If you want to see what a hedge hog looks like as it come for you, look below!
This was the second attack with Hedgehogs (you can see the marks of the first), two of the ones shown hit, it didn't kill me, but the depthcharges that followed did!
Hedgehogs in most cases didn't kill subs, they damaged them to the poit where they would have to surface, or made them an easier target. Escort captains liked it as they could keep contact with a possible contact throught the engagement, so the enemy couldn't dodge as well. They also didn't go off unless they hit the sub, so there was no disturbance in the water that could mask a sub getting away