This World War II-era anti-submarine mortar system was so successful, the Russian Navy still uses it. 

Dive, Dive!

Though depth charges are now obsolete thanks to modern torpedo technology, during the Second World War, they were one of the only means of hitting submarines from the surface. Depth charges varied somewhat between navies, but were, in essence, an explosive-packed barrel that was dropped off a ship and into the water. Conventional depth charges are typically carried at the rear, or stern of a ship. 

Though they produced a terrific explosion, depth charges were limited by their direction of fire—they could really only be used for attacking submarines that were behind a ship. They typically were not able to detonate on contact with a submarine, but were rather equipped with a timer fuze that would set off an explosion at the depth sailors guessed a submarine to be. Their success rate was rather haphazard. 

U-boats could be virtually invisible if they kept relatively close to the fore, or in front of a ship. At close distances, sonar could not easily distinguish between the initial sonar pulse, and the ping of a return signal bouncing off a submarine hull back to the ship. In this blindspot, submarines thrived. So the Royal Navy invented the forward-firing Hedgehog. 

Bombs Away

The Hedgehog was essentially a spigot mortar system, in which multiple projectiles are fitted onto thin tubes. When the mortar assembly was empty, its rows of thin tubes resembled a hedgehog’s spikes, hence the Hedgehog designation. 

The Hedgehog projectiles were aimed to land about 600 feet in front of the ship’s bow, in a roughly circular arrangement, more or less 150 feet in diameter. The mortars themselves were quite large, and weighed 65 pounds, or about 30 kilos. Packed with Torpex, an explosive 50% more powerful than TNT, the Hedgehog was deadly.  

Importantly, they were fitted with contact fuses, which triggered on contact. Although a successful hit against a submarine would be evident from an explosion, a miss could be demoralizing to a ship’s crew. As dispiriting as a miss no doubt was, the lack of an explosion meant that the ship’s sonar could stay active, as underwater explosions would disturb the water disrupting onboard sonar, and giving U-Boat commanders a window of underwater invisibility. 

Success

Hedgehogs were considerably more successful than depth charges. The Allies scored more kills using the Hedgehog system than they had with depth charges, once Hedgehogs were properly calibrated for range and spread pattern. 

Variants

The successful British-developed Hedgehog was also installed on U.S. Navy ships. Several variants were built, by both the American and British navies, including several unique systems that launched larger depth charges forward over the bow. Post-war, a version was developed by the Soviet Union. A modernized variant is still in service on Russian ships to this day.

Caleb Larson holds a Master of Public Policy degree from the Willy Brandt School of Public Policy. He lives in Berlin and writes on U.S. and Russian foreign and defense policy, German politics, and culture.

Image: Reuters