: Does anyone know if stringers are needed to give stiffness or increase
: bonding area?

This would be VERY dependent on your design. are you working with any particular plans?

If you build a frame similar to that used in a skin-on-frame kayak then your structural issues are moot. The frame would carry any loads, and the plastic sheeting would simply keep out the water.

At a minimum you would probably want three areas to be reinforced: the keel, the gunwales (OK, so I count both left and right sides as ONE area) and the center support for the deck. If you use deck beams to support he deck then you definitely need some reinforcement on the gunwales, as that is where the deck beams will rest. Depending on how acrobatic your entry can be, (or isn't) you will probably want to renforce the area around the cockpit coaming, too.

A rolled up section of this material, or a "box" shaped piece made by laminating several this strips, should be something you could use for the keel and gunwales. If you go with curved deckbeams, consider trying to bend this material by gently heating it first, and then wrapping it around a form until it cools.

If you look at the plans for a stressed or "tortured" plywood canoe you'll see that with a large enough piece ( 10 feet long or longer) you can just cut the profile of the ends and roll the canoe into shape. That would minimize seaming and sealing of the ends of the material. You could then add gunwales and a keel made fom wood. In this case use thin pieces of wood (about 3/8 to 1/2 inch, or 9 to 12 mm thick) for the gunwales and sandwich the upper edge of the plastic bwtween two of these wood strips. Screws can pull the woodstrips together, firmly pinching the plastic between them.

The keel can then be suspended from the gunwales by use of a strip at bow and stern Seats can also be suspended from the gunwales in the conventional manner. Depending on the length of the boat, one or more thwarts should be used to maintain the width of the boat. If the keel is very flexible, vertical pieces can join the keel to the thwarts, reinforcing the keel.

: The stock I am thinking of using is 5mm, Liquid nails construction adhesive
: and silicon sealant to fill exposed cores.

I'd strongly consider heat sealing the cut end to eliminate exposed cores, and heat sealing/welding the panels together, eliminating the need for Liquid Nails (r).

As I see it, you would want to cut your panels about an inch oversize on all edges to be joined, then heat those edges and apply pressure to cause the corrugations to collapse and weld the ends of the channels shut. You might be able to do this with a household electric iron. Place a sheet of aluminum foil between the iron and the plastic to keep the plastic from melting onto the iron itself.

That should give you a flat lip about an inch wide which you can align with a similar lip on an adjacent panel. If you use glues here at least you'll have a large bond area and a flat surface to clamp. You might also use a gasket and pull the parts toether with bolts and nuts, or rivets. Or, you can overlap the lips and melt them together with the same household iron setup, or perhaps use a propane torch and a wide-jaw pliers. In the latter case you would heat the plastic to its melting point and then squeeze the pieces together until they cooled.

Just some thoughts

PGJ