Convoy OB 318 OB 318 was a North Atlantic convoy which ran during the Battle of the Atlantic in World War II . During Operation Primrose Royal Navy convoy escorts , " Broadway " and " Aubretia " captured " U - 110 " with an intact Enigma machine and a wealth of signals intelligence , which led to the Allied breakthrough into cracking the German naval Enigma code . Prelude . By the Spring of 1941 the battle of the Atlantic was starting to have an increase in German U - boat losses . This forced Vizeadmiral Karl Dönitz to change his strategy and he now moved his wolf packs further west , in order to catch the convoys without their anti - submarine escort . OB 318 was a west - bound convoy of 38 ships , either in ballast or carrying trade goods , and sailed from Liverpool on 2 May 1941 bound for ports in North America . The convoy commodore was R . Adm . WB MacKenzie in SS " Colonial " . It was escorted by 7 EG , an escort group led by HMS " Westcott " ( Cdr . Bockett - Pugh ) and comprising ten warships ; these were joined in md - ocean by 3 EG , a force of eight warships led by HMS " Bulldog " ( Cdr J Baker - Cresswell ) . Opposing them was a force of nineteen U - boats , though in the event only six were in a position to pose a threat . One of those was " U - 110 " under the command of Kapitänleutnant Fritz - Julius Lemp . Lemp notoriously had been in command of " U - 30 " in 1939 which had controversially sunk the 13 , 581 ton passenger ship " Athenia " . Action . OB 318 was sighted on 7 May 1941 by " U - 94 " , which reported its position and commenced shadowing while U - boat Command ( BdU ) alerted other U - boats in the area . There were six U - boats witin striking distance , and these were ordered to close with " U - 94 ' s " position . Meanwhile during 7 May the escort force was joined by five ships from Iceland and the destroyers of 3 EG , which were to take over escort duties from the Western Approaches to a dispersal point at 34 West , a location south of Greenland . Three ships and the destroyers of 7 EG left for Iceland during 7 May , leaving the escort force still at ten warships . " U - 94 " . At nightfall on 7 May " U - 94 " attacked the convoy , sinking two ships . Her skipper , Herbert Kuppisch , was able to enter the convoy by submerging ahead of the convoy and letting the lead escorts pass . he was then able to fire at close range on the ships in the centre of the convoy , hitting " Ixion " and " Eastern Star " . However " U - 94 " was found by the sloop " Rochester " and counter - attacked by her , " Bulldog " and " Amazon " for four hours . " U - 94 " was damaged and forced to retire , though she was able to effect repairs later and continue her patrol . On 8 May the remaining vessels of 3 EG , three corvette s and two trawlers , with the Armed Merchant Cruiser " Ranpura " in company , joined the convoy , and the remaining ships of 7 EG departed . " U - 110 " . On the evening of 8 May " U - 110 " and " U - 201 " also made contact , tracking the convoy until morning . In an unusual move , the two skippers made rendezvous in order to co - ordinate their attack ; Lemp in " U - 110 " would make a submerged attack from ahead , while Adalbert Schnee in " U - 201 " would do the same from the rear . It was expected that the escort would have departed by this time , leaving the convoy vulnerable to their assault . During the morning of 9 May " U - 110 " moved into position and commenced her attack . Lemp was surprised to encounter the escort still in place , but succeeded in penetrating the convoy , sinking two ships . Lemp was targeting a tanker with his fourth torpedo tube when " Aubretia " spotted the periscope and then located " U - 110 " with ASDIC . The " Aubretia " gave the alert to other ships while she unloaded and dropped a pattern of depth charges . " U - 110 " performed a deep dive and managed to survive the initial onslaught . " Aubretia " was joined by the destroyers " Bulldog " and " Broadway " , and the attack was delivered with such force that Lemp was forced to surface . As he came up a dozen men on " U - 110 " rushed to man the guns but were shot by the waiting British ships . Lemp also saw that " Bulldog " was preparing to ram so Lemp gave the hurried order to abandon ship . Commander Joe Baker - Cresswell decided to try to capture the U - Boat instead and hove to . The survivors went over the side including Lemp . It turned out that Lemp realising that " Bulldog " was not going to ram