[James Tonthat]

All my info is based upon my experience on cardboard and duct tape boats; so a bit different than your situation. Take what you can, feel free to contact me.

I wish I had a picture (yahoo email deleted the original) of the cardboard boat we built in 6th grade. The picture showed the boat with the two hulls seperated and there being about half a soda can worth of water. I don't think any water leaked through, the only water that got in was water that splashed in from the top. We had a double hull design with the inside being slightly smaller than the outside, only the outer facing walls were covered with tape and inner faces bare. (Time constraints) Between the hulls we had ribs that spaced the two hulls as well as making the boat ridgid. Our boat was way ahead of any of the other 6th graders, easily winning the competition.

Couple tips
Use Duck Brand tape; the Home Depot Nava-something brand peels after any exposure to sunlight or heat (not sure which but we discovered this after we had to work in Texas outdoors)

Use a consistant tape layering technique, think roofing tiles.

Avoid seams, use duct tape mainly as a protectant not as a structural material

I'm sure you might be able to do more than what we did since it seems you have a bit more flexability. We had to use two sheets (4' x 6') of supplied cardboard, some cardboard scraps, and unlimited amounts of duct tape (had to buy your own) and that was all we could have used.

Good luck!

Edit: I forgot to explain the attachments. The first attachment is how we used our cardboard (solid for folds, dotted for cuts), after folding into the shape we wanted. We had to make an end cap for the rear that looks like the cutaway.

Edit 2: Heh, I probably started typing this post before you posted. Maybe the shape of the boat will be of help.