“The Hedgehog had several advantages over the depth charge.”

By Mark Lardas

THE U.S. NAVY’S Mk. 10 and Mk. 11 ahead-throwing spigot mortar projector was the deadliest submarine killer of World War Two. The proper name was a mouthful, so it was generally known as the Hedgehog.

The Hedgehog was developed by the Royal Navy in 1941. It came to the United States as a form of reverse Lend-Lease, entering service in the U.S. Navy in late 1942.

The Hedgehog consisted of 24 projectiles mounted six deep and four across on a steerable launcher. Each of the anti-submarine projectiles was 46.5 inches long and had a diameter of 7.2 inches. They weighed 65 pounds (29.5 kilograms) with a 35-pound (15.9 kilograms) Torpex warhead. Each projectile was fired from its launch tube by a small propellant charge. The collection of projectiles on the launcher gave the assembly the appearance of a bristling hedgehog, hence the name.

A vessel armed with Hedgehog launchers, typically a destroyer or escort vessel, would fire the projectiles forward to either port or starboard. Projectiles were fired in pairs from between one-tenth or two-tenths of a second apart. All 24 were discharged in under two seconds. A Hedgehog launcher could be reloaded in three minutes. Six reloads were typically carried.

Each row of launch tubes had a slightly different angle. The tubes with the highest trajectories were fired first and those with the flattest trajectories last. This ensured all 24 bombs entered the water simultaneously. Each had a sink rate of 23.5 feet per second (fps). Projectiles landed in a pattern. Mk. 10 Hedgehog projectiles splashed down in an oval — 95-foot wide (59 meters) and 168-foot long (51 meters), with each one approximately 18 feet apart. Those fired from the Mk.11, landed in a circular pattern with an 800-foot diameter, with each bomb roughly 100 feet apart. Both the Mk. 10 and 11 had a range of 275 yards.

Hedgehog projectiles were equipped with a contact-fuse; they exploded when they hit either a U-boat or the ocean floor. A single Hedgehog bomb could sink a U-boat. Since a Type IX U-boat was 252.8 feet (76.76 meters) long and 22 feet (6.76 meters) across, an accurately placed Mk.10 salvo would inevitably score at least one hit, while the Mk. 11 projector had a better-than-even chance of striking the target at least once.

 

The Hedgehog (seen in the clip above being loaded and fired) had several advantages over the depth charge. Since it fired ahead of the ship, sonar contact could be maintained with the target throughout the attack. With depth charges, a ship passed over the target, creating a sonar ‘blind spot’ just as the depth charges were dropped. A Hedgehog attack was more accurate than a depth charge attack.

Hedgehog projectiles also had a faster sink rate than depth charges. A projectile would reach 200 feet in under 10 seconds and a U-boat’s test depth (750 feet) in just over half a minute. By contrast Mk.6 and Mk.7 depth charges, used through 1943, had sink rates of eight and nine fps respectively. The late-war Mk. 9 had a sink rate of 14.5 fps. The Hedgehog reached the target in half the time of the older depth charges and 50 per cent faster than the Mk.9. That gave U-boats less time to evade.

Depth charges exploded at pre-set depths, regardless of their distance to the target. The crew had to set the depth prior to release. If they guessed wrong, the charge exploded above or below the U-boat, leaving it unharmed. Depth charge explosions temporarily deafened sonar; an attack could not be resumed (or an assessment of target damage made) until the water settled, also allowing the U-boat an opportunity to escape. Since a Hedgehog projectile exploded only if it made contact, if an attack was unsuccessful sonar performance was unaffected.  

The Hedgehog had one other critical advantage over depth charges. It had no depth limit. The maximum depth a U.S. Navy depth charge could be set was 600 feet. The test depth of the Type VII and Type IX U-boat was 750 feet (230 meters). A U-boat below 600 feet was safe from U.S. Navy depth charges. At that point the only option a depth-charge armed destroyer had was sit on top of the U-boat. Once the submarine ran out of breathable atmosphere and its batteries were depleted it would be forced to surface, but that could take many hours, during which time, a clever U-boat captain could slip away.

Since the Hedgehog was developed in wartime, launchers had to be retrofitted onto vessels. Because the mounts were large, it generally required a bow turret to be removed to make room for the system. While this was done on a few of the Benson-class destroyers, for example, the Hedgehog was not installed on most destroyers in the class. Instead, 12 Atlantic-service Bensons were fitted with Mousetrap launchers. Similar in principle to the Hedgehog, Mousetraps were instead rocket-propelled weapons fired from smaller four- or eight-rail launchers. The Mousetrap had a smaller footprint and less recoil than Hedgehog launchers and could be mounted on the sides of the ship.

The Hedgehog launcher was standard equipment on destroyer escorts. These ships were optimized as anti-submarine warfare vessels. Smaller than World War Two-era destroyers, they were roughly the same size as American destroyers built between 1919 and 1934. They proved themselves to be excellent U-boat killers.

The Hedgehog was not without its drawbacks, however. The system required a well-trained crew to operate it and those were in short supply in early 1943. Additionally, Hedgehog teams needed very precise target information to be effective.

