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Naval Gazing Main/Anti-Submarine Warfare - WWII Weapons
April 15, 2018

Anti-Submarine Warfare - WWII Weapons

Last time, we discussed detecting submarines, but that was only the start of a process. Once a submarine had been detected, it still needed to be destroyed. At the start of the war, the only weapon was the venerable depth charge, essentially an explosive thrown over the side and fuzed to go off at a specific depth. However, as the war went on, the traditional depth charge, unchanged over the previous 20 years, became increasingly inadequate, and new and improved weapons were developed.

K-Gun with traditional "ashcan" depth charge

One of the biggest problems, even during WWI, with the traditional depth charge rack was that it only produced a narrow pattern. Submarines could estimate when a ship would lose contact, and then maneuver violently, hoping to dodge the pattern. During WWI, the USN developed the Y-gun, a device mounted on the centerline which threw two depth charges between 50 and 80 yards on either side of the ship. The biggest problem was that it consumed valuable centerline space, which was also in demand for guns and torpedo tubes. To solve this problem, the USN introduced the K-gun, which was essentially half a Y-gun, mounted on the railings. These were mounted in great numbers on destroyers and destroyer escorts during WWII.

Mk 9 depth charge

As submarine technology advanced, another problem arose. Depth charges sank relatively slowly (8-9 ft/sec), which meant that a deep submarine had plenty of time to dodge, even if the pattern was expanded by the use of K-guns. The first solution was to raise this about 50% by reducing the warhead by a third and adding lead weights. However, this was not enough, and the USN soon developed the Mk 9, which had a teardrop shape and a sink rate of 22 ft/sec. Most of the improved depth charges also had their hydrostatic pistols modified to include deeper settings, and by the end of the war, a 600 ft maximum depth was standard, twice what it had been in 1939.


Of course, the ideal weapon is one that would attack targets ahead of the ship, before they passed into the sonar's blind spot. These were primarily developed by the British, and the first to reach active service was the Hedgehog. Hedgehog was a spigot mortar, firing a total of 24 65-lb projectiles at a fixed angle ahead of the ship. The 35 lb of explosives in each projectile would have been inadequate as a depth charge, but the projectiles were fused to go off on contact with the target, where it was more than adequate. A second bonus of using contact fuses is that, unlike conventional depth charges, a failed Hedgehog attack did not cloud the water with spurious echoes from clouds of bubbles. The US developed a rocket-powered version, known as Mousetrap, for ships which could not withstand the substantial forces generated by a Hedgehog firing. In British service, Hedgehog sank 47 submarines in 268 attacks, nearly 10 times the success rate of depth charges.

A Squid on HMCS Haida

The British also developed Squid, a more advanced weapon that threw a trio of 390-lb depth charges 275 yards ahead of the launcher, where they formed a triangle 40 yards on a side. They were fused like depth charges, but the sonar system automatically updated the depth settings until the moment of launch, reducing the risk of getting the charges at the wrong depth.1 Ultimately, Squid sunk 13 submarines in 50 attacks. The British preferred the Squid, the Americans the Hedgehog, probably as a result of the Americans having generally better sonar operating practices.

Mk 24 FIDO

The most influential ASW weapon developed during the war was very different. It was an aircraft weapon, never used from surface ships: The Mark 24 mine. This weapon, also known as FIDO, was not a mine of any sort, and was only called that as a cover. It was instead an air-dropped homing torpedo, a truly remarkable weapon for the time. 19" in diameter and weighing 680 lbs, it was a viable substitute for the depth charges which had previously been the only option for aircraft attacking underwater targets. After being dropped into the water, it would circle, four hydrophones in the nose listening for the target. If it detected propeller sounds, it would steer towards them, attempting to equalize the signal between both the left-right hydrophone pair and the up-down hydrophone pair. The 340 torpedoes dropped by American and British aircraft were responsible for sinking 68 submarines and damaging 33 more, an incredible success rate for the time.

