Mark. I am not very conversant with the torpedosystem, I never interfered with the torpedo people. We have the same word "Umhullung" in Norwegian and I guess your assumption is correct, you flood the torpedotube, launch the torpedo, the surrounding water is ejected and you compensate for the totalweight torpedo plus surrounding water.
Exhaustblowing of the ballast tanks always seemed to me to take a long time. I Guess I never recorded the time and it varied depending upon trim, exhaustpressure adjusted by the outer flapvalve,weather and how much the ballasttank were blown by air etc.
Tore