sunworksco wrote:The scale size submarine that you are building needs to use Norbert Bruggen's piston diving tanks and bajonett rings.
While a piston-based system can provide you the best level of control, it is definitely not something that "needs" to be done. Any of the dive system types in use today can be used...even for a boat this large. Heck, even if you do decide to sue a piston-based system, bayonett rings are not even needed....though you do need some way to hold the end caps in place in the presence of the hifger internal pressure.
I would suggest evaluating the various trade-offs between systems (which I think you are doing) and choose the one that interests you and fits your needs/budget and then build it.
The way I do this is to first understand how large the ballast tank needs to be and then figure out for each ballast system type what it would take to allow the boat to dive/surface at speeds that are like the prototype.
For my various boats, I've made a point of building the dive module for each new boat of a different type. My large boats use the piston, hybrid RCABS, sealed pressurized ballast...and yes even gas.
p.s. even the system made popular by Skip Asay (and now being offered in slightly(?) different form by another vendor as the 'snort system' can work for a large Type VII as long as you get the correctly sized pump.