Well, I've read Norbert's book and I have a better idea of how a submarine works although I can't claim to understand everything in the book, my eyes glazed over at some of the math. However, it was a good read and will make a good reference book even if some of the electronics is a bit dated.
As for U-570 one of the decisions is made for us (it's a joint project between myself and teenage son Michael. Michael is the brains and chief driving force and I'm the chief sponsor and transport provider) the watertight box is coming away from hull so we will replace this with a cylinder. The ballast system is not decided yet but it will be either a piston tank or air bag. We had a look at the Engel piston tanks, very nice but a lot of money. If anybody knows where we can find a design we could build our own piston tank to that would be very nice. The ballast tank of whatever type we decide on will be fitted at the c of g to prevent any unwanted trim changes when diving.
For the WTC we can get some 100mm x 3mm acrylic tube on ebay for about £15 and a friend is willing to let me use his lathe to make some end caps.
Lots more to plan design yet but we are getting closer to actually making a start.
Many thanks to all the people who have helped and made many suggestions here on Mayhem and at Brockwell
Phil.