Flak Trap

The concept of the "U-flak" or "Flak Trap" originated on 31 August 1942, when U-256 was seriously damaged by aircraft. Rather than scrap the boat, it was decided to refit her as a heavily armed anti-aircraft boat intended to combat the losses being inflicted by Allied aircraft in the Bay of Biscay. Two 20 mm quadruple Flakvierling mounts and an experimental 37 mm automatic gun were installed on the U-flaks' decks. A battery of 86 mm line-carrying anti-aircraft rockets was tested (similar to a device used by the British in the defense of airfields), but this idea proved unworkable. At times, two additional single 20 mm guns were also mounted. The submarines' limited fuel capacities restricted them to operations only within the Bay of Biscay. Only five torpedoes were carried, preloaded in the tubes, to free up space needed for additional gun crew.

Four VIIC boats were modified for use as surface escorts for U-boats departing and returning to French Atlantic bases. These "U-flak" boats were U-441, U-256, U-621, and U-953. Conversion began on three others (U-211, U-263, and U-271) 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 U-953.)