Mines carried by the VIID
TMA (Torpedo mine A) Mines were pre-war standard design floating mines, phased out by TMB from mid-September 1939. Stored aboard U-boats in the same manner as torpedoes, TMA mines were shorter than “eels”, measuring only 3.64 metres long. Their detonator was of the magnetic influence type. However because the complete mine comprised the anchor by which to moor it to the seabed plus the connecting cable, the actual warhead was relatively small at 215kg.
TMB was also designed especially for submarine use. Again deployed via the torpedo tubes in the same manner as a torpedo, they were slightly smaller than TMA mines — measuring 2.31 meters — thereby allowing the submarine carrier to load even more into its confined storage space. Up to three could be fired from a single torpedo tube as a single shot. This was an unearthed “ground mine” that utilized a magnetic firing pistol, activated on a timer allowing the U-boat to depart before the mine became “live”. It carried an explosive TNT charge of 567kg — twice that of period torpedoes. The mine was laid on the bottom in shallow seaways and detonated by the magnetic field generated by large ships passing above. The explosive force beneath a ship's keel carried by shock waves through incompressible water was usually more effective than a contact hit against a hull side, often snapping a vessel's spine and dooming her instantly.
These mines were originally ordered laid at a depth of 30 metres. However questions arose as to the weapon's reliability at that depth and further trails were conducted. Following these it was established that 25-metres was a more suitable depth, and more likely to prove fatal to shipping above. It began to be noticed that often the magnetic trigger was itself overly sensitive and smaller ships were sunk than those intended. The blast from such a mine against smaller trawlers was particularly devastating. U-boat crews therefore began sowing their minefields in progressively shallower waters as well as de-magnetizing them so as to reduce their sensitivity
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