This instructable is about building a 12 foot rowboat from very cheap materials: 20 pvc electrical conduit tubes (5/8"), 2 sheets of 9mm underlayment (3/8") and polyester tent fabric for the skin. The only hard part is you need access to a large cnc router. Perhaps you can use one at a fablab nearby?
Step 1: Design
There are a few free ship modeling tools out there(like hulls and carene), but most of them only work on windows and are not open source. Freeship is open source, but is only compiled and released for windows. It runs fine however with wine, so on either Linux or windows you can use this nice tool for designing hulls and ships etc. Do not make the mistake to take the newer release of delftship however: the file format is not backwards compatible, delftship has fewer features and is not open source anymore (great shame really).
By far the easiest way to get started with freeship, is to create a new generic model, and adapt that to a linesplan you have found on the internet. Even with this tool designing a nice looking boat takes a lot of effort, copying the effort of others will make your learning curve less steep. Freeship works with nurbs-surfaces, which means in practice you can not alter your model with an exact amount. You can however import a lot of file types to carry on with a previous design mad in another progam.
When you are done with your design, make sure all your surfaces that you want to make in plywood, should be put in their own layer that is marked as developable. That way, you can export them from the "tools->develop plates" menu item. in here you can mark which surfaces you want to export. It might be best to export each surface in its own dxf file. Not every program can understand this dialect of dxf (it uses a vertex to describe the surfaces), but you can open it in most 'real' cad programs or in Rhino. The stations (the cross-sections at the hull can be exported when you choose file>export>2D polylines.
With these, I went to inkscape (not a real CAD program yes, but it was good enough for my purposes). I drew some help-lines on the bottom layer, to help determine where the notches for the conduit tubes should go. I know now that it would have been easier to define as many hard edges on the design as you want stringers, and then use the resulting ridge as a guide for the tube locations.
A trick in inkscape is to determine the shape of your notch, duplicate it, select your station and then subtract the first from the last (Ctrl -).
Of course, if you don't want to spend much time on design, you can just take mine and perhaps adapt it to your materials circumstances.