Most everybody who reads about WWII knows that Depth Charges were used to destroy enemy submarines. They also know that Depth Charges explode at a preset depth. They probably do not realize the explosion creates a massive temporary change in density of the water and a "scrambling" of the sonar sound waves as the operator is trying to maintain or reacquire sound contact on the submarine. The "scrambling" is worsened by as many as 15 depth charges exploding in rapid order and at varied locations and depths. The resulting static usually caused loss of the sonar contact.
To eliminate this problem the British developed the 24 projectile forward-throwing spigot mortar. The U. S. Navy quickly adapted it for our anti-submarine warfare (ASW) vessels and every ASW ship built from early 1943 to the end of WWII was so equipped. 24 projectile forward-throwing spigot mortar is such a tongue twister the U. S. Navy simply called it a "Hedgehog Launcher". The projectiles became "Hedgehogs".
The Launcher fired 24 individual Hedgehogs each of which contained a 20 pound TNT or 35 pound Torpex charge. They were fired sequentially (actually with split-second delays) to spread evenly over an oval pattern about 140 feet by 120 feet with the center of impact about 180 to 230 yards ahead of the ship. Hedgehogs were contact fused; i.e., they had to hit something in order to explode. If there is no contact the hedgehog simply sinks to the bottom with either no explosion or an explosion so deep that the sonar sound wave static was minimal. The need for a contact fuse made them very tricky to handle and store since accidental impact could detonate the explosive charge. An ingenious arming system solved the safe handling problem. The contact fuse is armed after it's in the water by a small propeller in the nose of the Hedgehog which rotates as it sinks through the water. The propeller is covered with a metal cap and the propeller is locked against rotation by means of a cotter pin, until removal of both just seconds prior to launching. The cotter pin, which prevents rotation of the propeller, is much too small to remove with a bare hand so each cotter pin was placed on a key ring big enough to pull with a gloved hand.
The "Key Ring" shown here was from 1 of 96 Hedgehogs that sank the Japanese Submarine RO 42 in 2600 fathoms of water 60 miles northeast of Roi in the Marshall Island Group 11 June, 1943. I picked it up off the deck at the Launcher, discarded the cotter pin, put one of my dog tags on it and have used it as a key ring ever since. It is a little heavier and not as nicely finished as a regular key ring -- but, that's okay
HeD 28 June 2002