I\'ll take a stab at answering this. you touch on a number of different subjects, each with its own technical foibles. in brief:

1) Acoustic homing torpedoes were developed by both the US and Germany during the war. The US \'Fido\' was an air dropped anti-submarine weapon. Homing in on the slow moving U-boat in the silent depths of the ocean proved to be a relaitvely simple acoustic homing problem, and the Fido was very successful, sinking scores of U-boats.

An equivelant version was fielded in the Pacific by the US. It enjoyed a good hit ratio, but had much less strategic impact: the IJN (japanese navy) was on the ropes by the time it came out.

The Germans fielded an acoustic homing torpedo, the T-5, for use against escort ships. Basically it was a flop. It was perhaps confused by the noise from the heavy seas of the north Atlantic, and was easily foiled by Allied countermeasures, like towed noisemakers. The T-5 sank few ships, and did little to stem the allied onslaught on U-boats.

Acoustic torpedoes, besides being difficult to build and not always reliable, were easily fooled by countermeasures, and had trouble hunting objects that were faster than 10 or 12 knots, or hunting in heavy seas, which in the North Atlantic, was almost all the time.

Basically a good, reliable acoustic torpedo for routine anti-convoy ship use was beyond the state of the art in WWII.

2) Magnetic exploders for torpedoes were invented by all major powers, and abandoned when they proved unreliable. What worked in a test often failed in the different magnetic patterns of the front. The magnetic exploder designed to detonate under a 40,000ton battleship often failed around 2000 or 4000 ton merchantmen. They prematured, or failed, more often than not. By mid war everyone had abandoned them in favor of the simple contact detonators that actually worked.

The magnetic field of most ships are too weak for long range detection and homing. Additionally, all navy ships, and most merchantmen, were being \'degaussed\' to avoid detonating magnetic mines.

The US did deploy MAD detectors on aircraft. These could detect a submerged sub by its magentic field. It was not possible to mount it in a torpedo.

3) The Germans, who faced large convoys, invented torpedoes that looped and circled to try and hit more ships. By the time they were deployed U-boats were hard pressed, and these FAT torpedoes did not significantly hinder the allies. The US rarely faced large convoys in the Pacific (though, contrary to popular mis-conception, the Japanese did send many, if not most, merchantmen by escorted convoys, just small ones) and did not develop a looping torpedo till after the war.

4) firing on just a sonar signal, without a periscope sighting to get exact bearings, was beyond the capability of the sonar equipment of the day. Try it and you\'d miss for sure. to get a hit from a thousand or so yards and you really needed to know bearings down to the degree, while data from the sonar was barely better than a rough direction, more like hours on a clock face than degree marks on a sextant.

Pre-war US tactical doctrine, in stark contrast to the German\'s, held ASW aircraft in awe, and the \'blind\' sonar attack was emphasized in training, with the attack pewrsicope used in only \'safe\' conditions. It was quickly realized that no one could hit a thing with just a sonar contact, and those commanders who persisted in using it were sacked as cowards.

The German XXI sub had sonar that was quite good, and was supposed to do its fiighting, and firing, without coming up to persicope depth or any persicope sightings. It was deployed too late to see service in WWII.

Hope this helps.