When the German Navy developed an acoustic homing torpedo, torpedo countermeasures such as the Foxer acoustic decoy were deployed. Intelligence One of the best kept Allied secrets was the breaking of enemy codes including some of the German Naval Enigma codes (information gathered this way was dubbed Ultra) at Bletchley Park in England. This enabled the tracking of U-boat packs to allow convoy re-routings; whenever the Germans changed their codes (and when they added a fourth rotor to the Enigma machines in 1943), convoy losses rose significantly. By the end of the war, the Allies were regularly breaking and reading German naval codes. To prevent the Germans from guessing that Enigma had been cracked, the British planted a false story about a special infrared camera being used to locate U-boats. The British were subsequently delighted to learn that the Germans responded by developing a special paint for submarines that exactly duplicated the optical properties of seawater. Tactics Many different aircraft from airships to four-engined sea- and land-planes were used. Some of the more successful were the Lockheed Ventura, PBY (Catalina or Canso, in British service), Consolidated B-24 Liberator (VLR Liberator, in British service), Short Sunderland, and Vickers Wellington. As more patrol planes became equipped with radar, U-Boats began to be surprised at night by aircraft attacks. U-Boats were not defenseless, as most U-Boats carried some form of anti-aircraft weapon. They claimed 212 Allied aircraft shot down for the loss of 168 U-boats to air attack. The German naval command struggled to find a solution to the aircraft attacks. 'U-Flak' submarines, equipped with extra anti-aircraft weapons, were tried unsuccessfully. At one point in the war, there was even a 'shoot back order' requiring U-boats to stay on the surface and fight back, in the absence of any other option. Some commanders started charging batteries during the day to gain more warning from air attack, and perhaps gain time to submerge. One solution was the snorkel, which allowed a U-boat to stay submerged and still charge its batteries. A snorkel made a U-boat more survivable and losses to aircraft went down. However, the low snorkeling speeds of greatly limited the mobility of the U-Boats. The provision of air cover was essential. The Germans at the time had been using their Focke-Wulf Fw 200 Condor long range aircraft to attack shipping and provide reconnaissance for U-boats, and most of their sorties occurred outside