The Hedgehog was a forward-throwing anti-submarine weapon that was used
during the Battle of the Atlantic in 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 fuzes, detonation always occurred directly against the hull of a
submarine making it more deadly than depth charges which relied on damage
caused by hydrostatic shockwaves. Statistics show that in 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. Development
The "Hedgehog", so named because the empty rows of its launcher spigots
resembled the spines of a hedgehog, was a replacement to the unsuccessful
Fairlie Mortar that was trialled aboard HMS Whitehall in 1941. Although a
failure, the Fairlie was designed to fire depth charges ahead of a ship when
attacking a submarine. This principle of forward-firing projectiles was
considered viable. It was from this, secret research undertaken by the
Directorate of Miscellaneous Weapons Development 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 varying size. The propelling charge was part of the main weapon and
worked against a rod set in the baseplate which fitted inside a tubular
tail of the 'bomb'. This principle was first used on the Blacker Bombard 29mm
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 Army. The weapon fires a salvo of 24 bombs in an arc, aimed to
land in a circular or elliptical area about 100 feet in diameter at a fixed
point about 250 yards 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 ships ASDIC when the ship came within 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 that of the initial sound pulse emitted by the sonar - the
so-called 'Instantaneous echo', where the output sound pulse and returning
echo merge. This 'blind spot' allowed the submarine to make evasive manoeuvres
undetected while the ship was still out of range for depth charge attack, the
submarine being effectively invisible to the sonar as the ship came within the
sonar's minimum range. The solution was a weapon mounted on the foredeck that
discharged the projectiles up and over that carrying ship's bow, to land in the
water some distance in front of the ship while the submarine was still outside
the sonar's minimum range. History
Hedgehog entered service in 1942. Carrying a Torpex charge weighing 16 kg,
each mortar had a diameter of 18 cm and weighed about 29.5 kg. The projectiles
were angled so they would land in a circular shape with a diameter of 40 m
about 180 m ahead of a stationary ship. The projectiles would then sink at about
7 m/s. They would reach a submerged U boat, for example at 200 ft in under 9
seconds. If one struck the submarine, it was hoped the subsequent explosion would
trigger all the other bombs to explode as well. Hedgehog remained in use as
anti submarine weapon until the end of the war.
Initial success rates - of about 5% - were only slightly better than depth
charges. The results were blamed on crew inexperience and low confidence in the
weapon. However, after an officer from the DMWD was sent to Londonderry Port,
where the convoy crews 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 for every five attacks made by Hedgehog
resulted in a kill. In response to this new deadly threat to its U Boats, the
Kriegsmarine brought forward its program on homing torpedoes in 1943. These could
be fired at greater distances from a convoy; after running straight for a
predetermined distance, an internal guidance system would take over guiding
it to a target. 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 Mynol charges. = Variants=
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.
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 with increased range of 600 meters
Operational usage 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, if they miss, the submarine can still be tracked
by sonar. The depth of the target does not need to
be known. Proximity weapons need to be set for the
target's correct depth to be effective. Contact fused charges do not have that
limitation. In addition, any explosion indicates a "hit".
The weapon gives no warning of the attack.
Until depth-finding sonar became available, 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 one or two 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. 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.
Former operators United Kingdom
Royal Navy United States
United States Navy General characteristics
For a single bomb Calibre: 7 in
Weight: 65 lb Shell Diameter : 7.2 in
Shell Length: 3 ft 10.5 in Explosive charge: 30 lb TNT or 35 lb
Torpex Range: about 250 yd
Sinking speed: 22 to 23.5 ft/s Fuse: Contact, High Explosive
Firing Order: Ripple in pairs, one every tenth of a second
Reload Time: ~3 minutes = Variants=
Mark 10: elliptical pattern measuring about 140 by 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. See also
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. USS Solar, accidentally destroyed by a
dropped Hedgehog charge. Notes
External links Anti-Submarine Projector Mks 10 & 11