Escorts Insane detection (GWX)

[BigWalleye]

Originally Posted by UKönig

ASDIC detection is not dependent on sound but magnetic anomaly, or, the mass of the U-boat in water.

UKönig, could you please provide a source that explains how ASDIC actually worked. I was always under the impression that it was a form of echolocation, based on reflected sound waves (the way a bat flies at night). I had no idea that it was magnetc in nature. Could you provide any more information? Thanks.

[banryu79]

Originally Posted by BigWalleye

UKönig, could you please provide a source that explains how ASDIC actually worked. I was always under the impression that it was a form of echolocation, based on reflected sound waves (the way a bat flies at night). I had no idea that it was magnetc in nature. Could you provide any more information? Thanks.

This sounds strange for me too, I already red or believed ASDIC was based on generating sound waves and detecting the echo produced by the hull of the uboat when hit.

Btw, BigWalleye, thanks for your explanation.

[Zosimus]

http://uboat.net/allies/technical/asdic.htm

ASDIC / Sonar

ASDIC was the primary underwater detection device used by Allied escorts throughout the war. The first versions, crude to say the least, were created near the end of World War One and further developed in the following years by the Royal Navy.

How it works
The ASDIC, known to the Americans as Sonar, was basically a transmitter-receiver sending out a highly directional sound wave through the water. If the sound wave struck a submerged object it was reflected back and picked up by the receiver. The length of the time from transmission until the echo was received was used to measure the range, which was shown as a flickering light on the range scale. By mounting the transmitter head so that it could be directed almost like a searchlight, the bearing of the target could be read from the compass receiver.

The transmitter (sound) head extended beneath the ship, and was encased in a large metal done to minimize the noise of the water rushing past the ship while at moderate speed. This dome was filled with water, through which the sound passed, although this water was stationary and acted almost like a bumper. Noise level remained relatively low at moderate speeds, but anything above 18 knots resulted in too much noise and good contacts were difficult to find. The same results also resulted from bad weather when the ships were rolling, pitching and heaving.

The search pattern
During screening operations the ASDIC operator searched through an arc of roughly 45 degrees each side of the base course of the vessel. The ASDIC had to be stopped at regular intervals on this arc long enough to allow the relatively slow underwater sound waves to return should they locate a submerged target. Normally the head would be stopped on a bearing and a sound pulse would be transmitted, which would be heard as a "ping" noise. If no echo was received after several seconds the head would be rotated a few degrees (usually 5) and the process repeated throughout the watch.

If the outgoing impulse stuck a submerged target the echo would be heard as a distinct "beep". If this occurred the ASDIC operator would sound the alarm, feed the range and bearing to the bridge and then immediately start left and right cuts to try to determine the width of the target and trying to see if it was moving from one side to another. He could also determine if the target was closing or opening the range.

Echoes would bounce back from many things besides the U-boats such as whales, schools of fish, vertical sea currents and ship's wakes. This caused many false alarms, especially with the inexperienced operators. The veteran operator was much better at figuring out these bad signals and hunting down the intended target. The commanding officers quickly learned which operators were reliable.

Another problem was that often a real U-boat could not be detected due to water conditions. ASDIC was not very reliable in rough water, nor when layers of different temperature deflected the sound waves. U-boats could dive beneath such layers to avoid detection. Modern nuclear and diesel submarines use this tactic to this day.

ASDIC in a passive role
The device could also be used to listen as well as pinging. The propeller noises of the U-boat would sometimes be heard as well as its operation of various machinery and its use of compressed air in the ballast tanks to change depths. This was not very usual as one of the standard German tactics, when located, was to dive deep, rig for silent running and hide beneath a thermal layer at speeds slow enough to eliminate any cavitation from the propellers.

The attack
When the U-boat was located the attacking vessel would rush directly towards contact, usually at the speed of 15 knots. This run was used to determine the final movements of the target and further plot the final attack. The attacking vessel had to very sure where the boat was and estimate where it would be when the depth charges (or Hedgehogs) would reach its depth. Thus the attacking vessel would have to take a lead on the U-boat much as a hunter does on a bird. At 500 yards the allied commander hoped to know what the U-boat was doing and then he finalized his attack.

As the range closed the U-boat would pass under the beam of the ASDIC and be lost to the escort. The deeper the U-boat was the longer the range of the lost contact and thus more difficult to attack accurately. Normally a good and firm contact was lost at 300 yards. This did not affect the forward-throwing Hedgehogs as much as the depth charges.

