Original challenge: build a canoe with a transparent skin in two days (two Saturdays, hence the '2xSat' name), for under NZ$300. Version 1 was not a total success in that the hull form was not very stable in the water. Version 2 will address this with a ground-up re-design...
The hull form of version 1 was flawed in that it did not put enough buoyancy at the points furthest from the centre-line. This resulted in a very 'tippy' hull.
Version 2 will fix this by focusing on a variation of a Dory hull form. This hull form has a number of advantages: it is much more stable (see image, right); it is formed from conic sections, which are easy to form using sheet building materials (e.g. plywood); it should be relatively cheap to build (which might significantly reduce the NZ$300 budget). I'm also going to use a mixture of plywood and polycarbonate for the hull (instead of an 'all polycarbonate' hull), as this will allow me to re-use some of the materials from version 1. Version 3 will return to the 'all polycarbonate' hull.
To help me design my hull I used OpenSCAD. I wrote a small OpenSCAD script that generates a parameterized hull form from three conic sections (2 sides + bottom). The hull dimensions are set at the top of the file, and you can tweak them to fit your needs. When I build this hull, the sides will be made from 6mm [¼"] plywood, and the bottom from the same polycarbonate sheeting I used in version 1.
To help fabricate the frames around which the hull will be formed, I used OpenSCAD and Inkscape to produce a set of router templates, and submitted them to Ponoko for laser-cutting. My templates are available as a free download. The cost to cut the templates (making + materials) was US$85.06 (+ tax) in August, 2013. Note that in order to keep the cutting costs down, each template represents only half of each frame (and will need to be mirror-flipped when tracing/cutting).
The final piece in the design puzzle was the shape of the 6mm [¼"] plywood side panels. I was keen to make an accurate template instead of taking a more empirical approach (e.g. laying cardboard onto the erected frames and tracing). I tried a few tools to help create this template (OpenSCAD, MeshLab, Pepakura, and even had a quick attempt via a friend's copy of SolidWorks), but there was enough variation in the outcomes that I decided to do the maths and spit out a mathematically accurate description of the flattened conic section. I got the resulting half-panel template CNC cut in MDF at my friendly, local Makerspace.
Update: I built a Dory Hull Calculator to allow you to design your own hull form, and produce a set of plans therefrom.
An important aside here was how cost effective the CNC cutting turned out to be (significantly more affordable than the last time I'd checked this out). For only a few tens-of-dollars more than getting the half-frame templates laser-cut I could have got the full set of frames CNC cut in 25mm [1"] plywood and saved myself about 5-hours of labour. It was too late for me, but I'd recommend this option to other makers. I've not created the required DXF, but I've got you started by laying out the full frames in SVG. If you go down this route, you'll need to decide on whether to follow my method to build the hull in two halves (and, if so, you still need to get the centre frame cut in two 12mm [½"] slices, rather than the full 25mm [1"] thickness (see below)).
Bill Of Materials (plus costs as at Oct, 2013)
Clear polycarbonate, 0.8mm [0.03"] -thick, 4m [13'] -long, from 1m [39"] -wide roll. Bunnings Warehouse SKU: 00314020. NZ$23.80/m x 4m = NZ$95.20. I'm going to re-use the skin from version 1 of my canoe.
Plywood: 1 x sheet 12mm [½"] treated ply. Used to fabricate the frames. NZ$106.00. Or, if you get the frames CNC'd, half a 25mm [1"] sheet will do you.
Plywood: 2 x sheet 6mm [¼"] marine ply. Used to fabricate the sides. NZ$46.00 per sheet.
Glue: this time around I'm going to use Aerodux (Resorcinol) for the gluing, and thickened epoxy for the filleting. I like working with the former as it has a far longer 'pot life' than the latter.
The original intention of this design/build was to replicate the "quick'n'dirty" approach used in version 1. Halfway through the build, however, I decided that the hull that was taking shape deserved more care and attention, so you'll notice that the speed drops and the level of finish rises about halfway through the process, below. If I'd not changed tack, the build would easily have come in under the 16 - 18 hours I'd allotted (although probably spread over three or four half-days, rather than the two full days used on version 1 - epoxy = waiting/curing time). As it was, upping the quality added about 50% onto the build time.
