Type 206 submarine
The Type 206 is a class of diesel-electric submarines (U-boats) developed by Howaldtswerke-Deutsche Werft (HDW). Its design is based on the preceding Type 205 submarine class. These small and agile submarines were built during the Cold War to operate in the shallow Baltic Sea and attack Warsaw Pact shipping in the event of military confrontation. The pressure hulls were built out of non-magnetic steel to counter the threat of magnetic naval mines and make detection with MAD sensors more difficult. The low emission profile allowed the submarines in exercises to intrude even into  well-protected opposing forces such as carrier formations with their screen. Programme. Ten Type 205 submarines were constructed between 1962 and 1968 with hulls constructed of a new non-magnetic steel. The early boats, however, suffered from cracking due to stress corrosion and an urgent programme was initiated to develop a new steel which overcame these problems, which received much publicity at the time. The new high-strength, non-magnetic, austenitic steel has greater elasticity and good dynamic strength, and has proved very satisfactory in service, thoroughly overcoming the doubts that were raised by those early problems, although it has  never been selected by IKL's many export customers and thus remains unique to German Navy submarines. Subsequently, the Type 206 was designed by Ingenieur Kontor Lübeck (IKL) in 1964–65 and a production order was placed on 7 June 1969 for eight from HDW in Kiel and ten from Nordseewerke in Emden. Of the 18 submarines built for the Bundesmarine (West German Navy), 12 were modernized in the early 1990s and were re-designated as Type 206A; the others have been decommissioned. The current German Navy is starting to decommission some 206A vessels, with more of the new Type 212  submarines being commissioned. In June 2010 the Ministry of Defence announced that all six remaining vessels were to be retired from active service immediately and to be decommissioned by the end of 2010 to cut costs. There are no type 206 submarines left in active service with the German Navy. A slightly modified variant of the Type 206 (which includes the distinctive dome, or bulge, in the front of the boat), the "Gal"-class submarine for the Israeli Navy was built to Israeli specifications as the Vickers Type 540 in the UK rather than Germany for political reasons. Three such boats  were built, the first being commissioned in 1976. When the Israeli navy received its new "Dolphin"-class submarines (also built by HDW), the "Gal"s were retired. As of 2006, one had been scrapped and two had been sent to HDW in an attempt to find a buyer for them. When no buyer was found, one of the submarines was returned to Israel for display in the Clandestine Immigration and Naval Museum. Modernisation. A major mid-life modernisation was conducted on twelve of these submarines, the boats concerned now being officially designated Type 206A. The work started in mid-1987 and completed in  February 1992, being carried out by Nordseewerke, Emden; this upgrade includes: The STN Atlas DBQS-21D sonar has been fitted, together with new periscopes, and a new weapon control system (LEWA). The ESM system has been replaced and GPS navigation installed. The rebuilt submarines are armed with new torpedoes ("Seeaal"), and the propulsion system has been comprehensively refurbished, and improvements have been made to the accommodation. List of units. Notes: Exports. The Colombian Navy purchased four decommissioned Type 206A submarines to boost its submarine force. Two submarines, named "Intrépido" (ex-U23) and "Indomable" (ex-U24) were commissioned into the Colombian Navy  on 28 August 2012. Two submarines (ex-U16 and ex-U18) were acquired to be cannibalized for spare parts. On 5 December 2015, "Intrépido" and "Indomable" entered active service after a lengthy refit in Germany.