Taranto's defenders had shore-based listening posts that could detect aircraft approaching the harbour. An Italian fleet at sea lacked radar and could not carry that fixed warning network with it. Manoeuvring ships would be harder torpedo targets, but their own noise might also mask low aircraft engines until the attack was close. The comparison shows that leaving harbour exchanged a static target's vulnerability for a weaker night-warning picture.
Notable Point · 2:28:09-2:28:54
The warning tradeoff between harbour and open sea
Ships at sea could manoeuvre, but they would lose the shore listening posts that warned Taranto of approaching aircraft.