Oh, and a 1980s project for which I have no pictures.
A 3 foot cardboard rubberband sub.
It had a U shaped cross section with top of U squeezed a bit.
Construction material was brown cardboard dividers between layers of candy bar counter display boxes at the F.W. Woolworth store where I worked.
It was free-flooding like model in book.
Sub had an inner skin and an outer skin, I forget what width of U frames was.
3/8 inch, maybe?
All cardboard was soaked with varnish as sub was constructed.
Space between inner and outer skins was sealed to not flood.
Middle of hull could be described as modular U-channels which were the assembled.
Bow and stern each tapered, were similarly based on U-channels. Exact shapes are not remembered but bow shape was inspired by typical USN and Kriegsmarine 1940s subs.
Took almost 5 pounds of BBs poured between skins to ballast the boat to float deck awash.
Don't remember how I made dive planes, rudder, or what was used for prop.
Pretty sure a test prop was made form sheet metal, but not used for actual running as my friends had an above ground vinyl-lined backyard pool and it would have been rude to cut their pool liner.
Prop was much bigger than above, was several inches.