Dale,
Welcome to the list.
Your idea of a plastic bag sounds ok. I think there
is one problem. You need a regulating system that keeps your
internal pressure a little higher than the pressure of the surrounding
water. If not the hull will become smaller as you dive. Ballancing the sub
will become verry difficult. You might end up sinking like a rock. A view months ago I suggested an inflatable (foldable) sub
that has a constant internal pressure. I think it
has some special advantages:
1) You only have to deal with internal pressure.
One could build a submarine balloon from non stretching materials
such as used in modern sails.
2) If you want to build a "high
speed" ambient sub you have to suffer from some physical
discomfort and maybe even danger (going up to fast). An inflatable sub can
have a constant (high) internal pressure.
3) You do not need a lot of air for pressure
compensation.
In an inflatable sub you pressurise the hull to a
level depending on your maximum dive depth. After the dive you can let
the air escape in a rate corresponding the decompression tables.
Maybe i have overseen a lot of essential
problems. I am just a daydreaming amateur.
Greatings,
Thijs
Struijs