and returning to French Atlantic bases. These "U-flak" boats were , , , and . Conversion began on three others (, , and ) but none was completed and they were eventually returned to duty as standard VIIC attack boats. The modified boats became operational in June 1943 and at first appeared to be successful against a surprised Royal Air Force. Hoping that the extra firepower might allow the boats to survive relentless British air attacks in the Bay of Biscay and reach their operational areas, Donitz ordered the boats to cross the bay in groups at maximum speed. The effort earned the Germans about two more months of relative freedom, until the RAF modified their tactics. When a pilot saw that a U-boat was going to fight on the surface, he held off attacking and called in reinforcements. When several aircraft had arrived, they all attacked at once. If the U-boat dived, surface vessels were called to the scene to scour the area with sonar and drop depth charges. The British also began equipping some aircraft with RP-3 rockets that could sink a U-boat with a single hit, finally making it too dangerous for a U-boat to attempt to fight it out on the surface regardless of its armament. In November 1943, less than six months after the experiment began, it was discontinued. All U-flaks were converted back to standard attack boats and fitted with Turm 4, the standard anti-aircraft armament for U-boats at the time. (According to German sources, only six aircraft had been shot down by the U-flaks in six missions, three by U-441, and one each by U-256, U-621, and .) Type VIIC/41 Type VIIC/41 was a slightly modified version of the VIIC and had the same armament and engines. The difference was a stronger pressure hull giving them a deeper crush depth and lighter machinery to compensate for the added steel in the hull, making them slightly lighter than the VIIC. A total of 91 were built. All of them from onwards lacked the fittings to handle mines. Today one Type VIIC/41 still exists: is on display at Laboe (north of Kiel), the only surviving Type VII in the world. Type VIIC/42 The Type VIIC/42 was designed in 1942 and 1943 to replace the aging Type VIIC. It would have had a much stronger pressure hull, with skin thickness up to 28 mm, and would