Magnetic Anomaly Detection

As early as 1917, alternatives to the detection of submarines by hydrophones were being studied. One such area for consideration was the use of magnetism. In 1918 the U.S experimentally tried a ship towed magnetic detection device. This device was found to have too limited a detection range and also suffered from the presence of the magnetic signature of the towing ship. For the time being, magnetic detection was abandoned as impractical.
With the outbreak of WW II, renewed interest occurred in alternative detection systems for anti-submarine warfare. For aircraft there was a pressing need to devise a means for them to be able to detect a submerged submarine. One of the devices that received renewed attention was the use of magnetic anomaly detection.
As early as 1941 magnetic detection devices (which measure changes in the Earth's magnetic field) had been developed in both Britain and the U.S. The first airborne use of these devices was in U.S. K type blimps. This was followed by much wider installation of MAD devices in ASW patrol aircraft. By 1943 most ASW aircraft were equipped with MAD.
Initially, the U.S. thought that MAD would be a primary means of detecting submerged submarines. In use MAD was found to be a system of limited usefulness. This was due to its very limited range and, its inability to distinguish between sources of magnetic variance. Frequently, wrecks or local magnetic disturbances were classified as submarines. This was particularly true earlier in the war before experience with the system had discovered its limitations.
By late war, MAD in combination with sonobuoys proved more useful. In combination, MAD allowed an aircraft to localize a contact made with sonobuoys and, the sonobuoys provided confirmation that the contact was, indeed, a submarine. In this combination MAD became the secondary system to the sonobuoy, the reverse of what was originally expected and it double the threat for a u-boat.
Source:www.uboat.net,with necessary changes.

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ASDIC

ASDIC / Sonar

ASDIC, developed through the work of the Anti-submarine Detection Investigation Committee , from which its name is derived, 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 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 loosing 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.

Source : www.uboat.net ,with necessary change.