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The huge numbers of depth charges required to kill a submarine made them very inefficient. Any miscalculation meant that the charges would detonate in the wrong place and an enemy sub could survive, slip away and live to fight another day. An evolution in depth charge technology created a ruthless sub killer. Called hedgehog this weapon system was a prickly problem for any submarine that encountered it. During the Battle of the Atlantic in World War II hedgehog increased submarine kill rate from 7% to 25%.
