German weather ship Lauenburg

HMS Tartar's boarding party prepares to board the weather ship Lauenburg north east of Jan Mayen
History
Nazi Germany
Name:	Lauenburg
Namesake:	Lauenburg/Elbe
Laid down:	1 July 1936
Launched:	1938
Acquired:	1940
Commissioned:	November 1940
Fate:	Sunk 28 June 1941
General characteristics
Class and type:	Converted trawler
Complement:	19-21 crew 8 meteorologists

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.

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 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 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 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.