Depth charges and other ASW weapons

Part two

And now it's time to continue our summary of anti-submarine weapons (ASW) of the Battle of the Atlantic. Today we move on to advanced depth charge throwers and their replacements.

The K Gun Depth charge projector

The K-gun was a significant improvement in anti-submarine warfare (ASW) during World War II, largely replacing the earlier Y-gun. Its primary function was to launch depth charges to the sides of a ship, complementing the charges dropped from stern racks.

The K-gun (officially various Marks, such as the Mark 6, 7, and 9) was a single-barrel depth charge projector.

Unlike the Y-gun's fixed, centerline mounting, the K-gun could be mounted individually on the periphery of a ship's deck. This freed up valuable centerline space for other equipment and allowed for a more flexible and wider pattern of depth charges.

Ships like destroyers typically carried four to six K-guns, while destroyer escorts often had eight.

Each K-gun consisted of a smooth-bore barrel attached to an expansion chamber, fitted with a breech mechanism.

A depth charge was secured to an "arbor" (a launching shuttle) that was inserted into the barrel.

A black powder propellant charge (an "impulse cartridge") was loaded into the breech.

When fired (either electrically from a remote control station or by percussion with a lanyard), the expanding gases from the propellant charge propelled the arbor and depth charge out of the barrel.

A mechanical release mechanism automatically detached the depth charge from the arbor during flight, allowing the depth charge to sink unimpeded.

The range of the depth charge could be varied by using different weights of the black powder charge, typically achieving ranges of 60, 90, or 150 yards (55, 82, or 137 meters).

Advantages over the Y-gun:

Flexible Mounting: Being a single-barrel unit, K-guns could be installed on the port and starboard sides of a ship's deck, allowing for a broader and more customizable depth charge pattern. This was a major advantage over the Y-gun's fixed, centerline position.

Wider Pattern: By using a combination of K-guns (firing to the sides) and stern racks (dropping charges directly behind the ship), a much more extensive and effective "carpet" of depth charges could be laid over a suspected submarine's position. This increased the chances of a hit.

Easier Reloading: While still manual, the single-barrel design often made reloading somewhat simpler than the two-barrel Y-gun.

Effectiveness and Context:

K-guns, in conjunction with stern racks, significantly improved the effectiveness of depth charge attacks during WWII.

Hydrostatic Detonation: Depth charges were not contact-fuzed. Instead, they used a hydrostatic pistol that could be set to detonate the charge at a pre-selected depth, based on the estimated depth of the submarine.

Shock Wave Damage: The primary method of destruction was the powerful shock wave generated by the underwater explosion. This shock wave could buckle a submarine's pressure hull, even at some distance. Near misses could cause significant internal damage to equipment, loosen joints, and severely impact crew morale, often forcing the submarine to surface.

Limitations:

Accuracy: Despite improvements, placing a depth charge close enough for a kill remained challenging. Post-war analysis suggested that a 300 lb depth charge needed to be within about 14 feet (4 meters) of a submarine to buckle its hull, and within 30 feet (9 meters) for disabling damage.

Sonar Limitations: Early sonar (Asdic) systems had limitations, particularly as a ship closed in on a submarine, often losing contact directly overhead. This meant that depth charges were typically deployed over the last known position.

Submarine Evasion: Experienced submarine commanders could sometimes use the time between the "bang" of the initial shock wave and the "boom" of the full explosion to estimate the depth charge's distance and take evasive action.

Success Rate: While vital, depth charges (including those from K-guns) had a relatively low success rate compared to later, more advanced ASW weapons. In the early part of WWII, the chance of sinking a U-boat with depth charges was very low (around 1-3%). By 1943-44, with improved sonar and tactics, this figure rose to about 5-6%. However, multiple charges were almost always required. For example, during one hunt of U-427 in April 1945, 678 depth charges were dropped!

Evolution of ASW: K-guns and depth charges eventually gave way to more sophisticated forward-throwing weapons like the Hedgehog and Squid (which fired multiple projectiles ahead of the ship, allowing continuous sonar contact), and later, homing torpedoes. However, K-guns played a crucial role in Allied anti-submarine efforts during World War II, especially during the Battle of the Atlantic.

The Hedgehog weapon system

The Hedgehog was a forward-throwing anti-submarine mortar. This means it fired its projectiles ahead of the attacking ship, rather than dropping them over the stern or to the sides like depth charges.

It was named "Hedgehog" because its launcher, with its many spigots (short, upright rods), resembled the spines of a hedgehog when loaded.

The system typically consisted of a steel cradle with 24 spigots arranged in four rows of six. Each spigot launched a small, contact-fuzed bomb, firing all projectiles in rapid succession (under two seconds).

Each bomb weighed about 65 pounds (29.5 kg), with a 35-pound (16 kg) Torpex warhead, and was equipped with a contact fuze—detonating only upon striking a submarine or the sea floor.

Sink Rate: Hedgehog bombs sank much faster than depth charges (about 23 ft/s or 7 m/s), reaching 200 feet in under 10 seconds—making it harder for submarines to evade.

