Hedgehog (weapon)

• 1. Introduction

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  The Hedgehog's kill record tells the story of the Battle of the Atlantic in a single comparison. During the Second World War, British depth charge attacks had a ratio of 60.5 attacks for every submarine sunk. The Hedgehog, a forward-throwing naval mortar developed by the Royal Navy, achieved the same result in 5.7 attacks. One weapon needed eighty chances; the other needed fewer than six. Those numbers were stark enough to raise an obvious question. If the Hedgehog was so much deadlier, why did escort crews use it so rarely in its first year? And what problem did it solve that no depth charge could? The answer begins with a fundamental weakness in how sonar worked at close range.

• 2. The Sonar Blind Spot

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  ASDIC sonar, the underwater detection system fitted to Royal Navy escort ships, had a hard limit at close range. When a submarine came within the minimum detection distance, the outgoing sound pulse and the returning echo merged into one indistinguishable signal. An operator could not separate them. The submarine, still beyond depth-charge range, effectively vanished from the screen.

  U-boat commanders learned to exploit this gap, known as the blind spot. When they knew the attacking ship had lost track, they made sharp changes in course and speed, breaking contact. No stern-dropped depth charge could address this. By the time the ship passed over the last known position, the submarine was somewhere else.

  The Directorate of Miscellaneous Weapons Development found the solution in direction. A weapon mounted on the foredeck could arc its projectiles over the bow and into the water ahead of the ship. The bombs would enter the water before the submarine disappeared from sonar, landing where the target still was. That engineering requirement pointed the DMWD toward a spigot mortar design that had begun its life as an infantry weapon between the wars.

• 3. Spigot and Spines

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  Lieutenant Colonel Stewart Blacker of the Royal Artillery had spent the interwar years developing the spigot mortar, drawing on early trench mortar principles. Instead of dropping a bomb down a tube, the propelling charge sat inside the launcher. Each projectile fitted over a central rod, the spigot, set in a baseplate inside the bomb's tubular tail. A single launcher could fire warheads of different sizes, since the tube's width was not the constraint. Blacker first applied this to the 29 mm Spigot Mortar, also known as the Blacker Bombard. The same mechanism later appeared in the PIAT anti-tank weapon.

  The Hedgehog's immediate predecessor was the Fairlie Mortar, secretly tested aboard ship in 1941. The Fairlie was designed to throw charges forward ahead of an attacking vessel. It failed. The Directorate of Miscellaneous Weapons Development judged the forward-firing concept sound despite that failure and transferred the work to Major Millis Jefferis of MIR(c). Jefferis had already adapted Blacker's design for Army use and now brought it to the anti-submarine problem.

  Jefferis's launcher arranged 24 spigots in rows across a mounting. When loaded with warheads, the rows of projectiles stood upright from the frame. Strip the warheads away and only the bare metal rods remained. Their resemblance to the raised spines on a hedgehog's back was distinctive enough to give the entire weapon its name.

  Jefferis's design fired a salvo of 24 bombs in an arc. They were aimed to land in a circular area about 100 feet across at a fixed point roughly 250 yards ahead. The question was whether wartime crews would trust it enough to use it.

• 4. Slow Acceptance

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  Each mortar bomb weighed about 29.5 kg, measured 18 cm across, and carried a 16 kg Torpex charge. Angled spigots spread the salvo into a 40-metre circle landing roughly 180 metres ahead of the ship. Sinking at about 7 metres per second, a bomb could reach a submarine at 200 feet in under 9 seconds.

  The prototype launcher was tested aboard ship in 1941, but no submarine kills were recorded until November 1942. By that point the weapon had already been installed on one hundred ships. Initial kill rates sat around 5 percent, only marginally better than depth charges.

  Heavy North Atlantic swells and spray frequently soaked the launcher before a crew could fire. Wet firing circuits then failed to complete the salvo, sending fewer than 24 bombs rather than the full pattern. Each incomplete salvo reduced the spread and the chance of a contact hit.

