Thermal layering (or salinity) is dealt with in the U.Kdt.Hdb. (U-boat Commanders Handbook) that were written by a collection of U-boot commanders during the first year of WWII.
Its located under item 57)
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As regards the conditions of ASDIC in relation to the trasmitting capacity of the water, it has been ascertained that the efficiancy of the submarine-detecting gear is considerably reduced in sea areas with numerous layers of water.
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examples are mentioned as:
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North Sea, in the Skagerrak and Kattegat, Straits of Gibraltar, Gulf Stream, near mouth of rivers (ref. Atlas of water Densities of the oceans).
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Continual observation and measuring of water densities and temperatures are therefore important and indispensable for establishing the presence of "stratification" when submerging to considerable depth as a means of evading pursuit by position finding.
In addition, position finding is very difficult, and almost impossible, in shallow water of varying depths (sand banks), where there are many wrecks, as well as in narrow bays (Norwegian fjords), as it is usually not an echo that is produced, but numerous echoes, which make it difficult to keep, but more especially to locate, the target.
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