Interesting that book claims that target speed could be derived from engine RPM and type but there is not a single reference during the war as to that method ever being used outside of a general "target is speeding up" or "target is moving slow." I've never seen a speed in knots tied to a hydrophone report in any report made during the war.
There were two thermal conditions that were quite interesting. The first was the usual situation where the top layer was warmer than an underlying cold layer. In this case there was a sometimes wide range of densities where the sub would sink in the lower density warm layer, but float on the higher density layer below. There were a couple of times when submarines were saved by this floating on the layer. They could shut down all engines and work on the sub without worrying about depth control. This condition wasn't really rare.
But the rarer condition is an inversion, where a cold surface layer overlays a warmer layer at depth. They would have to make the sub heavier to sink under the surface, and when it encountered the warm water at the thermocline, the bottom would just drop out. The sub would drop like a rock off a cliff and it took time to pump water out to lighten the sub enough to regain control. It was a rare, but very dangerous situation: a race between the pumps and reaching crush depth.
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