U-673 Type U-VIIC Turm II

First launched in June 1936, the Type VII was not the best submarine in any particular aspect, but it was the most successful of the war and formed the backbone of the U-boat force. Like many other military designs, the original Type VII was quickly modified to provide longer range, better performance and revised armament. These additional variants resulted in an improved torpedo attack boat (VIIC), minelayer (VIID), supply boat (VIIF) and other variants such as Flak and repair boats.

Boat Bridge Conversions or Bridge Designs referred to the different bridge layout used by the Type VII U-boat during the war. In order to accomodate a variety of different anti-aircraft weaponry, the original bridge design was modified, giving rise to at least seven different bridge conversions during the war.

At the beginning of the war, the standard U-boat anti-aircraft armament is a single 20mm Flak intended as a defensive measure against aircraft. Mounted on the main upper deck, aft from the Bridge, this position limited the arc of fire due to the obstructing bridge. It was eventually relocated to a platform just aft of the bridge.

By mid-1942, the very rapid increase in hostile aircraft made it clear that the single gun was an inadequate defense against enemy air attacks. This gave rise to numerous bridge conversions to improve the boat's air defense capability.