Challenge: build a canoe with a transparent skin in two days (two Saturdays, hence the '2xSat' name), for under NZ$300. This project article documents the design and build process so that anyone can duplicate the project...
Please note: This project was a mixture of success and failure. The materials and construction method are highly 'fit for purpose' and work very well. However, the hull shape used was not a success - it is not very stable and has too much buoyancy in the bow and stern. The following article should still be highly instructive for any would-be maker, but care should be taken when selecting a hull form. Version 2 of this project will focus on a greatly improved hull form. See Sea Trials And Project Evaluation for more on this.
Ribs/keel: 4mm [⅛"] -thick marine plywood, cut into 40mm [1.6"] -wide strips and laminated (typically 4 layers, with an extra couple along the gunwales). The entire frame (excluding the bow/stern keel extensions) should be able to be cut from less than two sheets of ply.
Bow/stern keel extensions will be fabricated from solid timber.
Canoe dimensions will be approximately: 4m [13'] long; 67cm [26"] wide (at widest point); 33.5cm [13"] deep.
Seat will be a block of foam, shaped to fit snugly inside the hull.
In general, this is a quick-and-dirty project designed to get a cheap boat in the water as quickly as possible. It may involve gaffer tape. It will not win any beauty contests.
Hull Cross Section
Most of the plywood frame will be bent into whatever shape it takes to match the design. One exception is the ribs, which will be laminated into a specific shape over a forming jig/mold. Of the regular shapes on offer (see image, left) I chose the catenary curve. It lies midway between a sine curve (high form stability, low volume) and an ellipse (low form stability, high volume) (see here for more about this design aspect). A further advantage of the catenary curve is that is the easiest curve (after a circle) to construct on a large scale - it is the shape taken up by a hanging chain. To mark out my rib jig/mold, I will use 96cm [38"] (i.e. a little less than the width of the polycarbonate) of fine chain and slide things around until I get a curve that is twice as wide as it is deep*. The jig/mold has a fractionally smaller 'arc length' as the lamination layers will 'fatten' the ribs (out to a 'arc length' of about 102cm [40"])
Plywood: 1 x sheet 4mm [⅛"] 'standard' marine ply. Used to fabricate the gunwales and keel (excluding bow/stern). NZ$46.00.
Plywood: 1 x sheet 4mm [⅛"] 'flexi' marine ply (you'll only use about a third of this sheet). Used to fabricate the ribs. NZ$63.25. A second sheet of 'standard' marine ply could be used instead, but you'll probably need to steam bend it to make the ribs.
1.5m x 65mm x 32mm [5' x 3" x 1½"] dressed, clear, treated (H3) timber. NZ$25.30. Used to fabricate bow/stern keel extensions.
1.4m x 45mm x 45mm [4½' x 2" x 2"] dressed, clear, treated (H3) timber. NZ$5.54. Used to fabricate thwarts.
A solid bench to work on (including screwing jigs/molds to). Alternatively construct a 'strong back' out of a plank and saw horses. Your bench/'strong back' should be between 1m [39"] and 2.4m [8'] -long (note that the bow and stern will bend down below the level of the bench/'strong back'). As the canoe is 4m [13'] long, ensure your bench/'strong back' is located somewhere with enough space.
96cm [38"] fine chain, e.g. bath chain (to define jig/mold shape).
12 x 50mm [2"] brackets to secure jigs/molds to bench/'strong back'.
As many clamps as you can lay your hands on. To be able to clamp the frame lamination every 20cm [8"], you're going to need at least 50 clamps - that's a lot of clamps (more than I've got, anyway). If you can't beg/borrow/steal that many clamps, you can do the lamination process in phases (detailed below), but bear in mind that this will impact your timetable.
A good, sharp scraper (for removing excess glue, especially if you're using a polyurethane wood glue).
Optional: router + flush-cutting bit (for shaping jigs/molds).
We need three copies of our rib jig/mold. The expensive-but-most-accurate method would involve a laser-cutter. The quickest-but-least-accurate method would only involve a jigsaw (i.e. no router). My method falls somewhere in between those two options. Choose a method that suits your budget and timetable...
This process requires you to work over an unguarded router head. Follow all appropriate safety procedures, especially securing any loose clothing or hair.