I built my hull with 4mm plywood, which resulted in a very 'limber' hull. As a result I had to fibreglass the outside to stiffen the hull (which you'll notice in the photos, below). As a result of this learning I can recommend using 6mm [¼"] plywood, especially if you're going "quick'n'dirty" (and these instructions now refer to using 6mm [¼"] ply). The 'glassed 4mm plywood did work, and makes for a lighter hull, but going down this route will impact the build time, and result in a less robust hull.
Stage 1: Prepare Parts
Each frame will be fabricated from two layers of 12mm [½"] plywood - in my experience, 12mm [½"] is about as thick as you can reasonably work with on a home router table. A double-layer, glued/screwed construction should also be stronger than a single 25mm [1"] layer. However, if you do elect to get the frames CNC cut from 25mm [1"] plywood, you can skip the first few steps and pick-up at the skin cutting step (and save yourself about 5-hours of labour)...
This process requires you to work over an unguarded router head. Follow all appropriate safety procedures, especially securing any loose clothing or hair.
Drill a Ø2mm [Ø⅛"] hole at each 'cross' etched onto each frame template - these will be used to 'register' templates on the plywood (using a nail through each hole) and keep everything aligned when you mirror-flip each half-frame template. Note that each template represents just over half of each frame, such that there is a good bit of overlap at the centre when you mirror-flip it. Trace each frame template onto 12mm [½"] plywood. If we number each template from bow to hull centre (i.e. narrowest to widest) you will need: four copies of frames #1 - #4, and two copies of frame #5 (the widest frame). I found that I could get everything onto a single sheet of plywood by cutting it across-ways into 7 x 320mm [13"] strips and laying the frames out as follows: #5 + #3; #5 + #3; #4 + #4; #4 + #4; #3 + #2 + #1; #3 + #2 + #1; #2 + #2 + #1 + #1.
Roughly cut out each frame (leaving a good 5mm [¼"] border outside the traced lines). Two of each frame size need to be shaped using your router + flush-cutting bit + template: screw each template to its frame; route the first half; mirror-flip and re-attach the template; route the second half. Screw/glue a 'rough' partner to each routed copy (frames #1 - #4 only). Note that we are not creating a double-thickness of frame #5 at this time - I'm planning to build the hull in two halves, so each half will get a slice of #5.
The last job for this stage is to cut out four copies of the side panel template (which I had CNC cut from 6mm MDF). When we cut timber on a router table, the router is underneath; the timber-to-be-cut is in the middle; and the template is on top. But when the timber-to-be-cut is large (as is the case here), it is easier to move the tool, not the timber. To do this, the template goes on the bottom; the timber-to-be-cut is in the middle; and the router is on top. Having the template on the bottom means it needs to be mounted onto a slightly smaller (rough) copy of itself so that is elevated above your workbench (otherwise the router head will drag on the workbench). I traced my template onto 12mm [½"] plywood, cut it out slightly undersized, and then screwed my template to it. Instead of rough-cutting my 6mm [¼"] plywood and then cleaning up with the router (as we did for the canoe frames), here we can use the router to do the cutting in one step. As you can see in the first photo, I free-cut some flaps onto the ends of my panel pieces - these will be used to hold the panel's shape during the build and will be cut off later. In the second photo you can see I drilled a Ø5.5mm [Ø¼"] holes every 5cm [2"] along the leading edge of the bow/stern - these will be used to 'stitch' the panels together during the build. One final thing I did here was use the top edge of the side panel template to cut four 5cm-wide strips of 6mm [¼"] plywood that match the curve of the side panel top - these will be used to reinforce the top edge of the side panels after they've been assembled.
Stage 2: Build
Once the frames have cured, you're ready to proceed to stage 2...
Use your router + flush-cutting bit to trim the rough portion of each frame. The end result is frame members with a 25mm [1"] -square cross-section. Note that several of the frames will have the flat crossbar cut out at a later step in the build - we'll keep them all for now to help keep the frames 'true and square'. Frames #1 and #2 need to be bevelled along the straight edges of one face so that the side panels will follow their shape better. Lightly sand all the frames to remove any burr. For extra aesthetic appeal, you could consider using a round-over router bit around the inside edges of each frame (I wish I had).
