U-864, animation of the project outside Fedje, Norway

Action of 9 February 1945
Further information: Action of 9 February 1945
On 6 February 1945, U-864 passed through the Fedje area without being detected, but on 9 February Venturer heard U-864's engine noise. Launders had decided not to use ASDIC since it would betray his position and spotted the U-boat's periscope as her captain looked for his escort. In an unusually long engagement for a submarine, and in a situation for which neither crew had been trained, Launders waited 45 minutes after first contact before going to action stations. Launders was waiting for U-864 to surface and thus present an easier target. Upon realising they were being followed by the British submarine and that their escort had still not arrived, U-864 zig-zagged underwater in attempted evasive manoeuvres, with each submarine occasionally risking raising her periscope.

Venturer had only eight torpedoes as opposed to the 22 carried by U-864. After three hours Launders decided to make a prediction of U-864's zig-zag, and released a spread of his torpedoes into its predicted course. This manual computation of a firing solution against a three-dimensionally manoeuvring target was the first occasion on which techniques were used and became the basis of modern computer-based torpedo targeting systems. Prior to this attack, no target had been sunk by torpedo where the firing ship had to consider the target's position in three-dimensional terms, where the depth of the target was variable and not a fixed value. The computation thus differs fundamentally from those performed by analogue torpedo fire-control computers which regarded the target in strictly 2D terms with a constant depth determined by the target's draught.

The torpedoes were released in 17 second intervals beginning at 12:12, and all taking four minutes to reach their target. Launders then dived Venturer suddenly to evade any retaliation. U-864 heard the torpedoes coming, dived deeper, and turned away to avoid them. The first three torpedoes were avoided, but U-864 unknowingly steering into the path of the fourth. Exploding, U-864 split in two, and sank with all hands coming to rest more than 150 metres (490 ft) below the surface. Launders was awarded a bar to his DSO for this action.
https://en.wikipedia.org/wiki/HMS_Venturer_(P68)