Hi Joe. I went through the same learning process a few years back.
As you noted the ama gets strength by being a closed box. The vaka with its open top would distort easily unless measures are taken to prevent it. With fore- and aft decks it will be stiffer in those locations especially if they are closed boxes.
These boxes are connected by the midships segment: an efficient way to stiffen it is to add narrow side-decks, gunwales are less effective but simpler. My sailboat has a foredeck but is too small for an aft deck, so I enclosed the volumes under the side decks to form longitudinal sealed floatation chambers which provide torsional stiffness back to the transom, and double as seats.
As others have noted, for ply-on-frame construction the frame provides a support for a plywood skin during assembly. In glued assembly the skin is stronger and stiffer than the frame. The main function of the longitudinal frame members is then to provide adequate area for the glued joints between the ply planks.
In stitch and glue this function is performed by glass tape and epoxy, which saves significant weight. The glass tape can be viewed as a long hinge, although epoxy fillets provide some stiffness, but the strength and stiffness mostly comes from two pieces of ply attached together at an angle, like the corners of a cardboard box. This is further improved in a boat by the multi-dimensional curves of the planks, thus preventing the “hinges” from opening or closing.
I moved away from stitch and glue some time ago. For small boats where the thin planks are easy to break during assembly, I glue longitudinal members or chine logs to the planks while flat. The planks are easier and stronger to handle during assembly, and the chine logs eliminate the need to use glass. It adds a few percent weight but saves a great deal of time drilling, wiring, taping and filleting the plank-to-plank joints.
A card model of a design is an excellent method for understanding how the various elements of the hull work together. Flex it this way and that and you will easily see how forces applied at one location result in flexing in another place, and this reveals where reinforcement such as decks and gunwales can most usefully be added.
