At the beginning of the war, allied airpower was weak or non-existent. Hence the standard anti-aircraft armament of a single 20mm flak was sufficient to ward off most air attacks. The 20mm gun was not waterproof, which meant that it had to be stowed away in a vertical watertight casing before diving, increasing the amount of time required for the boat to dive.
When surprised on the surface, U-boat crews usually put up a good fight and shot down many aircraft, but as the war progressed, allied airpower increased rapidly and it became clear that the single 20mm flak gun was inadequate. Urgent steps had to be taken to increase the boat’s anti-aircraft capability, giving rise to a number of conversions.
The first improvement consisted of the addition of extra anti-aircraft guns mounted in twin and even quadruple configuration. These were mounted on a slightly lowered platform on the forward and aft of the conning tower. These platforms came to be known as “wintergarden”. When these also proved inadequate, an even heavier twin 37mm flak was tried, but even this was not enough to stop a determined pilot.
Finally, a specialized Flak U-boat was designed as the ultimate solution to counter the air threat. Similar to role played by the mechanized anti-aircraft division of the German army – which was to provide air defense for surrounding units. The Flak U-boats were intended as mobile air defense units for other U-boats in the vicinity.
A total of seven U-boats, all of the Type VIIC class were converted to Flak variants. First launched on May 22 1943, their orders were to shoot down any allied aircraft encountered. Though they had some early successes, it was soon realized that the operational concept of the Flak U-boat was flawed and they were of no value in the North Atlantic.
A number of problems were encountered with the Flak U-boats. First, the bridge structure had to be enlarged so that multiple anti-aircraft emplacements could be mounted. The enlarged structure added considerably to the top weight, causing stability problems, especially in rough sea conditions. Further, it added to the hydrodynamic resistance, increasing diving time and reducing underwater speed. More men on the top deck also meant that more time was needed to clear the deck during a dive. The additional anti-aircraft armament took up considerable space, so that only five torpedoes could be carried, while their reduced fuel bunkers meant that they could not operate in more distant waters. Finally, the final proof that Flak U-boats were conceptually flawed came when allied pilots developed new tactics to overcome these defenses. Appreciating that the AA defenses had been strengthened, allied pilots would circle just out of range of the flak guns while staying close enough bomb it effectively if it tried to dive. Rest assured that if the standoff continues long enough, other aircraft or surface ships will appear. As a result, the design was scrapped and all existing Flak U-boats were converted back to their original VIIC designs.
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