USS The Sullivans Hedgehog Anti Submarine Mortar Projector
Number of Projector Charges: 24 Mk 6 Rounds
Shell Diameter : 7.2 inches
Shell Length: 3 feet 10.5 inches
Shell Weight: 75lbs
Warhead: 30lbs TNT or 35lbs Torpex
Shell Type: Contact, High Explosive
Horizontal Range : 200yds
Sink Rate: 24 fps
Pattern: Oval 140' X 120'
Firing Order: Ripple in pairs, one each 0.10 seconds, 2.4 seconds to fire all 24 charges
Reload Time: 3 minutes
Entered US Service: 1942
The hedgehogs are an anti-submarine mortar launcher. When the ship contacted a submerged submarine using sonar, the boat would be steered towards the sub's position. When the submarine was directly ahead the hedgehogs would be fired, landing in an oval grid pattern and sinking straight down at 24 feet per second, arming after reaching a depth of 15 feet. If one contacted a submarine, all would go off together. If not, then there would be no explosion and the sonar contact with the sub would not be broken as it would be when using depth charges.