Physical clues revealed only part of the shallow-water method. An Italian detachable wooden fitting was intended to stop air-dropped torpedoes diving too deeply, while British weapons used a related arrangement that also involved tension wires. Investigators noticed strange wooden pieces, but the harbour already contained many cables and balloon wires, so the critical control lines were not obvious. John Opie's report apparently did not mention those wires, perhaps because nobody pointed them out or he did not recognise their function. British caution is one possible operational-security explanation because the United States was not yet at war and detailed communications could be intercepted by Germany. Technical intelligence here depended on recognising the significance of debris, not merely gaining access to a damaged harbour.
Notable Point · 33:38-36:05
Torpedo debris reveals only part of the shallow-water method
Observers could study wooden fittings associated with shallow-running torpedoes, but the British wire-control details were easy to miss and may have been withheld for security.