Even if the attack was delivered with the correct lead angle and firing time there was no guarantee of damage to the U-boat since its depth could differ from the settings at which the depth charges were set to explode. The correct depth of the U-boat could only be guessed or estimated based on the range at which contact was lost.

U-boat's evasive maneuvers
The U-boats of course used tactics to evade the depth charges and Hedgehogs. The best time to act was when the attacking vessel had taken its lead angle and the ASDIC contact was just lost.

A very common German move was to run away from the escort and force it on a stern chase pinging through the wake of the U-boat which could give the ASDIC a hard time. Then at the moment of the ASDIC [losing] contact the U-boat took a radical turn to left or right and more often than nor escaped out of the attacking pattern.

Another one was to turn radically with great power and disturb the water in order to confuse the ASDIC sometimes causing the attacker to be shaken off. The Germans also often released chemical pellets, which would produce clouds of bubbles to reflect the sound waves of the ASDIC.

Yet another tactic was to dive very deep and under a thermal layer or beneath the depth at which depth charges were normally set to explode. From 1942 onwards depths of 200 meters (600 feet) were not uncommon in an evasive tactic.

Although it is often claimed that the name ASDIC is derived from "Anti-submarine Detection Investigation Committee", there is no trace of such a committee in the Admiralty's archives (see Willem Hackmann, Seek & Strike: Sonar, anti-submarine warfare and the Royal Navy 1914-54 (HMSO, London, 1984) xxv). Hackmann explains that the name almost certainly relates to the "Anti-submarine Division or Anti-submarine Division-ics" - the department that initiated the research into underwater detection in World War One.

[BigWalleye]

Thanks for the explanation, Zosimus.

[Sailor Steve]

Originally Posted by Zosimus

ASDIC / Sonar

Thank you for the link. I would ask that when you copy and paste something like this that you use quote brackets so everyone can see that the words are someone else's and not yours. Also please do not quote more than the opening paragraph. This has led to legal difficulties in the past.

[UKönig]

Sorry gang, magnetic anomaly detection is another piece of equipment on its own. With all the u-boat books (factual and otherwise) I've been reading lately, it's not surprising that I'm mashing things together.
MAD is/was a valid system, but was exclusive to sonar. I beg everyone's pardon.

Here's some more info on MAD...

Magnetic anomaly detectors employed to detect submarines during World War II harnessed the fluxgate magnetometer, an inexpensive and easy to use technology developed in the 1930s by Victor Vacquier of Gulf Oil for finding ore deposits. MAD gear was used by both Japanese and U.S. anti-submarine forces, either towed by ship or mounted in aircraft to detect shallow submerged enemy submarines. The Japanese called the technology jikitanchiki (磁気探知機, "Magnetic Detector"). After the war, the U.S. Navy continued to develop MAD gear as a parallel development with sonar detection technologies.
There is some misunderstanding of the mechanism of detection of submarines in water using the MAD boom system. Magnetic moment displacement is ostensibly the main disturbance, yet submarines are detectable even when oriented parallel to the Earth's magnetic field, despite construction with non-ferromagnetic hulls. For example, the Soviet-Russian Alfa class submarine, whose hull is constructed out of titanium to give dramatic submerged performance and protection from detection by MAD sensors, is still detectable.
This is due in part to the fact that even submarines with titanium hull will still have a substantial content of ferromagnetic materials as the nuclear reactor, steam turbines, auxiliary diesel engines and numerous other systems will be manufactured from steel and nickel alloys.
First used in World War II, MAD uses the Earth's magnetosphere as a standard, detecting anomalies caused by large metallic vessels, such as submarines. Modern MAD arrays are usually contained in a long tail boom (fixed-wing aircraft) or an aerodynamic housing carried on a deployable tow line (helicopters). Keeping the sensor away from the plane's engines and avionics helps eliminate interference from the carrying platform.

At one time, reliance was placed on electronic warfare detection devices exploiting the submarine's need to perform radar sweeps and transmit responses to radio messages from home port. As frequency surveillance and direction finding became more sophisticated, these devices enjoyed some success. However, submariners soon learned not to rely on such transmitters in dangerous waters. Home bases can then use extremely low frequency radio signals, able to penetrate the ocean's surface, to reach submarines wherever they might be.

Again I apologize for that error, I was tired...