HMS Grampus (SO4) was built by Cammell Laird at Birkenhead between 1955, launched in 1957 and commissioned in 1958. She was one of eight Porpoise class diesel-electric submarines, with dimensions of 88 metres length and 8 metres beam, of 2400 tons. She carried a complement of 71 officers and men.
The Porpoise class consisted of a series of submarines: Porpoise, Rorqual, Narwhal, Cachalot, Grampus, Finwhale, Sealion and Walrus. The Porpoise class boats were launched in 1958 during the ever increasing threat of the Soviet Union's submarine fleet. They were initially roled as hunter/killer SSKs although this changed as the benefits of silence for clandestine operations and intelligence was noticed. Submarines of this type later served as trials platforms for the development of Passive Sonar type 2007 and the UK Towed Array. They were also employed in acoustic intelligence gathering (on Soviet subs) missions. HMS Grampus has ASDIC kit comprising sets 169 and 168.
The Porpoises were built from carbon manganese molybdenum steel known as UXW, and had aluminium casing. UXW steel was low alloy with higher carbon equivalents that gave rise to some weldability problems. In order to alleviate these problems, QT28, an electrode with a lower carbon equivalent, was introduced in 1956. This, the first fully quenched and tempered low alloy steel, was used for pressure hull plating for the successful Oberon class of boats which succeeded the Porpoise class. Use of UXW steel combined with improved design and construction techniques allowed much deeper diving to a maximum of 300 metres.
The Porpoise boats had 2 - 16 cylinder ASR (Admiralty Standard Range) supercharged diesel engines developing c16700 bhp with indirect drive, that is, they were not coupled to the propeller shafts. Propulsion was provided by electric motors. Designed with a top speed of 18 knots (33 km/h; 21 mph), the boats were capable of 17 knots (31 km/h; 20 mph), or 16 knots (30 km/h; 18 mph) once fitted with silenced propellers. It was found in tests that the unusually long engine room of Porpoise class subs such as HMS Grampus was liable to collapse, so there were extra large frames in this section, which proved to be something of an operational inconvenience.
The Porpoise class were exceptionally quiet underwater, more so than their NATO counterparts and far more so than the Soviet Whiskeys. They generated 97% less noise than previous submarine types. The silent running abilities made their sonar equipment particularly effective. The Porpoise class were far more capable than previous submarine classes in operating for prolonged periods, thanks to much improved air recirculation and cleaning systems. HMS Grampus demonstrated this, spending weeks under the polar icecap looking for holes in the ice in spring 1963 under the command of Lieutenant-Commander Compton-Hall. She returned to Gosport on 1 April 1963 having superficially damaged her hull on the ice. The Porpoise class also performed excellently in clandestine operations, such as surveillance and inserting special forces, due to their quietness.
On 11 January 1968, the French trawler Fomalhaut caught Grampus in her nets in the English Channel. Under the command of Lieutenant-Commander T D A Thompson, Grampus surfaced and both crews spent over three hours disentangling the nets. In 1968 she was part of the First Submarine Squadron based at HMS Dolphin and in that year was present during 'Navy Days' in Portsmouth Dockyard. In November 1968, HMS Grampus participated in a NATO exercise in the Mediterranean, Exercise Eden Apple, which saw British Naval and RAF contingents participate including aircraft carriers, frigates, warships and guided missile destroyers. The weaponry of the Porpoise class was updated in 1970 to operate the Mark 24 Tigerfish torpedo. In 1972, HMS Grampus operated with USS Tigrone in a joint American-British oceanographic operation in the eastern Atlantic.
The Porpoise class boats were all decommissioned by 1988, with some vessels being sold to be broken up as scrap and other being sunk to use as targets. HMS Grampus (SO4) was intended to be scuttled and sunk as a sonar target within the Ardnagowan submarine exercise area. However, she sank prematurely on the 18th September 1980 whilst being towed to her intended scuttling location in upper Loch Fyne. Nothing more is known about how she came to sink when she did.
HMS Grampus was surveyed by MV Seaforth Clansman on 25 November 1980.