German U-Boat

At the start of the war, the standard U-boat antiaircraft armament was a single 20mm Flak (Flugabwehrkanon) gun mounted on the main deck, far from the conning tower. The setback was that the conning tower itself created a large dead zone, and the gun had to be removed and stowed before diving, increasing the amount of time needed to dive. Eventually the weapon was relocated to a special platform just aft of the bridge.

By mid 1942, allied airpower had increased significantly, and it became clear that a heavier air defense armament was required. As a result of this, twin and even quadruple 20mm mountings were installed. But as a result of delays, it was not until the middle of 1943 that the first twin and quadruple mountings appeared. At about the same time, new 20mm shells were also introduced, which not only carried three times more explosive than previous shells, but the explosive itself was new and much more powerful. By late 1943, the new and much more powerful 37mm Flak gun made its debut. This was a navalized version of the German army Flak 37 which was quickly adapted and installed on U-boats. Problems were encountered with the 37mm gun as well, as it was very sensitive to seawater and frequent stoppages meant that it had to be serviced regularly. Finally when this too proved inadequate to thwart air attacks, it was recognized that U-boats were unlikely to win combat engagements with aircraft, and the best defense was to spot an aircraft in time to crash dive to safety.

U-118, a Type XB minelayer under attack from ASW aircraft of the USS Bogue. She sank shortly after this attack.	A U-boat crew mans the 20mm flak on a Type IX.