Aside the radar, maybe the most ingenious defensive measure used by German submarines was the Focke-Achgelis. The ‘Focke’ was basically a manned rotary glider with a triple blade rotor. It was as simple to operate as it was to assemble. Housed in a storage cylinder on the afterdeck, the Focke was quickly armed and launched. It remained connected to the U-boat by an umbilical cord. From its advantageous position high above the sub (10-12,000 feet), the pilot could spot any target approaching the boat. Unfortunately for the Focke, if the U-boat came under direct attack, there was no time to reel it in, thus the sub cut the cord and left the pilot to defend himself until all was cleared to surface back again.
More effective than the Focke-Achgelis was the Aphrodite. It was a basic devise consisting of a large (one meter diameter) hydrogen-filled balloon from which dangled small strips of metal foil. It was attached to the sub by way of an anchor weight. Its main purpose was to confuse allied aircraft utilizing radar navigational systems.
