The J class was designed by the Royal Navy in response to reported German submarines with surface speeds over 18 knots (33 km/h; 21 mph).^[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 275 feet (84 m) in length overall, with a beam of 22 feet (6.7 m), and a draught of 14 feet (4.3 m).^[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 19 knots (35 km/h; 22 mph) on the surface (the fastest submarines in the world at the time of construction), and 9.5 knots (17.6 km/h; 10.9 mph) underwater.^[1] Range was 4,000 nautical miles (7,400 km; 4,600 mi) at 12 knots (22 km/h; 14 mph).^[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 60 feet (18 m) 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 Sydney and Brisbane, and the tender Platypus.^[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,^{[citation needed]} and was sold on 26 February 1924^{[citation needed]}. 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.^[4]

• Bastock, John (1975). Australia's Ships of War. Cremorne, NSW: Angus and Robertson. ISBN 0207129274. OCLC 2525523.

• Map of Sandringham yacht club marina Archived 20 July 2008 at the Wayback Machine

37°56′36″S 144°59′41″E / 37.943381°S 144.99471°E / -37.943381; 144.99471