a cloak of secrecy by Vickers at Barrow-in-Furness in a guarded shed. HMS D1 was also launched in secret, with only the departmental heads and some officers from HMS Mercury in attendance. She was screened off during fitting out as well.
HMS D7 was laid down on No 7 slip on 14th Feb 1910. She was launched into the Medway on 14th January 1911 by Mrs Ricardo, the wife of Captain Arthur Ricardo RN, Captain of the Dockyard at the time. After fitting out, she commissioned at Chatham on 14th December 1911.
As completed, HMS D7 was 163 ft long, 13ft 6" wide at the beam and displaced 483 tons surfaced and 595 tons dived. She was armed with 3 21" torpedo tubes, 2 in the bow and 1 in the stern. She also carried a 12pdr (3") deck gun.
Model of HMS D1. D7 would have been identical.
In May 1915, while patrolling in the German Bight, she was attacked from the air, but all 3 bombs dropped on her missed and she escaped without damage or casualties. On 15th May 1915, she departed Great Yarmouth for a patrol in the North Sea, but sprang a leak and returned home for repairs. After repairs she continued regular patrols in the German Bight, accompanied by HMS D8.
On 12th September 1917, HMS D7 torpedoed and sank the German submarine U-45, with a single torpedo fired from her stern tube at a range of 800 yards. On 10th February 1918, HMS D7 was depth-charged by HMS Pelican but survived the attack. In May 1918, HMS D7 collided with a U-boat while passing underneath it. Her periscopes were damaged but the boat was otherwise unscathed.
Obsolete at the end of the Great War, HMS D7 was sold for scrap on 19th December 1921.