Not until the Combat Information Center (CIC) appeared did the Hedgehog finally deliver on its full potential. Under the CIC model, a ship’s captain remained on the bridge directing the battle, while a subordinate, usually the executive officer, would be in a separate room through which all information flowed. This improved combat performance. By 1944, more advanced technology was further integrated. Leading-edge sonars and radars fed targeting information directly to the weapons, contributing to the dramatic improvement in Hedgehog accuracy from mid-1944 on.

The anti-submarine projectiles led to massive improvement in ASW effectiveness. Eleven of the last 16 sinkings of U-boats by U.S. Navy destroyers and destroyer escorts in World War Two were achieved using such projectiles. A methodical Hedgehog attack, such as that mounted by the USS Eugene Elmore against U-549, was almost always successful.

U-549, a schnorchel-equipped, long-range Type IXC/40 boat commanded by Kapitäleutnant Detlev Krankenhagen, left Lorient on May 14, 1944 for a cruise in the South Atlantic. Task Group 21.11, made up of Bogue-class escort carrier USS Block Island accompanied by Escort Division 60 (destroyer escorts USS Ahrens, USS Barr, USS Eugene E. Elmore and USS Robert I. Paine) had been unsuccessfully hunting U-549 off the Cape Verde Islands since May 23. Instead at 20:15 on May 29, an hour before sunset, U-549 torpedoed the Block Island, which sank almost two hours later.

The four destroyer escorts in the task group responded. Barr made the first attack. Seeing U-549’s periscope, the vessel ran over the target dropping 11 depth charges. U-549 dove and evaded. Krankenhagen fired one homing torpedo at Barr, which hit near the stern, damaging the destroyer escort.

U-549 then went deep, probably to test depth, which was below the maximum depth setting of U.S. depth charges. The surviving ships of TG 21.11 lost contact with U-549.

Paine and Ahrens moved in to pick up Block Island’s survivors. Ahrens stopped its engines to recover sailors. At 21:02, with its engines off, Ahrens sonar operator picked up U-549 on its sonar.

With Barr crippled but still floating, and Ahrens and Paine rescuing survivors, the destroyer escort Eugene E. Elmore was the only ship available to chase down the enemy submarine. Commander Morgan Harris, captain of Ahrens, directed Elmore to the contact bearing 275 degrees from Ahern’s bow. Lieutenant Commander G. L. Conkey, commanding Elmore, steamed towards Ahren’s contact. Elmore soon found U-549 with its own sonar.

The rest of the battle unfolded quickly. U-549 was deep; Krankenhagen likely thought his vessel was safe. Detection was difficult at that depth, and U-549 was running as silently as it could. The U-boat had only to wait until the Americans lost contact or got tired of futile attacks.

There were two problems for the U-boat, however. Elmore was fitted with a Type 144 sonar. Extremely sensitive, its normal range was 2,500 yards. However, under ideal conditions, it could detect a U-boat at 3,000 yards. Having locked onto U-549, Elmore was unlikely to lose contact. Moreover, the sonar was integrated into the fire control of Elmer’s Hedgehog launcher and the projectiles could reach a target at depths greater than 600 feet.

At 21:13, moving slowly to keep from breaking sonar contact, Elmore made a Hedgehog run. Listening for the destroyer escort through its hydrophones, U-549 sought to evade the danger by turning to port and reversing course. Krankenhagen expertly timed his doubling back maneuver to coincide with the sonar disruption that would occur during a depth charge run.

Although the Hedgehog projectiles missed U-549, silently continuing down towards the ocean floor, Elmore maintained sonar contact and reloaded its launcher. The destroyer made a second run over U-549, again firing the launcher. Thirty seconds later, sonar picked up three underwater explosions. Of the 24 projectiles fired, a trio had hit U-549. Four minutes later Elmore picked up the sound of a heavy crunching “explosion,” most likely the sound of U-549 imploding as it reached crush depth.

If the German crew was lucky, one of the projectiles cracked the pressure hull open, killing all inside instantly. If however the bombs only ruptured the ballast tanks, the crew would have been forced to endure four hellish minutes of waiting as U-549 sank before the pressure hull finally yielded to the intensifying pressure.

Conkey was not completely sure U-549 was dead. To make certain, Elmore made two more runs at U-549, firing Hedgehog salvoes. It used the roiling waters marking the site of the underwater explosions as its aiming point on the first run and the crossed wakes of its previous two attempts for the second.

Nothing was heard either time. Elmore slowly circled for another 20 minutes, sweeping the area with its sonar, seeking any trace of a U-boat. Results were negative. It finally gave up the search at 21:50. By that point it was dark. Ahrens and Paine had recovered 951 of the 957 men aboard Block Island.

Elmore went to assist Barr, taking aboard 14 wounded and any crew deemed unnecessary to saving Barr. Damage control parties eventually patched the leaks and Elmore took Barr under tow. The four destroyer escorts and Block Island survivors reached Casablanca three days later. U-549 was never heard of again, its disappearance credited to Elmore.

Mark Lardas is the author of U.S. Destroyers vs German U-Boats: The Atlantic 1941–45. Educated as a naval architect, worked for over 30 years as an engineer, space navigator and technical writer in the space, aviation and energy industries. He has an abiding interest in aerospace, maritime, naval and military history, with an emphasis on ship construction and engineering from the 18th to the 21st centuries. Mark is the author of over three dozen published books on these subjects.