Submarine K5 left Torbay on 19 January 1921 in company with her sister submarines K8, K10, K15 and K22 as part of the Atlantic Fleet for a mock battle in the Bay of Biscay.
The submarine was commanded by an experienced officer, Lieutenant Commander John A Gaimes, DSO, RN, but had a new crew. The other officers on board were Lieutenant F Cuddeford, Engineer-Lieutenant E Bowles, Acting Engineer-Lieutenant G Baker, Lieutenant B Clarke & Acting Lieutenant Robert Middlemist. The full complement included 51 ratings on board.
All 57 hands were lost on 20th January, about 120 nautical miles south-west of the Isles of Scilly. She had signalled that she was diving but she did not surface at the end of the exercise. An oil slick was discovered and some after planks from the battery covers and a sailor's "ditty box" were recovered, it was presumed that she had somehow gone past her maximum depth and been crushed.
On return from her exercises in the Mediterranean in 1922, HMS Hood and the rest of the fleet dropped wreaths and held a memorial service where K5 had gone down.
