- For the area denial weapon, see Czech hedgehog.
The Hedgehog was an anti-submarine weapon developed by the Royal Navy during World War II, that was deployed on convoy escort warships such as destroyers to supplement the depth charge. The weapon worked by firing a number of small spigot mortar bombs from spiked fittings. Rather than working on a time or depth fuze like depth charges, the bombs exploded on contact and achieved a higher sinking rate against submarines than depth charges did.
Hedgehog received its name because when unloaded, the rows of empty spigots resembled the spines of a hedgehog.
Hedgehog was replaced in new construction for the Royal Navy by the Squid mortar in 1943. Squid was in turn replaced by the three-barreled Limbo. The United States produced a rocket version of Hedgehog called Mousetrap. The United States developed Weapon Alpha as a replacement for Hedgehog and Mousetrap. 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.
The Hedgehog was adapted into a 7 shot launcher form for use on the back of the Matilda tank serving with Australian forces.
From 1949, a copy of Hedgehog was produced in the USSR as MBU-200, developed in 1956 into MBU-600 with enhanced range of 600 m.
Technically the weapon was a multiple 'spigot mortar' or spigot discharger, a type of weapon developed in the interwar period by Lt-Col Blacker, RA. The spigot mortar was based on early infantry trench mortars. By using a spigot, the warhead and barrel size were no longer dependent in the design. 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 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 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 launcher had 4 "cradles", each with 6 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 3 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, if they miss, the submarine can still be tracked by sonar.
- The depth of the target does not need to be known.
- Proximity weapons (such as depth charges) need to be set for the target's correct depth to be effective. Contact fused charges don't have that limitation. In addition, any explosion indicates a 'hit'.
- The weapon gives no warning of the attack.
- 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 the 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, thus making the attack less accurate. Ahead-thrown weapons such as Hedgehog did not give the target the necessary warning of when to dodge.
- A direct hit by 1 or 2 Hedgehog bombs was usually sufficient to sink a submarine.
- Many depth charges were required in order 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 contact charge, on the other hand, had the cushion on the other side, actually increasing the explosive shock. However near-misses with the Hedgehog did not cause cumulative damage as depth charges did; nor did it have the same psychological effect as a depth charge attack.
The Hedgehog became much more successful than depth-charge attacks (the best kill rate was about 25% of attacks whereas depth charges never achieved more than 7%). It initially had a very poor record, although many of the factors had nothing to do with the design of the weapon. USS England sank six Japanese submarines in a matter of days with Hedgehog in May 1944.
For a single bomb
- Calibre: 7 in (178 mm)
- Weight: 65 lb (29 kg)
- Shell Diameter : 7.2 inches
- Shell Length: 3 feet 10.5 inches
- Explosive charge: 30 lb TNT or 35 lb (16 kg) Torpex
- Range: about 250 yards (200 to 259 m)
- Sinking speed: 22 to 23.5 ft/s (6.7 to 7.2 m/s)
- Fuse: 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 x 120 feet to a range of 200 yards
- Mark 11: circular pattern measuring 200 feet in diameter out to a range of about 188 yards.
- 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.
- 12px Media related to Hedgehog ASW mortar at Wikimedia Commons
- Hedgerow, a land version of Hedgehog for exploding landmines as used on landing craft during the D-Day invasion.
- The Blacker Bombard and PIAT infantry anti-tank weapons and the Churchill AVRE's Petard demolition gun also use the spigot discharger principle.
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- Fitzsimons, Bernard, ed. Illustrated Encyclopedia of 20th Century Weapons & Warfare (London: Phoebus, 1977), Volume 12, p.1283, "Hedgehog".
- Albrecht, Gerhard. Weyer's Warship of the World 1969. (Annapolis, MD: United States Naval Institute Press, 1969), pp.325-328 & 340
- Lanier, William D. and Williamson, John A., CAPT USN "The Twelve Days of the England" United States Naval Institute Proceedings March 1980 pp. 76-83