Now we need to erect the frames on 40cm [16"] centres, ready for the sides to go on. In order to keep everything on one workbench, I decided to build the hull in two halves, which will be joined later. Draw a centre-line, and then a line perpendicular to the centre-line every 40cm [16"] from one end. Use brackets to temporarily hold the frames in place (frame midpoints aligned to drawn lines). I also added a right-angle support to frame #1, and clamped a bar along the top of the frame set to hold all frames parallel to frame #1. Notice in the photo that I put my right-angle support facing the bow/stern - I can recommend doing this on the other face (i.e. facing towards frame #2) to make it easier to remove after the sides are glued in place. Also notice that I lined my workbench with baking paper to help keep the epoxy away.
Mix up a batch of thickened epoxy (I use West System products, and like their 406 Colloidal Silica Filler for it's dual gluing/filleting suitability). I like to work with no more than 150g resin + 30g hardener at a time - this should be plenty to glue up a half-hull, plus leave enough over to make a start on some filleting. 'Butter' all the frame edges with a good lot of thickened epoxy - lots of squeeze-out here is fine, as were going to fillet over it later. Starting from frame #5, lay each side in place and screw through to each frame, top and bottom (screws are temporary fixing, and will be removed after curing). Generously butter the inside of the leading edges of the bow/stern, press together and 'stitch' with cable-ties (cable-ties will be cut away after curing). Stand back and make sure everything is properly aligned (e.g. frames all meet the edge of the skin exactly). Use your epoxy mixing stick to smooth any squeeze-out (especially inside the bow/stern join). Use your epoxy mixing stick to begin filleting each frame/skin joint on both sides with any epoxy you have left over. In order to hold the shape around frame #5, I tensioned gaffer tape across the temporary flaps I'd free-cut onto the side panel ends (without this tension there is a danger of a discontinuity when the two half-hulls are joined) - see the bottom of the photo in the next step.
Once the first round of epoxy is cured you can remove all the temporary screws and cut away the cable-ties. Trim off the temporary flaps at frame #5. Mix up another batch (or two) of thickened epoxy and finish the filleting. Fill any screw and cable-tie holes, and build out any lumps/bumps in the leading edge of the bow/stern. I also glued a small triangle of ply forward of frame #1 at each end (this was oversized, and then trimmed back after the glue had cured).
Time to glue/screw the two halves together at the midline. You can also glue on the gunwale strips (cut in Stage 1, Step 3). I used a whole strip centred on the midline join, and a half strip at either end. I backed up the gunwale strip gluing with a screw at each frame, excluding both #4 frames (other stuff is going to be screwed here later). Select either frame #4 and nominate it as your future seat back-rest. Glue/screw a length of 25mm [1"] -square timber on the aft side of the top of this frame (just visible in the upper, middle of the right-most photo) - this is to strengthen the back-rest. You'll notice in the photos that I was forced to fibreglass the 4mm plywood I used to make my side panels - you won't need to do this if you go with the 6mm [¼"] plywood I've subsequently recommended.
Once the above has cured, give everything an extremely thorough sand, in preparation for painting. Now is the time to decide which crossbars you want to remove - I took out frames #2, #4 (not the one you're using for a back rest) and frame #5. I also chose to 'box in' forward of frames #1 to create a sealed buoyancy chamber at each end - this is not mandatory, and will add to the build time. When sanding, ensure all epoxy 'quiffs' are gone, and that every edge and surface is smooth . Now is also the time to install the seat plate (300mm x 825mm x 12mm [11.8" x 32.5" x ½"] plywood) - screw it securely across the middle three ribs (which have been notched for this purpose). Any exterior paint will suffice - my preference is for a darker colour inside to help reduce glare when you're out on the water.
The penultimate job is skinning the bottom with the polycarbonate sheeting. Roughly cut the sheet to size and then use screws+cup washers to join the skin to the ribs. Crop the skin flush to the edge of the hull and then use transparent, outdoor-grade tape to seal the skin/hull join. The idea is that the skin can replaced with relative ease should it become too scratched up. Lastly, attach any deck hardware you need - I put a pad-eye at each end, plus a pad-eye at each frame #4/gunwale junction - and pad the seat plate and backrest with some closed-cell foam. That's it - we're done!
The new design was significantly more stable than version 1. It has to be said, however, that my ninety-mumble-kilogram body was a little heavy for this hull (it was 'well down on its lines' in the water) - I'd say it would be better suited for a person of no more than 80kg [180 lbs]. All-in-all I'm pretty happy with this one...