For all the wolfpack’s power and menace, it had one key weakness. In order to organise their U-boats for an attack, the captains needed to communicate at regular intervals, initially to report the sighting of a convoy, and then to invite other boats to join. Even in the midst of battle, requests often came in for U-boats to announce the result of specific attacks, fuel and torpedo stocks and, thanks to those helium-filled condoms, to issue weather reports. These signals, when picked up by Allied shore stations, acted as clues to the U-boats’ whereabouts. By triangulating the signal from two
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