HMS Porpoise (S01) was a Porpoise-class submarine

British Porpoise class submarine

The Porpoise class was an eight-boat class of diesel-electric submarines operated by the Royal Navy. This class was originally designated patrol submarines, then attack. They were the first conventional British submarines to be built after the end of World War II...

 of the Royal Navy

Royal Navy

The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

. She was launched on 25 April 1956, commissioned on 17 April 1958, and was sunk as a target in 1985.
She was decommissioned in 1982 and sunk as a target in 1985 in trials for the Spearfish torpedo

Spearfish torpedo

The Spearfish torpedo is the heavy torpedo used by the submarines of the Royal Navy. It can be guided by wire or by autonomous active or passive sonar, and provides both anti-submarine warfare and anti-surface ship warfare capability.It replaces the unreliable Tigerfish torpedo, which was...

, for which purpose she was painted bright red. She had been used as a target while still serving with the Navy, in 1979 her casing, ballast tanks and vents were reinforced so that unarmed torpedoes could be fired at her without the risk of sinking her.