Making a battleship-sub....

[Walkiria]

I have totally implemented a battlecruiser, everything is in place and workin' properly. As I was interested in the possibility of using it as a submarine , I added the diving and surface systems. It works fine, and it keeps quite stable until it reaches 30ft deep. But, from 30ft, it sinks forward and it starts an unrecoverable fall . My other problem is that, altought it sinks smoothly and it reaches the bottom at 0 speed, it starts suffering quite severe damages in its central structure after a while resting.
My two main questions are:
1-How could I level the fall under 30ft (which parameters should I adjust)?
2-When I use the same parameters in a submarine, it works perfectly. What should I change in the battlecruiser files in order to achieve the same behavior?

Thank you everybody!