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.
Step 2: Prepare for Cutting
After the initial design, you need to conform your drawing to your cutting method. What routing bit will you be using? It pays off to modify the shapes of your slots in such a way that a square piece will precisely. You can do this by extending the corners with a little notch. This will remain visible on the outside as two part-circles, but your slot will fit without trouble.
For generating the g-code I used Deskproto, because that is part of the toolchain we use at protospace. Any other g-code generator probably works just as good.Once you get to the gcode stage: make sure you drill your holes and insides cut first. Also make sure that you have two or three screw holes in each separate piece that you can use to fixate it on the router.
Step 3: Assembly
So, did you make sure you know for each station where it goes exactly? I did not, and mixed up two stations while building. Please, either mark the station in the file, or keep a computer handy to mark each station while it is still on the router.
For the prototype, I did not create a strongback for building and, with hindsight, I really should have. I think it pays off to have a dedicated support structure for it that you can assemble and move around your boat on while you are building it.
Otherwise just start with putting the floor on something like a bench or a stool. make sure you allow for the rise of the keel + floor aft. now put the keelsom part on it and fix it by putting one middle station in its slot. Put the rest of the stations in their slots and move it around a little until you are satisfied about the trueness of it all. Before you commit to it, check the fairness by inserting a tube in the notches nearest to the gunwhale. If it snakes around, you have the stations swapped and you should correct that before continuing (Yes I know, I was in a hurry with the prototype and regretted not correcting it).
Attach the stations to the floor with some hotglue. The idea is to fix their position while you insert the tubes and turn the boat, it is not intended as final fixation. If your deck is slotted to accept the stations, install them now, to make sure things are aligned well. Then comes the trick: installing the conduit tubes. With a rubber mallet and a firm tap, you can just click them into their slot. try to do this with one good whack; I believe you weaken the tube too much if you keep hammering it. make sure you alternate between sides when installing the tubes, to avoid building a skewed boat. A later version of the design will feature slots in the stations, floor and deck to ease truing up the boat... When they are all in, you can turn the boat and install the inner tubes (there is four of them on each side) and the decks. Also fix the floor to the keelsom with screws.
Step 4: Skinning
With the right materials, skinning a boat is easy. You can look at this tutorial at gaboats.com. What I did was slightly different, and had possibly less good results.
Since a rowboat has a wider hull than a canoe or kajak, skinning it with one piece of fabric will mean you have to make some folds or seams to acomodate the curves. Polyester fabric will shrink about 10%, so you can leave some wrinkles before shrinking.
I started a the middle section of the boat gluing the fabric with contact glue to the edge and topside of the deck. At stations 2 and 6 I made a seam to take in some excess fabric. The bow needed its own section of fabric, partly because the length of my fabric was not sufficient. So the bow section got one big seam running along station 1, and a part seam around the round part of the bow. I glued the fabric with 'Pattex tranparant' contact cement, which makes the strongest bond in tests I did previously. stresses on the fabric are not that high, and the sheer strength is quite high, so I think that any contact cement that is waterproof and adheres to the polyester will do.
Once all the fabric had been glue to the sides, I heat-shrunk the fabric with a ho-air gun at the medium setting. GABoats recommends against using a hot-air gun, as it has no constant temperature. I found it to be quite workable, though you need to keep the gun moving: I had to patch one small hole above the waterline. ;)
As for strength: yes the fabric is strong enough to keep the water out. I have rowed around in the boat for 45 mins with two persons in it, and it had only minimal water in it. The fabric is not resistant to scraping though: at lift out we touched the side of the boat and tore the fabric where it ran over the stringers :( So you will have to add some kind of rubrails there, or be very careful when lifting her out.