Had this happen recently in one of the shallower areas of the north sea in a Type II late 1944. IIRC the depth was maybe 180 meters.
Surprisingly every one of the hedge hogs missed me and sank deep exploding beneath me... the warhead is smaller so the shock wave is nothing like a depth charge.
I did go as deep as i could in an attempt to evade the destroyers, at one point i was only a few meters above the sea bed. yet i was doomed, they were dropping hedge hogs and depth charges with some serious accuracy.
In fact, during one depth charge / hedge hog run I swear my II sub was rolled completely over.
they eventually destroyed me, but yes, if these hedgehogs hit the bottom they blow up. its just not a very forceful explosion unless you are really close
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