RE: New ballast system
In my casings the ballst tank is a true tank ,strong and always watertight since it is also inside of the casing.
Why build it that way one it is imparitive that a reliable means to contain the water which will have pressure both coming in and laeving will require a strong tank as is seen on the full size subs.I could also use a bag but I would set it up just like the TT system and would cost alot more.
In my ballast system the tanks pressure is kept within contol by the 2 one way valves and that way the entire tank is used with no hi PSI or vacume induced.
A very inexpensive and reliable system anyone can make,