How Did Old Magnetic Sensing Torpedoes Detect Submarine Hulls?

[Rothiemurchus]

What size of induced magnetic field strength were the torpedoes homing in on and how were they detecting it?

 

[Marijn]

As far as i know, the torpedoes used in WWII were not really homing in on the magnetic field of a vessel.

The were fired and detonated them selfs once under a vessel.
This the "knew" because of the disturbance in the magneticfield becouse of a big chunk of steel above them (being the vessel).

It was so important that the torp's detonated right under the vessel because rather than blowing a hole in the hull, a detonation right under the keel will break the vessel in two because the vessel first is "lifted" out of the water at a point on the keel, putting tremendous stress on the keel, then because of the vacuum (spelled right??) that remains after a blast it is stresses the other way.
A "snapping" motion able of destroying any vessels keel.

 

[ray b]

homing torpedos in late WW2 used sound to locate targets
some other types ran random paths or circled after a timed delay kicked in

some of both types some times returned to sub that shot them with bad effects

 

[Gokul43201]

What size of induced magnetic field strength were the torpedoes homing in on and how were they detecting it?

What the old magnetic sensing torpedos detected was the local disturbance in the Earth's magnetic field by all the steel in a surface vessel's hull. Typically the magnetic field lines run nearly parallel to the surface, but as they approch a large ferromagnetic object, the lines of force curve upwards towards it. The torpedos sensed this change in direction and moved toward/detonated beneath a calculated source.