  Depth charges, even when they missed, still produced a loud explosion. Crews could tell themselves the blast had rattled the U-boat's crew or at least shaken the vessel. A missed Hedgehog attack produced nothing of the kind: no explosion, no sound, no visible evidence of effect. The Royal Navy later traced part of the weapon's poor uptake to this discouraging silence.

  By early 1943 the weapon was fired so seldom that the Royal Navy issued a formal directive. Captains equipped with the Hedgehog were required to report in writing why they had not used it on any underwater contact. Investigators placed the blame on crew inexperience and low confidence in the weapon. An officer from the Directorate of Miscellaneous Weapons Development was then sent to Londonderry, the base where escort vessels were stationed. Better training followed, along with briefings drawn from accounts of successful attacks. Kill rates climbed considerably.

  In 1943 the Kriegsmarine accelerated its acoustic torpedo programme in response to the weapon's growing effectiveness. The Falke, a homing torpedo that submarines could fire without using a periscope, was the first result.

• 5. Four Advantages

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  Fifteen minutes was how long a depth charge's explosion could prevent sonar from tracking a target. In that window after every unsuccessful depth charge attack, submarines escaped. Hedgehog's contact fuzes created no comparable disruption; they detonated only on impact with a hard surface, leaving the water undisturbed.

  400 feet was the rough depth beyond which doctrine discouraged using the Hedgehog against deep-running targets. For shallower contacts, its contact fuzes bypassed the need to estimate a submarine's depth before attacking. A depth charge set to the wrong level would explode harmlessly above or below; the Hedgehog detonated on first contact with the hull.

  Before the Royal Navy's Q attachment appeared in 1943, there was no way to measure a target's depth in real time. In the final approach before a depth-charge attack, contact with the target was routinely lost. Experienced U-boat commanders timed sharp course changes for precisely those moments. The Hedgehog remained usable for as long as the submarine was visible on sonar, giving the target no time for evasive manoeuvres.

  One submarine withstood 678 depth charges dropped against it in April 1945 before escaping. That endurance was possible because each depth charge detonated at a set depth, separated from the hull by a cushion of water. That water rapidly absorbed the explosive force. A Hedgehog charge exploded in direct contact with the hull. There was no cushion of water between the bomb and the steel.

  A direct hit by a single Hedgehog bomb was usually sufficient to sink the target. That lethality carried the weapon's design far beyond convoy escort work.

• 6. Beyond the Atlantic

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  Three specialized flotillas of Landing Craft Assault boats, designated 'Hedgerow' units, carried the weapon to the Normandy landings in place of troops. Nose extensions were fitted to each projectile so the warhead would detonate above ground rather than on the seabed. Those modified bombs cleared 100-yard-wide paths through mines and barbed wire on the beach.

  In May 1944, Hedgehog attacks in the Pacific Theatre sank six Japanese submarines within a two-week period.

  The Australian Army adapted the marine launcher into a seven-shot land-based version and mounted it on the back of Matilda tanks.

  In 1949 the Soviet Union produced its own version of the Hedgehog, designating it the MBU-200. A developed variant, the MBU-600, followed in 1956 with an extended range of 644 metres.

  The United States developed a rocket variant called Mousetrap, then a more capable successor called Weapon Alpha. Both eventually gave way to ASROC. In Russia and allied states such as India, weapons in the Hedgehog lineage were replaced by anti-submarine rocket launchers.

  In late 1943 the Royal Navy introduced the Squid, a three-tubed mortar that fired depth charges. The Squid was used first as a single launcher, then paired as a 'double squid' when that proved insufficient. In 1955 a three-barrelled successor called Limbo entered service, firing 400 lb Minol charges.

  The Mark 15 variant of the Hedgehog could be aimed and fired remotely from the ship's plotting room. No crew member was needed at the launcher itself. The RBU-6000, which replaced Hedgehog-derived weapons in Russia and allied states including India, carried that lineage into a new era.