i believe that there is a depth where the watch crew automatically goes below deck.
this is something like 8 meters depth.
every time you pass 8m the watch crew disappears - you will notice this in game if you have free cam enabled.
this is the point at which the game engine considers the u-boat to be "submerged"
i believe this is also the point at which DD hydrophones can begin to hear you underwater.
DD Hydrophones cannot hear you while surfaced above this 8 meter demarcation.
if memory serves me correctly, 300 meters depth is the maximum sensor range for hydrophones and it is also the maximum depth for depth charges.
theoretically, if your boat can survive at 301m depth or deeper, you can run ahead flank all day and never be detected.
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