German weather ship Lauenburg The Lauenburg was a German weather ship used in the early years of the Second World War to provide weather reports for German shipping , particularly German U - boats . Her capture and subsequent sinking on 28 June 1941 allowed the Royal Navy to acquire important German code books and parts of an Enigma machine , and came after the German use of such vessels had been identified as a weakness that could be exploited to break the Enigma code . Early life . The " Lauenburg " had been built in 1938 as a fishing trawler , named after the town of Lauenburg , and with the identification number ' PG 532 ' . She operated out of Geestemünde for her owners , H . Bischoff & Co , of Bremen . She was acquired by the Kriegsmarine in 1940 , and entered naval service in November that year , having been converted into a weather ship , but retaining the name " Lauenburg " . In her new guise she carried a crew of between 19 and 21 , as well as eight meteorologists . She was to be used to provide detailed weather reports for naval units , including Germany ' s U - boat fleet . The weather ships and Enigma . The British cryptologist Harry Hinsley , then working at Bletchley Park realised at the end of April 1941 that the German weather ships , usually isolated and unprotected trawlers , were using the same Enigma code books as were being used on the heavily armed U Boats . The trawlers , which were transmitting weather reports to the Germans , were in turn being sent naval Enigma messages . Although the weather ships did not transmit enciphered weather reports on Enigma machines , they still needed to have one of the machines on board if they were to decode the Enigma signals transmitted to them . Hinsley realised that if the code books could be captured from one of these vulnerable trawlers , the naval Enigma system could be broken , with British intelligence able to decipher messages to U - boats and discover their locations . The problem remained that if the navy were to attempt to capture one of the weatherships , the German crew would have time to throw their current Enigma settings into the sea before they were boarded . Hinsley instead reasoned that the following month ' s Enigma settings would be locked in a safe aboard the ship , and could potentially be overlooked if the Germans were forced to hastily abandon ship . On being informed , the Admiralty despatched seven destroyers and cruisers to the northeast of Iceland at the beginning of May 1941 . The target was the " München " , one of the weather ships operating in the area . In the course of the raid , the weather ship , and the Enigma settings for June 1941 were captured . As a result , naval Enigma messages transmitted during June 1941 could be quickly deciphered . Halfway through June 1941 the Germans replaced the bigram tables used in Enigma . This would have resulted in a codebreaking blackout unless further settings could be captured . Hinsley and the Admiralty were concerned that capturing another weather ship might alert the Germans to their vulnerability and cause them to immediately alter them again . It was eventually decided to take the risk and on 25 June 1941 four warships , the light cruiser HMS " Nigeria " and the destroyers HMS " Tartar " , HMS " Jupiter " and HMS " Bedouin " , were despatched from Scapa Flow to capture the codebooks from the " Lauenburg " , another weather ship operating north of Iceland , which Hinsley had selected . The capture of the " Lauenburg " . At around 7pm on 28 June , a lookout aboard HMS " Tartar " sighted the " Lauenburg " off Jan Mayen , and " Tartar " began firing . The " Lauenburg ' s " crew quickly abandoned the ship in two lifeboats . Minutes later , " Tartar " steamed alongside and a boarding party seized the " Lauenburg " . A large amount of material was collected and transferred to the " Tartar " . The allied warships then fired on and sank the " Lauenburg " . The recovered material allowed further understanding of the Enigma codes and resulted in faster decoding of encrypted messages , as well as providing an up - to - date set of codes .