Ahead-Throwing: Unlike depth charges, which required the attacking ship to pass over the submarine (losing sonar contact in the critical final moments), the Hedgehog fired its projectiles in an arc ahead of the ship. This allowed the attacking ship to maintain continuous sonar contact with the submarine right up until the moment of impact.

Salvo Firing: A salvo of 24 projectiles was fired simultaneously, with a staggered launch sequence designed to make them all land in a circular or elliptical pattern (about 100-130 feet in diameter) approximately 200-250 yards (180-230 meters) directly ahead of the ship.

Contact Fuzing: This was a key innovation. Hedgehog projectiles only exploded on contact with the submarine's hull. This had several crucial advantages:

No Sonar Disruption: Since a miss meant no explosion, there was no massive underwater disturbance to "blind" the sonar, allowing the attacking ship to immediately re-engage or adjust its attack if the first salvo missed. Traditional depth charges, even near misses, created massive explosions that temporarily deafened sonar.

Confirmation of Hit: An explosion meant a definite hit, providing immediate confirmation of success.

No Depth Setting: Unlike hydrostatic depth charges that needed to be set to a specific depth (which was often guesswork), the contact fuze eliminated this variable.

Key Features and Advantages over Depth Charges:

Higher Kill Probability: While individual Hedgehog projectiles carried a smaller explosive charge (around 35 lbs of Torpex) compared to a depth charge, the ability to maintain contact and the contact fuze significantly increased the chance of a kill. Statistics show a much higher kill-to-attack ratio for Hedgehog compared to depth charges (e.g., 5.7 to 1 for Hedgehog vs. 60.5 to 1 for depth charges in British service during WWII).

Faster Sinking Rate: Hedgehog projectiles had a higher sink rate than conventional depth charges, making evasion more difficult for the submarine.

Psychological Impact: While traditional depth charge attacks were terrifying due to the concussive blasts, the Hedgehog's silence on a miss, followed by a sudden explosion on a hit, was also psychologically impactful for submarine crews.

The Hedgehog was developed by the Royal Navy's Directorate of Miscellaneous Weapons Development (DMWD) under Sir Charles Goodeve, entering service in 1942.

It was widely adopted by Allied navies, including the U.S. Navy through reverse Lend-Lease, and became a crucial weapon in the Battle of the Atlantic.

The USS England (DE-635) famously used the Hedgehog to sink six Japanese submarines in a record 12-day period in May 1944, showcasing its deadly effectiveness.

The Mousetrap System

The Mousetrap was a forward-throwing anti-submarine rocket launcher.

It was conceptually similar to the British Hedgehog in that it fired projectiles ahead of the ship, allowing the attacking vessel to maintain sonar contact.

It typically consisted of a bank of four or eight short, parallel rails mounted on a swiveling base. These rails guided the rockets.

It was a relatively light and simple system, often seen on smaller escort vessels like patrol craft, subchasers, and some destroyer escorts, particularly in the early to mid-war period.

How it worked:

Rocket Propulsion: Instead of being propelled by an explosive charge like a K-gun or Y-gun, the Mousetrap fired self-propelled rockets. This eliminated the need for heavy breeches and recoil absorption systems, making the launcher lighter.

Small Projectiles: The rockets themselves were relatively small, essentially a depth charge warhead (often around 30-60 lbs or 14-27 kg of Torpex) attached to a rocket motor.

Contact Fuzing: Like the Hedgehog, Mousetrap rockets were contact-fuzed. They only detonated upon impact with the submarine, offering the same advantages:

Salvo Firing: The Mousetrap launched its rockets in a salvo, creating a pattern of explosions ahead of the ship.

Range: The effective range was relatively short, typically around 200-300 yards (180-275 meters).

Key Features and Role:

American Equivalent to Hedgehog (in principle): While not as sophisticated or as powerful as the British Hedgehog, Mousetrap served a similar purpose for the U.S. Navy in providing an "ahead-throwing" capability, which was a significant improvement over reliance solely on stern-dropped or K-gun launched depth charges.

Simplicity and Ease of Installation: Its relatively simple design made it easier and quicker to produce and install on a wider variety of vessels, especially those that might not have had the space or weight capacity for more complex systems.

Supplemental Weapon: Mousetrap was often used in conjunction with stern racks and K-guns, providing another layer to a ship's ASW arsenal. It was particularly useful for smaller vessels that lacked the heavy armaments of destroyers.

Effectiveness: While less celebrated than Hedgehog or Squid, Mousetrap contributed to the Allied ASW efforts. Its contact fuzing and forward-firing nature increased the probability of a kill compared to depth charges, by eliminating the "blind spot" and the need for depth guesswork.

Decline:

As the war progressed, and more advanced and powerful ASW weapons like the British Hedgehog (which the U.S. also adopted) and later the Squid became more widely available and integrated into fire control systems, the Mousetrap's role diminished. More powerful rockets and torpedoes eventually became the preferred ASW solutions.