Rockets being loaded onto a Fairey Swordfish aboard an escort carrier

Depth charges remained an important part of the aircraft weapon inventory, although a serious limitation was the inability for most aircraft to set their fuses after takeoff.2 A submarine on the surface might be able to ride out a depth-charge attack, as the charges would be fused against a diving submarine. The primary weapon used against surfaced submarines was the 30-lb rocket, usually fitted with a solid-steel "warhead" to pierce the heavy pressure hulls of submarines. These would be fired just short of the target, hopefully punching holes below the waterline. Rockets were also used on the "retrobombs" fitted to MAD-equipped aircraft. MAD only detects targets directly below the airplane, which is a problem because bombs continue to travel forward with the plane's momentum after they're dropped. A retrobomb was fitted with a forward-facing rocket to more or less stop it in place, allowing it to hit MAD contacts.

Depth charge track containing Mk 9 depth charges3

So far, we've covered the material factors of the anti-submarine war. However, these are only half of the story, and other, less tangible factors played an equally large part. Next time, we'll look at the methods the British came up with to make most efficient use of their forces.

1 The Japanese famously set their depth charges too shallow until mid-1943, when an idiot in Congress revealed this in a press conference, and other idiots published it.

2 I believe the Short Sunderland was an exception, as the bombs were stored internally and cranked out onto the wings by the crew.

3 My photo, from USS Cassin Young in Boston.

For more on WWII ASW, see Hunters and Killers, Volume 1 and Volume 2.


  1. April 16, 2018ADifferentAnonymous said...

    Wait, what? This is the first I'm hearing about homing torpedoes in WWII. Why wasn't the same system used for ship-to-ship torpedoes? Or, if it was, why wasn't it paradigm-shiftingly effective?

  2. April 16, 2018Directrix Gazer said...

    @ ADifferentAnonymous

    The speed of the Mk 24 was only 12 knots. That's not a problem for a weapon designed to be dropped relatively close to a submerged diesel-electric submarine, but it pretty much eliminates the possibility of being used against surface targets. There was a (slightly) modified version of the Mk 24, designated Mk 27, made for use against surface vessels by submarines, but it wasn't any faster and was thus useful only against slow moving, un-alerted targets at close range.

    The Germans also developed a series of acoustic homing variants of their G7e electric torpedoes. These were faster at around 20 knots (IIRC), and had some success when used against escorts, but it was... far from a perfect weapon, especially in the guidance department. The Allies, of course, rapidly introduced a towed decoy called Foxer which was pretty successful. There are some fun stories about Foxer's development, but I'll let Bean tell them.

    The main reason the early homing torpedoes were relatively slow was to reduce flow noise, which otherwise would have deafened their hydrophones.

    Incidentally, the US and German acoustic homing torpedoes were introduced within a few months of each other in 1943.

  3. April 16, 2018bean said...

    Directrix Gazer pretty much nailed this one. They didn’t have the tech to home at high speed, which limited the utility of the torpedoes quite a bit. They were also more complex. The Mk 27 Cutie’s average success rate is pretty much identical to that of contemporary conventional torpedoes. I suspect that some of this was the increased complexity of the equipment. They also weren’t particularly easy to use, as you had to get in really close, and I suspect there were reliability problems. I don’t know that much about G7e.

    By the end of the war, they were getting close to 28-kt acoustic torpedoes, but the tech wasn’t quite there yet.

    @Directrix Gazer

    Feel free to tell the stories of Foxer. I have memory of some vague hijinks, but nothing specific, and I don’t see Naval Gazing covering that any time soon.

  4. April 17, 2018ADifferentAnonymous said...

    Ah, got it.

    Also, I assume the Mark 24's electronics were way too delicate to fire out of a gun, but I kind of love the idea of Iowa firing a salvo of those at a crash-diving sub 30000 yards away.

  5. April 17, 2018bean said...

    Also, I assume the Mark 24′s electronics were way too delicate to fire out of a gun, but I kind of love the idea of Iowa firing a salvo of those at a crash-diving sub 30000 yards away.

    I’m not sure if I’m really amused or horrified at that image. Besides the obvious problem that it would be pulped, it’s also 19″ in diameter, which means that you couldn’t fit it into a 16″ gun. The obvious solution is to use rockets.

  6. April 17, 2018ADifferentAnonymous said...

    Bah, don't pretend you're any kind of ambivalent about turning battleships into ASW powerhouses :p

  7. April 17, 2018bean said...

    You're right. I'm not. Hunting submarines is beneath the dignity of a battleship. That's what destroyers are for. A noblewoman does not go around hunting dishonorable rogues, but leaves it to her servants.

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