The idea is that the sub should be bakable by anyone with a shed full of tools.
The first version is the one illustrated. It's rather complicated and I haven't fully resolved exactly how each valve would be controlled.
Air from the 'full' tanks is used to control the depth and power the motor.
The depth is controlled by flooding or vacating the area between the inner and outer oil drums (the envelope). The volume of this area (eh?) is around 115 litres. It would be neutrally bouyant when half full, providing +- 50kg of lift/sink. The ballast is concrete, fine tuned with diving weights. The motor is taken from a compressed air bolt drill.
Air returning from the motor and from the envelope goes into the regulator tank, which in turn supplies a constant flow of air to the cabin. Before being released into the cabin, the air from the regulator tank powers a pump which pushes air from the cabin into the sea. If the pressure of the regulator tank drops below a certain level due to lack of sub movement, air is taken directly from the 'full' tanks.
Manouvering is done using a control bar at the front, which when rotated turns the elevators and when slid sideways, controls the rudders. it might be better to use radio control for these so that you don't need to pierce the hulls.
Nice big domes of plexiglas are somewhat hard to find so you could use washing machine doors instead for the window. Either way, this is where you get in. It wouldn't be a quick process and would probably involve someone bolting you in.
The 2 hulls are not fixed together. Although the sub is not intended to go very deep, it's still worth having some safeguards. If the inner hull starts to buckle or leak, you can increase cabin pressure or reduce the pressure in the envelope to relieve the strain as you head to the surface. Attaching the hulls together by glueing thousands of elastic bands between them would be ideal but would seriously increase construction time.
The simple version is more wasteful. It has 3 tanks and no return valves. There is a tank to inflate the envelope, a tank that supplies breathing air and pumps old air out, and a tank for the motor, but the last tank could easily be replaced with a battery. The only advantage of using a tank is that it's more useful than a battery in an emergency. Alternatively all of the tanks can be linked or only one tank used for all 3 porpoises.
Inspired by the 'subglider' idea and a sub built in Last of the Summer Wine.