Hi Brian, the large super-yacht subs looks really amazing. I am wondering how the technology is improving day by day.
Just ran across this interesting video, the development of the largest submarine in WW2. Its the Japanese I-400. Turns out to employ a little catamaran tech (2 hulls) (at about 12 minutes into the video) in order to gain stability. I remember seeing a number of references to this sub on various videos, but I think I missed this one.