Scanned from an album from a sailor who served on RCN escort ships on the North Atlantic during WW2
The Hedgehog on a Canadian River-class frigate
The Hedgehog was a mortar that fired up to twenty four anti submarine projectiles around 200 meters ahead of the ship as it tracked the submarine with ASDIC
Hedgehogs were more effective than depth charges with a kill ratio of 1 in every 5.6 attacks as opposed to 1 in every 60.5 for depth charges.