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The Focke-Achgelis Fa 330 Bachstelze (English: Wagtail) was a type of rotary-wing kite, known as a rotor kite. They were towed behind German U-boats during World War II to allow a lookout to see further. The Fa 330 could be deployed to the deck of the submarine and was tethered to the U-boat by a 150 m (500 ft) cable. The airflow on the rotors as the boat motored along on the surface would spin them up. The kite would then be deployed behind the U-boat with its observer-pilot aboard, raising him approximately 120 meters above the surface and allowing him to see much farther — about 25 nautical miles (46 km), compared to the 5 nautical miles (9 km) visible from the conning tower of the U-boat As Allied air cover in other theatres of the war was considered too much of a threat, only U-boats operating in the far southern parts of the Atlantic and the Indian Ocean used the Fa 330. excerpts from an ARTE documentary
