What obscure proposed air-defence system was particularly impractical?
Short answerAn attempt to put large acoustic aircraft detectors aboard a ship ran into interference from the ship's own engines.
NotesLarge acoustic collectors offered a way to listen for approaching aircraft. An attempt to adapt this equipment to a ship encountered interference from its engines. That noise undermined the listening system's purpose. The machinery aboard a working ship made acoustic warning more difficult than at a stationary land position.