See main article: Oberon-class submarine. The Oberon class was a direct follow on of the Porpoise-class, with the same dimensions and external design, but updates to equipment and internal fittings, and a higher grade of steel used for fabrication of the pressure hull.[1]
As designed for British service, the Oberon-class submarines were in length between perpendiculars and in length overall, with a beam of, and a draught of .[2] Displacement was 1,610 tons standard, 2,030 tons full load when surfaced, and 2,410 tons full load when submerged.[2] Propulsion machinery consisted of 2 Admiralty Standard Range 16 VMS diesel generators, and two electric motors, each driving a 7adj=midNaNadj=mid, 3-bladed propeller at up to 400 rpm.[2] Top speed was when submerged, and on the surface.[2] Eight 21inches diameter torpedo tubes were fitted (six facing forward, two aft), with a total payload of 24 torpedoes.[2] The boats were fitted with Type 186 and Type 187 sonars, and an I-band surface search radar.[2] The standard complement was 68: 6 officers, 62 sailors.[2]
Onslaught was laid down by Chatham Dockyard on 8 April 1959, and launched on 24 September 1960.[2] The boat was commissioned into the Royal Navy on 14 August 1962.[2]
Onslaught was first assigned to the 2nd Submarine Squadron, based in Devonport. She was reassigned to the Singapore-based 7th Squadron in 1966, and received a refit in Devonport from 1970 to 1972. She was then assigned to the 1st Squadron in 1972.[3] Her squadron assignemnt after 1983 is unknown.
Her patrols in the Mediterranean and Baltic from 1986 to 1988 are classified.[4]
Onslaught was paid off in 1990, and broken up in Aliaga, Turkey in 1991.[5]