The idea behind a hedgehog was that she was firstly forward firing which was a new and needed thing and also since the charges were impact only they made it so that the escort wouldn't lose contact unless there was a hit. The last thing was that usually one hedgehog hit was enough to make a boat go down. Thats a powerful point blank hit on the pressure hull.
Not sure myself as to its effectiveness as I have never been attacked by one.
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