Close but no cigar. Have you ever tried to get wet sand to move through a small hole? The ballast was steel shot held in two "silos". The shot was blocked from falling out by electromagnets, when you wanted to "blow tanks" you turned off the juice to the magnet and dropped some shot. Neat safety feature 'cuz if you lost power you automatically dropped all ballast and went positive buoyancy.
BTW the hull was sheet metal and it was filled with aviation gasoline to give the ship a fixed positive buoyancy without the shot in the silos.
Regards,
GtG