Worst. Submarines. Ever.

Britain’s answer to the German U-Boat were the K-Class submarines – 100-metre-long steam-powered submersibles that cost £340,000 a pop, and could do 24 knots on the surface. Minor design faults aside – travelling at speed could cause the front of the boat to plunge under the surface of the water, and big waves tended to pour down the funnels and put the boilers out – they were the pride of the fleet.
They saw heroic action in the First World War, and our seafaring nation is able to boast that not one was ever lost through enemy action. Six of the 18 built, however, sank by accident.
K1 crashed into K4 off the coast of Denmark and had to be scuttled.
K2 caught fire on her maiden dive. In 1924, she collided with K12 as they were leaving Portland harbour.
K3 plunged nose-first to the bottom in December 1916 (giving a thrill of patriotic pride, no doubt, to the future King George VI who happened to be on board at the time), got stuck in mud and took 20 minutes to free. The following January her boiler room flooded in the North sea. The year after that she again dived unexpectedly and crashed into the sea-bed, causing massive damage to her hull.
K4, having survived the collision that did for K1, was less lucky in the sarcastically-named “Battle of May Island”, the debacle that outright did for two of the K-class submarines, seriously damaged four more of them as well as dinging a couple of battlecruisers.
It was the last night of January, 1918, when a flotilla of around 40 vessels passed through the mouth of the Firth of Forth en route to exercises in Scapa Flow. The trouble started when the flotilla changed direction and K22 crashed into K14. Then the battlecruiser Inflexible crashed into K22 (Inflexible lived up to her name, but K22 bent round at right angles and sunk till only the conning tower could be seen above the surface of the water).
Fearless crashed into K17, which went straight to the bottom. Then, as the submarines behind took lumbering evasive action, things got worse. While attempting to avoid colliding with K3, K4 was nearly cut in half by K6 and then rammed by K7 before heading, too, with grim inevitability for the seabed.
K5 was not involved in the Battle of May Island. She went down with all hands for no apparent reason during exercises in the Bay of Biscay.
K10 foundered while being towed in 1922.
K13 sank during sea-trials in a Scottish loch when her hatches failed to shut properly on diving. Salvaged and recommissioned as K22, she would go on to take a distinguished role in the Battle of May Island.
K15 – anxious to cut to the chase – spontaneously sank at her mooring in Portsmouth Harbour in 1921.
Only one K-boat ever engaged the enemy. It hit a U-boat amidships with a torpedo. The torpedo failed to explode.