That is some great information, and thanks for sharing it.
I do notice two things, though. In that description he specifically says it is impossible for him to trim the boat accurately enough, and the boat
will rise or sink. So,
no underwater hovering. Yes, you can do it by constant trimming. Guess what? The pumps make just as much noice as the motors at slow speed.
The second point is the "hanging by the periscope". It's a nice trick, but he does say "...if the sea is calm enough and the boat's well trimmed..." This means that if the sea is perfectly calm you can sit and wait for a convoy, or sit perfectly still trying to avoid the escorts,
at periscope depth and with your periscope raised.
Good luck with that.
