The UPHOLDER and 206 class submarines are of the newer generation and learned their lessons from subs like the type 7. The type 7 was not streamlined and was very noisy with all the drainage slots and all of the other exteral clutter such as : AA guns,deck guns ,radio/jumping wire etc,etc. . The UPHOLDER and 206 class boats are very streamlined and smooth with no open drain slots all over the place , no AA guns and everything retractable,this is the key here: smooth ,sleak ,no excessive water movement over open slots etc, all of this equals drag and noise. You can have all the modern sensors you want ,but if your boat has poor underwater qualities you will be found first and sunk first!