The Hedgehog (also known as an Anti-Submarine Projector) was a forward-throwing anti-submarine weapon 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 barometric (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 WWII 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.
The "Hedgehog", so named because the empty rows of its launcher spigots resembled the spines of a hedgehog, was a replacement for the unsuccessful Fairlie Mortar that was trialled 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 secret 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 the baseplate which fitted inside a tubular tail of the 'bomb'. This principle was first used on the Blacker Bombard 29 mm Spigot Mortar and 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 point about 250 yards (230 m) directly ahead of the attacking ship. The mounting initially was fixed, but 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 when closer than the sonar's minimum range. Due to the speed of sound in water, the time taken for the 'ping' echo to return to the attacking ship from the target submarine became 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. This "blind spot" made the submarine 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.
The Hedgehog entered service in 1942. Carrying a 16 kg (35 lb) Torpex charge, each mortar projectile had a diameter of 18 cm (7.1 in) and weighed about 29.5 kg (65 lb). The spigots were angled so the projectiles would land in a circular pattern with a diameter of 40 m (130 ft), about 180 m (590 ft) ahead of the ship's position. The projectiles would then sink at about 7 m/s (23 ft/s). They would reach a submerged U-boat, for example at 200 ft (61 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 less than one in 80 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 two-week period with 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 subsequent and devastating explosions which wrecked the superstructure.
The launcher had four "cradles", each with six launcher spigots. The firing sequence was staggered so all the bombs would land at about the same time. This had the added advantage of minimising the stress on the weapon's mounting, so that deck reinforcement was not needed, and the weapon could easily be retrofitted to any convenient place on a ship. Reloading took about three minutes.
The Hedgehog had four key advantages over the depth charge:
- An unsuccessful attack does not hide the submarine from sonar.
- When a depth charge explodes, it can take 15 minutes before the disturbance can settle down enough that sonar becomes effective. Many submarines escaped during the time after an unsuccessful depth charge attack. Since Hedgehog charges only explode on contact, sonar tracking of the submarine is less likely to be disrupted by an unsuccessful Hedgehog attack.
- Although knowledge of target depth was less important, the Hedgehog was less successful against deep targets. Doctrine based on combat experience discouraged use on targets deeper than 400 feet (120 m).
- Proximity weapons (such as depth charges) need to be set for the target's correct depth to be effective. Contact-fuzed charges do not have that limitation, and an explosion at the time predicted for the contact-fuzed projectile to reach the target depth may indicate a "hit".
- There is no "blind period" allowing the submarine to escape undetected.
- Until depth-finding sonar became available (the first was the Royal Navy's "Q" attachment in 1943), there was a "dead period" during the final moments of a depth-charge attack when the attacker had no knowledge of what the target was doing. U-boat commanders became adept at sharp changes of direction and speed at these moments, avoiding the attack. Weapons such as Hedgehog, usable while the submarine was detectable by sonar, did not give the submarine a period of "invisibility" in which to dodge and evade the attack.
- A direct hit by one or two Hedgehog bombs was usually sufficient to sink a submarine.
- Many depth charges were required to inflict enough cumulative damage to sink a submarine; even then, many U-boats survived hundreds of detonations over a period of many hours—678 depth charges were dropped against U-427 in April 1945. The depth charge, usually exploding at a distance from the submarine, had a cushion of water between it and the target which rapidly dissipated the explosive shock. The Hedgehog's charge, on the other hand, exploded in direct contact with the hull. However, near misses with the Hedgehog were silent and did not cause any damage, unlike the cumulative damage caused by several depth charge near-misses; nor did it have the same psychological effect as a depth charge attack.
A large white upwelling of water from an underwater explosion just ahead of Moberly's bow following Hedgehog launch
Derivatives and successors
In late 1943 the Royal Navy introduced Squid. This was a three-tubed mortar that launched depth charges. Initially it was used as a single weapon, but when this failed to be successful, it was upgraded to the "double squid" that consisted of two launchers placed in parallel. In 1955 this system was upgraded to the three-barreled Limbo that launched 400 lb (180 kg) Minol charges.
The United States produced a rocket version of Hedgehog called Mousetrap, then Weapon Alpha as a replacement for both. Still, Hedgehog remained in service with the United States Navy into the Cold War until both Hedgehog and the less satisfactory Weapon Alpha were replaced by ASROC.
Three "Hedgerow" flotillas of specialized Landing Craft Assault boats carrying the Hedgehog instead of troops were used during the Normandy landings. An addition of impact fuse extensions in the projectile noses enabled detonating the warheads above ground. The bombs were used to clear 100-yard-wide paths through mines and barbed wire obstacles on the beach.
The Australian Army adapted the marine Hedgehog into a land-based seven-shot launcher that could be mounted on the back of Matilda tanks.
From 1949, a copy of Hedgehog was produced in the USSR as MBU-200, developed in 1956 into MBU-600 (also known as RBU-6000) which fired rockets with increased range of 600 metres (2,000 ft).
Weapons derived from the Hedgehog have been largely phased out from Western navies in favor of homing torpedoes. MBU-600 and its derivatives remain an important part of the Russian Navy's (as well as Russia's allies, such as India) anti-submarine arsenal to this day.
Live and practice projectiles – note the protective fuze caps (22) shown removed in the picture at the top of page.
- Weight: 65 lb (29 kg)
- Shell diameter : 7.2 in (183 mm)
- Shell length: 3 ft 10.5 in (1,181 mm)
- Explosive charge: 30 lb (14 kg) TNT or 35 lb (16 kg) Torpex
- Range: about 250 yd (230 m)
- Sinking speed: 22 to 23.5 ft/s (6.7 to 7.2 m/s)
- Fuze: Contact, high explosive
- Firing order: Ripple in pairs, one every tenth of a second
- Reload time: ~3 minutes
- Mark 10: elliptical pattern measuring about 140 by 120 feet (43 m × 37 m) to a range of 200 yards (180 m).
- Mark 11: circular pattern measuring 200 feet (61 m) in diameter out to a range of about 188 yards (172 m).
- Mark 15: pattern as for the Mark 11 but mounted on a platform adapted from that of a quadruple 40 mm Bofors gun mount. The Mark 15 could be fired remotely from the ship's plotting room.
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