The U-flak boats were 4 VIIC boats (U-441, U-256, U-621 and U-953) that were modified to act as surface escorts for the incoming/outgoing attack u-boats operating from the French Atlantic bases. They had greatly increased anti-aircraft fire-power and were intended as aircraft traps.
3 more U-boats were taken aside as additional U-flak boats (U-211, U-263 and U-271) but none of them was completed as Flak boats although conversion did certainly start on all of them. They were eventually returned to duty as traditional VIIC attack boats.
The modifications took place in 1943 and the boats became operational in June 1943 and had excellent successes against the surprised RAF aircraft. Dönitz realized their potential and ordered the boats to cross the Bay of Biscay in groups at max speed. The effort gave the Germans only about 2 months of still-limited freedom though until the RAF developed counter-measures where they called in surface hunters to assist the aircraft and the U-flak boats were withdrawn and converted back into fighting vessels.
U-flak - the origins of the idea
On 31/08/42 U-256 (on her first cruise sailing from Kiel on 28/07/42 to proceed to Lorient via the North Atlantic) was seriously damaged by Whitley 'B' of Sq 502. The boat was nearly scrapped but it was decided to adapt her into a U-flak - a heavily AA armed boat intended to lure unsuspecting aircraft to a deadly trap. It was expected to stop heavy losses in the Bay of Biscay inflicted by Allied aircraft by deploying a number of U-flaks.
Although U-256 was the first boat to be converted into a U-flak, the reconstruction was delayed. In the meantime, on 16/04/43, it was decided to convert U-441 in the same way. The third and fourth Flakvierling mounts available (20mm quadruple sets) and the first experimental 37mm automatic gun were installed on U-441. Also, a battery of 86mm line-carrying AA rockets was installed (but this idea proved unworkable).
It is sometimes indicated that two additional single 20mm guns were also carried. The fuel capacity was limited to Bay of Biscay operations only. Only 5 torpedoes were carried - in the tubes - for self-defence (room was needed for additional gunners taken aboard).
Flak-U1 or U-441 sailed on 22/05/43 from Brest on her 5th patrol commanded by KL Goetz von Hartman. On 24/05/43 U-441 was attacked by Sunderland 'L' of Sq 228, shot the aircraft down but got seriously damaged by aerial depth-charges and was forced to return arriving on 26/05/43.
The effectiveness of improved AA weaponry was overestimated and resulted in ordering U-boats to pass the Bay of Biscay on the surface in groups. This in turn resulted in heavy losses due to the group tactics adopted by the Allied aircraft.
U-441 as U-flak again sailed from Brest on 8/07/43. On 12/07/43 she was attacked by Beaufighters 'A', 'B' and 'V' of Sq 248 and ended up badly damaged with heavy casualties (10 men dead, 13 wounded) in spite of the initial heavy AA fire. U-441 returned on 13/07/43.
U-621 was converted into U-flak in June 43, after being damaged on her 4th cruise, by Liberator 'Q' of Sq 224 on 31/05/43. U-621 as U-flak sailed on 29/08/43 and scored no success on her 5th one month patrol. After being reverted to a normal flak armament she was damaged by aircraft on 6th cruise, by Liberator 'A' of Sq 59 on 13/01/44.
The number of U-flaks sometimes is given as greater than 7. The confusion may arise from the fact that many boats carried extended AA armament, including additional guns installed forward of the bridge (and therefore easily confused with U-flaks).
|5 1943||U-441||Operations in the Bay of Biscay|
|7 1943||U-441||Operations in the Bay of Biscay|
|8/9 1943||U-621||Operations in the Bay of Biscay|
|10 1943||U-256, U-271||AA cover in the refueling area north of the Azores|
|10 1943||U-953||Operations on the Bay of Biscay|
|10/11 1943||U-211, U-441 U-953||Group Schill against convoys MKS 28 an MKS 29, for U-953 it was just one long patrol.|
In November 1943 it was decided to convert all the seven U-flaks (not all completed) back to normal attack boats, fitted with Turm 4.
U-flaks were not able to fulfill their assigned mission in the Bay of Biscay - mainly because of the group tactics adopted by Allied aircraft (one can see the irony in this). According to the German war sources only 2 aircraft were shot down by U-flaks (and some damage inflicted on others) in 6 U-flak missions (3 by U-441, 1 by U-621, U-953 and U-256).
Because of the limited fuel capacity and torpedo load U-flaks were not suitable for normal operations. At the same time the standard AA armament for U-boats was no longer much inferior to U-flaks. This and lack of success made U-flaks somewhat redundant.