The Hedgehog (also known as an Anti-Submarine Projector) was a forward-throwing anti-submarine mortar that was used primarily during the Second World War. The device, which was developed by the Royal Navy, fired up to 24 spigot mortars ahead of a ship when attacking a U-boat. It was deployed on convoy escort warships such as destroyers and corvettes to supplement the depth charges. As the mortar projectiles employed contact fuzes rather than time or bathymetric (depth) fuzes, detonation occurred directly against a hard surface such as the hull of a submarine, making it more deadly than depth charges, which relied on damage caused by hydrostatic shockwaves. During World War II out of 5,174 British depth charge attacks there were 85.5 kills, a ratio of 60.5 to 1. In comparison, the Hedgehog made 268 attacks for 47 kills, a ratio of 5.7 to 1.
Development The Hedgehog, so named because the empty rows of its launcher spigots resembled the spines on the back of a hedgehog, was a replacement for the unsuccessful Fairlie Mortar, which was secretly tested aboard HMS Whitehall in 1941. The Fairlie was designed to fire depth charges ahead of a ship when attacking a submarine. The principle of firing projectiles forwards, instead of dropping depth charges over the stern, was considered viable, despite the failure of the Fairlie. This research by the Directorate of Miscellaneous Weapons Development (DMWD) led to the development of the Hedgehog. The weapon was a multiple spigot mortar, or spigot discharger, a type of weapon developed between the wars by Lieutenant Colonel Stewart Blacker, RA. The spigot mortar was based on early infantry trench mortars. The spigot design allowed a single device to fire warheads of different sizes. The propelling charge was part of the main weapon and worked against a rod (the spigot) set in a baseplate which fitted inside the tubular tail of the bomb. This principle was first used in the Blacker Bombard 1.1-inch (29 mm) Spigot Mortar, and it was also used in the later PIAT anti-tank weapon. The adaptation of the bombard for naval use was made in partnership with MIR(c) under Major Millis Jefferis, who had taken Blacker's design and brought it into use with the Army. The weapon fires a salvo of 24 bombs in an arc, aimed to land in a circular or elliptical area about 100 feet (30 m) in diameter at a fixed relative location about 250 yards (230 m) directly ahead of the attacking ship. The mounting initially was fixed, but it was later replaced by a gyro-stabilised one to allow for the rolling and pitching of the attacking ship. The system was developed to solve the problem of the target submarine disappearing from the attacking ship's ASDIC sonar when closer than the sonar's minimum range. The speed of sound in water is such that the time taken for the ping echo to return to the attacking ship from a close-by target submarine is too short to allow the human operator to distinguish the returning audible echo from the initial sound pulse emitted by the sonar – the so-called instantaneous echo, where the output sound pulse and returning echo merge, with the submarine still out of depth-charge range. A submarine in this blind spot became effectively invisible to the sonar, allowing it to make evasive manoeuvres undetected. The solution was a weapon mounted on the foredeck that discharged the projectiles up and over the ship's bow while the submarine was still detectable by the sonar, entering the water some distance in front of the ship.
History The Hedgehog entered service in 1942. Carrying a 35 lb (16 kg) Torpex charge, each mortar projectile had a diameter of 7.1 in (18 cm) and weighed about 65 lb (29.5 kg). The spigots were angled so the projectiles would land in a circular pattern with a diameter of 130 ft (40 m), about 200 yd (180 m) ahead of the ship's position. The projectiles would then sink at about 23 ft/s (7 m/s). They would reach a submerged U-boat, for example at 200 ft (60 m) in under 9 seconds. Sympathetic detonation of projectiles near those contacting hard surfaces was a possibility, but the number of explosions counted was usually fewer than the number of projectiles launched. The prototype launcher was tested aboard HMS Westcott in 1941, but there were no submarine kills until November 1942, after it had been installed aboard one hundred ships. Initial success rates, of about 5%, were only slightly better than depth charges. Swells and spray frequently covered the launcher during heavy North Atlantic weather, and subsequent attempts to launch from the soaked launcher were often hindered by firing circuit problems, launching an incomplete pattern. A depth charge total miss would still produce an explosion, leading crews to think that they might have damaged their target or at least demoralised its personnel; a Hedgehog miss was discouragingly quiet. The Royal Navy launched Hedgehog so seldom in early 1943 that a directive was issued ordering captains of ships equipped with Hedgehog to report why they had not used Hedgehog on an underwater contact. The results were blamed on crew inexperience and low confidence in the weapon. However, after an officer from the DMWD was sent to the base at Londonderry, where the escort vessels were based, with better training and shipwide talks on examples of successful Hedgehog attacks, the kill rate improved considerably. By the end of the war, statistics showed that on average, one in every five attacks made by Hedgehog resulted in a kill (compared with fewer than one in eighty with depth charges). In response to this new deadly threat to its U-boats, the Kriegsmarine brought forward its programme of acoustic torpedoes in 1943, beginning with the Falke. These new "homing" acoustic torpedoes could be employed effectively without the use of a periscope, providing submarines a better chance to remain undetected and evade counterattack. In the Pacific Theatre, USS England sank six Japanese submarines in a two-week period with the Hedgehog in May 1944. In 1946, the destroyer escort USS Solar was destroyed while unloading ammunition when a crewman accidentally dropped a Hedgehog charge near one of her main turret ammunition rooms, triggering three devastating explosions that wrecked the superstructure.
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