The "U-flak" boats were four VIIC boats (U-441,
U-256, U-621, and U-951) modified to be surface escorts for the attack U-boats
operating from the French Atlantic bases. They had greatly increased
anti-aircraft fire-power.
Conversion began on three others (U-211, U-263, and U-271)
but none was completed, and they were eventually returned to duty as
traditional VIIC attack boats.
The modified boats became operational in June 1943 and at
first appeared to be successful against the surprised Royal Air Force. Seeing
their potential, Dönitz ordered the boats to cross the Bay of Biscay in groups
at maximum speed. The effort earned the Germans about two more months of
still-limited freedom, until the RAF developed counter-measures. When the RAF
began calling in surface hunters to assist the aircraft, the U-flak boats were
withdrawn and converted back into fighting vessels.
The concept of the U-flak began the year before, 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 stop the losses in the Bay of Biscay inflicted by Allied aircraft.
Two 20 mm quadruple Flakvierling mounts and the experimental
37 mm automatic gun were installed on the U-flaks' decks. A battery of 86 mm
line-carrying anti-aircraft rockets was tested, but this idea proved
unworkable. At times, two additional single 20 mm guns were also mounted. The
submarines' fuel capacities were limited to Bay of Biscay operations only. Only
five torpedoes were carried, preloaded in the tubes, to free the space needed
for the additional gunners.
In November 1943 – less than six months after the experiment
began – all U-flaks were converted back to normal attack boats, fitted with Turm
4. The standard anti-aircraft armament for U-boats was no longer much inferior
to U-flaks, and the U-flaks had not been particularly successful. Even with
massive anti-aircraft firepower, a U-boat was still vulnerable to having her
pressure hull punctured; her best bet when encountering aircraft was simply to
dive. According to German sources only six aircraft had been shot down by
U-flaks in six missions (three by U-441, one each by U-256, U-621, and U-953).