Onslow held typical armament for an attack submarine with an armament suite led by 6 x 21" (533mm) torpedo tubes fitted to the bow (firing forwards). This was supplemented by 2 x 21" tubes in stern-facing mounts (later removed). The boat carried twenty torpedo reloads originally of the British Mark 8 type and then of the Mark 23 type before replacement by the American Mark 48 Mod 4 series. The boat was then modified to launch UGM-84 Sub Harpoon anti-ship missiles in place of torpedoes through an update in the early 1980s. It was at that time that she was also to lose her two stern-facing torpedoes which added no more tactical value due to the new generation of torpedoes being taken on for the bow tubes.
Onboard, Onslow utilized a mix of sensors and processing systems. An Atlas Elektronik Type CSU3-41 sonar system was fitted to the bow while a BAC Type 2007 array was added in a flank position. A rangefinding array was made up of the Sperry BQG-4 "Mircopuffs" system (a later feature installed) and radar included the Kelvin Hughes Type 1006 series system.
HMAS Onslow began her ocean-going career in 1970. In 1972, a disgruntled transfer disobeyed an order to close the forward ballast tank which led to the vessel taking a dangerously steep dive action. The valve was closed in time by another submariner and the boat was able to make it back to the surface unscathed. This incident led to the RAN submarine force accepting only volunteers in their ranks and not forced conscripts.
After assignment to ANZUK beginning July of 1972, Onslow was taken in for refit in May 1975 at the Cockatoo Island Dockyard. In 1980, she took part in "Exercise Kangaroo 3" which involved simulated attacks on moving surface warships. Onslow managed to "sink" all seven "enemy" warships in the scenario and proudly displayed the Jolly Rogers upon her return. In 1981, during a March joint exercise with the Royal New Zealand Navy (RNZN), one of Onslow's diesels began smoking with the boat submerged, causing carbon monoxide to fill the vessel. One crewmember died and a further eighteen blacked out while some 33% of the crew suffered from inhaling the gas. The cause of the engine malfunction was blamed on human error though the crew saw otherwise.
During 1982 to 1984 Onslow underwent a period of refit and had her weapons suite revised and updated systems and sensors installed. The support for Harpoon anti-ship missiles made her the first conventionally-powered boat to be given this useful attack feature. Onslow undertook several goodwill tours including visits to the United States and Canada in the period following.
During the early 1990s, the Oberon-class in Australian service was meeting its useful end. Four of the six boats were decommissioned by the middle part of the decade leaving just Onslow and Otama in active service. A hazing incident during 1995 brought about a stain to the Onslow's otherwise excellent career. The boat served in other exercises from then on including RIMPAC 1998 and was formally decommissioned on March 30th, 1999, HMAS Onslow. In April of that year, the boat was handed over to museum ownership for long-term preservation.
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