Why torpedoes mattered to a harbour strike

Setting a ship ablaze was different from breaching its hull badly enough to admit the water needed to sink it. Fire might leave a vessel a ruined hulk, while a sufficiently large underwater opening made loss much more likely. For a smaller attacking force, mines and torpedoes were the practical weapons for producing that flooding damage. Naval aircraft therefore had to progress beyond harassment bombing and deliver an underwater weapon if they were to destroy major warships in harbour.