In essence, the Mousetrap was a practical and important stepping stone in the evolution of U.S. Navy ASW, providing a much-needed "ahead-throwing" capability that enhanced the effectiveness of anti-submarine patrols during a critical period of the war.

FeatureMousetrap Mark 20Mousetrap Mark 22Number of Rails48Round Weight29 kgC29 kgRange280 Meters280 MetersPattern Width22 Meters22 MetersPropulsionRocketRocket

The SQUID weapon system

The Squid was another significant British anti-submarine warfare (ASW) weapon that emerged during World War II, building upon the "ahead-throwing" concept introduced by the Hedgehog. While it shared the general principle of firing projectiles forward of the ship, it differed from the Hedgehog in several crucial ways, making it even more effective.

What it was:

The Squid was a three-barreled mortar designed to launch large depth charges ahead of the attacking ship.

It was larger and more complex than the Hedgehog, and importantly, it was integrated with more advanced sonar systems.

How it worked:

Three-Barrel Salvo: The Squid fired a salvo of three heavy projectiles simultaneously.

Triangular Pattern: The barrels were precisely angled to ensure the three projectiles landed in a pre-determined triangular pattern (approximately 40 yards on a side) around the estimated position of the submarine, about 275 yards (250 meters) ahead of the ship.

Hydrostatic Detonation: Unlike the Hedgehog's contact fuzes, Squid projectiles used hydrostatic time fuzes. This meant they would detonate at a pre-set depth. However, a key difference was that Squid projectiles could be set to explode 25 feet (7.6 meters) above and below the target, creating a devastating three-dimensional pressure wave intended to crush the submarine's hull.

Automated Fire Control: A major advancement with Squid was its automatic linkage to the ship's sonar (ASDIC) system. This allowed for continuous, real-time depth and range updates to be fed directly to the Squid's firing mechanism. The weapon would then automatically fire at the optimal moment, when the submarine was within the calculated pattern. This eliminated much of the human error and guesswork involved in earlier ASW attacks.

High Sinking Speed: Squid projectiles had a very high sinking speed (around 43.5 feet per second or 13.3 meters per second), making it difficult for submarines to evade them once launched.

Key Features and Advantages over Hedgehog:

Larger Explosive Charge: Each Squid projectile carried a significantly larger explosive charge (207 lbs or 94 kg of Minol) compared to the Hedgehog's 35 lbs. This meant that a single hit or even a very close miss was more likely to cause catastrophic damage.

More Sophisticated Fire Control: The direct link to sonar and automatic firing made Squid far more precise and less reliant on operator reaction time. This greatly increased the probability of hitting the target within the deadly pattern.

Targeted Depth Detonation: While Hedgehog relied on contact, Squid could detonate its charges at specific depths around the submarine, aiming to create a crushing pressure field rather than just a direct impact.

Stabilized Mounting: Squid mountings were often stabilized to compensate for the ship's roll and pitch, further enhancing accuracy.

Development and Impact:

Designed by the same British Directorate of Miscellaneous Weapons Development (DMWD) responsible for the Hedgehog, Squid entered service in May 1943.

It was initially installed on HMS Ambuscade and later on many frigates and corvettes.

Squid proved to be exceptionally effective. Post-war trials showed it was nine times more effective than conventional depth charges. While Hedgehog achieved a kill rate of around 1 in 5.7 attacks, Squid reportedly had a success ratio of 1 in 2.9 attacks (17 kills in 50 attacks during WWII), making it arguably the most successful ASW weapon of the war.

The first successful kill by a Squid was HMS Loch Killin, which sank U-333 on July 31, 1944.

Successor: Limbo: Squid was eventually succeeded by the Limbo (Anti-Submarine Mortar Mark 10) in the 1950s. Limbo was an even more advanced three-barreled mortar that further extended range (up to 1,000 yards) and could use proximity fuzes, allowing for even greater flexibility and accuracy.

In summary, the Squid represented a significant leap forward in ASW by combining the "ahead-throwing" concept with larger charges, precise fire control linked to sonar, and a deadly three-dimensional attack pattern. It greatly increased the effectiveness of Allied escort vessels in combating the U-boat threat in the later stages of World War II.

With that we almost complete our coverage of the active ASW weapons. Next week we move onto a guided torpedo and anti-submarine minds.

Hope to see you there...

Knowledge detail

Minol

Minol is a military explosive that was developed during World War II as an alternative to standard explosives. Here are the key details:

Composition:

Minol consists of 40% TNT, 40% ammonium nitrate, and 20% aluminium powder. This combination was designed to stretch supplies of more expensive explosives like TNT and RDX.

Development and Purpose:

It was developed by the Admiralty early in the Second World War to augment supplies of trinitrotoluene (TNT) and RDX. The British military needed alternatives due to wartime shortages of conventional explosives.

Effectiveness:

The aluminum powder component acts as a fuel, making the explosive mixture more powerful than TNT alone, despite containing a significant portion of the less expensive ammonium nitrate. This made it both economical and effective for military use during the war.

The explosive was particularly valuable because it allowed military forces to maintain their explosive capabilities while conserving more scarce and expensive materials for other critical applications.