German submarine U-110 (1940)

9 May 1941: forced to the surface off Greenland

While protecting Convoy OB 318 in the North Atlantic off Greenland on 9 May 1941, the corvette HMS Aubretia dropped depth charges that badly damaged U-110 and forced the German submarine to the surface.

Its commander, Fritz-Julius Lemp, ordered the crew to abandon the boat without first ensuring that its secret material and equipment were destroyed.

Accounts differ over the precise circumstances of Lemp's death after the abandonment.

Six hours inside the boat

Sub-Lieutenant David Balme, aged 20, led a boarding party across from HMS Bulldog and entered the apparently deserted submarine.

Telegraphist Allen Long found a device resembling a typewriter, pressed its keys, saw peculiar results and passed it up through the hatch. The party recovered a working Enigma machine, cipher keys, keybooks and other cryptographic records.

They worked inside the damaged boat for about six hours before preparing it for towing.

Taken, and kept quiet

The Admiralty ordered the capture treated with extreme secrecy. Bulldog attempted to tow U-110 toward Iceland, but the submarine foundered on 10 May — after its intelligence material had been removed.

The machine by itself did not suddenly teach Britain to read every German code. The current key material, operating instructions and codebooks were the particularly valuable part, building on earlier Polish and British cryptanalysis.