See main article: J-class submarine. The J class was designed by the Royal Navy in response to reported German submarines with surface speeds over .[1] The submarines had a displacement of 1,210 tons surfaced, but J7 had a lighter submerged displacement than her sister boats, at 1,760 tons.[1] Each submarine was in length overall, with a beam of, and a draught of .[1] The propulsion system was built around three propeller shafts; the J-class were the only triple-screwed submarines ever built by the British.[1] Propulsion came from three 12-cylinder diesel motors when on the surface, and electric motors when submerged.[1] Top speed was on the surface (the fastest submarines in the world at the time of construction), and underwater.[1] Range was at .[1]
Armament consisted of six 18 inch (450 mm) torpedo tubes (four forward, one on each beam), plus a 4-inch deck gun.[1] Originally, the gun was mounted on a breastwork fitted forward of the conning tower, but the breastwork was later extended to the bow and merged into the hull for streamlining, and the gun was relocated to a platform fitted to the front of the conning tower.[1] The conning tower on J7 was sited 60feet further back than her sister boats, as the control room was located behind the machinery spaces.[1] [2] 44 personnel were aboard.[1]
J7 was built by HM Dockyard Devonport in Plymouth and launched on 12 February 1917.[3]
After the war, the British Admiralty decided that the best way to protect the Pacific region was with a force of submarines and cruisers.[1] To this end, they offered the six surviving submarines of the J-class to the Royal Australian Navy as gifts.[1] J1 and her sisters were commissioned into the RAN in April 1919, and sailed for Australia on 9 April, in the company of the cruisers and, and the tender .[1] The flotilla reached Thursday Island on 29 June, and Sydney on 10 July.[1] Because of the submarines' condition after the long voyage, they were immediately taken out of service for refits.[1]
By the time J7 was returned to service in June 1922, the cost of maintaining the boats and deteriorating economic conditions saw the six submarines decommissioned and marked for disposal.[1]
J7 was paid off on 12 July 1922, and was sold on 26 February 1924. The hulk was scuttled in 1930, for use as a breakwater at the Sandringham Yacht Club in Port Phillip.[4] Some years later a stone marina was constructed around the wreck, which was left in situ and visible, being too expensive to remove, where it continues to deteriorate.