had decided to go back and try to destroy the equipment along with the code books but he failed and died in the attempt . " U - 110 " itself survived the attack , but was seriously damaged and was taking on water . The " Bulldog " soon had pulled alongside and Sub - Lieutenant David Balme led a boarding party , and began stripping her of what they could find . Balme ' s team soon began to take off the valuables and these turned out to be codebooks , charts , ciphers and most significantly , a complete and undamaged Enigma machine . The crew hadn ' t any idea what it was they had captured . Before they transferred the equipment " Bulldog " had to depart , leaving the u - boat with the prize crew as it had received a message of another submarine in the area . Balme and his men were left onboard U - 110 for over an hour before " Bulldog " returned . " U - 201 " . Meanwhile Schnee in " U - 201 " made his attack , sinking one ship and damaging another . He was also counter - attacked , by " Amazon " , " Nigella " and " St Apollo " , and forced to retire . He was left damaged . " U - 556 " . In the early hours of 10 May OB 318 was found by " U - 556 " which attacked immediately , resulting in one ship damaged . As OB 318 was at its dispersal point , the convoy separated during the night , while the escorts departed to meet their next charge . However " U - 556 " was able to keep in touch with a group of ships heading south - west , and during the day sank two more . Aftermath and significance . U - boats sank five ships from convoy OB 318 ; and three , including the convoy commodore ' s ship " Colonial " , were sunk before arriving by U - boats patrolling near their destinations . Thirty - three ships arrived safely at their destinations over the next two weeks . Baker - Cresswell took " U - 110 " in tow , but she sank within hours due to the damage she had sustained . Lemp was lost with 14 members of his crew but a war correspondent , 4 officers and 28 men were rescued and sent to Scapa Flow as prisoners of war . At Scapa Flow experts from Bletchley Park were waiting and were exceptionally surprised with what they collected and took back with them . The capture of the Enigma machine was highly secret at the time and none of the crew knew of the significance . The machine itself significantly assisted the work in hand at Bletchley Park in breaking German naval codes . This was the first fully functioning machine and the first one used to break the naval codes along with the capture of codes from a number of German weather ships during the same year . Thanks to this Enigma machine though ; Bletchley were able to inform the Royal Navy and thus steered convoys away from where most groups of U - boat packs were present . The difference made was substantial ; from when the information began to pour through in June 1941 Allied shipping losses were around 432 , 000 tons but by August it had dropped to less than 80 , 000 tons . The most important find as well being the Reservehandverfahren cipher which was first solved at Bletchley Park in June 1941 by means of documents captured from " U - 110 " and then later on with the important capture of code - books and other important documents from " U - 559 " on 30 October 1942 . Thereafter it was solved using cryptanalysis led by Alan Turing for over three years . Some 1 , 400 signals were read during that period . Baker - Cresswell was awarded the DSO and promoted captain . King George VI told him the capture of the " U - 110 " cipher material had been " the most important single event in the whole war at sea " . U - 571 ( film ) . The film U - 571 was partially inspired by the capture of " U - 110 " . The film however caused irritation and anger in Britain whereby the films plot was Americanised . The anger came to such a point that at Prime Minister ' s Questions , Tony Blair agreed with questioner Brian Jenkins MP that the film was " an affront " to British sailors . In response to a letter from Paul Truswell , MP for the Pudsey constituency ( which includes Horsforth , a town proud of its connection with HMS " Aubretia " ) , U . S . president Bill Clinton wrote assuring that the film ' s plot was only a work of fiction . Despite the criticisms however David Balme was interviewed by the director of the film , Jonathan Mostow in which he described the capture of " U - 110 " and the capture itself was mentioned at the start of the end credits as recognition and response to the criticisms .