Probably the most existential and realistic book of submarine warfare from the point of view of a German crew that I have read. It reveals an experience most would rather not have had to live through.
Submarines need buoyancy to function. Salt water makes that extremely difficult because the specific gravity of salt water varies with depth, temperature, the amount of plankton, salinity, even the time of year. (Apparently fresh water is much easier.) Now let’s assume that the specific gravity changes by 1/1000, a small enough amount. If the weight of the sub is 800 tons, the weight must change by 1/1000 or 1600 lbs., not an insignificant amount. The weight has to be increased or decreased as the case may be.
Or let’s say the cook moves a 100 lb. sack of potatoes from the stern forward. That amount of weight has to be redistributed by pumping an equivalent amount of water back to the stern in the trim tanks. You will never think of submarines in quite the same way after you have read the description of their sub in the midst of a storm having to run on the surface for speed (if you could call it that) and to charge the batteries, the boat plunging and heaving through the waves. Can submarines capsize?
This kind of fascinating information adds such verisimilitude to one of the submarine classics to come out of WW II. The author served as a naval correspondent on U-Boots during the war and experienced much of what he then wrote about. I have seen the movie (in German) and listened to the audiobook in addition to reading the book (in English). Not recommended in any form if you have a heart condition.
“All doubts are silenced by the concept of duty.” Think about that.