Thanks guys. The notion of circular reinforcements in the ballast tank is a good one. They could serve a double purpose and be modified slightly to serve as slosh reducing baffles. Of course, the ballast tank can always be reduced to 2.5 inches and simply made a little longer...problem solved.
With the positive displacement pump type systems like the SubTech/Mikes Subworks, or a snort type, there is very little pressure on the tanks because the tank is essentialy empty when surfaced, maybe only an inch or so underwater, and filled entirely when submerged. The boat has to be at, or almost at, the surface to empty and fill the tank, with the air intake/outlet vent tube (ie. periscope) poking above the surface. They result in subs that are not true static divers, but are VERY close, with the added safety feature of being just slightly positive in terms of buoyancy in the event of power failure.
For the high cost stuff...the electronics and motor, I will continue to use the thick walled polycarbonate. I have a tendancy to build all of my boats, both subs and surface, more like locomotives than aircraft. Better safe than sorry.
Thanks to both of you, again.
"The Japanese solders would die for the Emperor, which made them formidable. The U.S. Marines would die for each other, which made them invinceable"...Iwo Jima