Use your 96cm [38"] of chain to plot a catenary curve that is approximately twice as wide as it is high (about 64cm [25"] wide by 32cm [13"] high). I'd recommend plotting this onto some cardboard for use as a template. Make sure whatever you're plotting onto is plumb and level. Transfer the curve to you MDF ensuring your leave a 15mm [⅝"] 'shoulder' either side of the widest part of the curve, and a 10cm [4"] deep 'footer' (see photos in steps 4 & 5). Once the jigs are erected, the 'footer' will elevate the curve above your table, and the 'shoulders' will facilitate positioning the gunwales.
Use a jigsaw to accurately cut out a 'master' jig/mold. Trace the 'master' onto MDF five more times. Roughly cut out the five copies (leaving a good 5mm [¼"] border outside the traced line). Screw each copy to the master in turn, and use your router to ensure each copy is a perfect duplicate of the master.
Each of the three pairs of jig/mold faces cut so far needs to have a third layer of material between them to give a total jig/mold thickness of 36mm [1.4"]. You are unlikely to be able to get three more whole copies out a single sheet of MDF. Instead, screw/glue scrap pieces of MDF around the edge of three whole faces and then attach a second whole face (i.e. scrap is sandwiched between a pair of whole faces). Ensure the whole faces are accurately aligned with each other, especially along the base.
Once the glue has cured, roughly trim the scrap to size and finish on the router. Route each jig/mold on both sides to ensure all the layers match each other.
Cut clamping holes as shown. I used a hole saw, but this does not play nice with multiple layers of glued MDF - use a jigsaw instead (the hole shape is not critical, just the hole location). Finally, cover the edge of each jig/mold with masking tape to help stop gluing the ribs to the jig/mold. It is also a good idea to mark a centre-line from the base of the jig/mold to the mid-point on the curve - this will aid in erecting the jigs/molds, and in positioning the plywood strips.
Day 1: Timber Frame Lamination
Ready, set, go!
The internal timber frame is made up of four laminated layers of plywood. The diagram on the right describes each layer, as viewed from above. The green segments represent 'standard' ply (gunwales/keel), and the blue segments represent 'flexi' ply (ribs). The outside/4m [13'] green runs are the gunwales, and the central/3m [9.8'] green run is the flat part of the keel (i.e. excludes bow/stern). The dimensions for each component are listed (a cut list is included in step 2, below). Components without dimensions are duplicates of a symmetrical part elsewhere in the diagram. Some dimensions include "+" or "++" - this indicates a component that lengthens slightly as the laminate is built up (e.g. the innermost rib segment is 96cm [38"] in length, but the outermost will be around 101cm [40"]). When cutting ply strips to length, ensure you allow for any component lengthening - "+" indicates allow a centimetre or two, "++" indicates allow as much as you can. The extremities of the gunwales and keel will be trimmed to length after the glue is dry.
Note that you will also need a total of around 16m [52'] (about 7 strips) of 'standard' ply to complete the gunwales on Day 2 - this can be partially made up of any offcuts ('standard' or 'flexi') from Day 1, if required.
Cut the full sheet of 'standard' marine ply and about a third of the 'flexi' marine ply lengthwise into 240cm [8'] -long, 40mm [1.6"] -wide strips. Keep the two types of ply in separate piles. You should be able to get 28 or 29 strips from the full sheet and will need another 8 or 9 from the 'flexi' ply sheet, to give a total of 37 strips. Both sets include a couple of spares in case you damage any during the lamination process.
Ply strip cut list (refer to lamination plan, above):
6 x [ 240 cm (uncut) ]
8 x [ 158+ cm / 80+ cm ] (share offcut between pieces)
2 x [ 108++ cm / 108++ cm ] (share offcut between pieces)
2 x [ 36 cm / 36 cm / 36 cm / 36 cm / 36 cm / 36 cm ]
1 x p[ 30++ cm / 30++ cm / 30++ cm / 30++ cm ] (share some offcut between pieces)
Total 'standard' ply strips = 19, cut into 42 components (plus offcuts).
3 x [ 96+ cm / 96+ cm ] (share offcut between pieces)
2 x [ 42+ cm / 42+ cm / 42+ cm / 42+ cm / 42+ cm ] (share offcut between pieces)
1 x [ 42+ cm / 42+ cm ] (share some offcut between pieces)
Total 'flexi' ply strips = 6, cut into 18 components (plus offcuts).
Erect the rib jigs/molds along your bench/'strong back', with centres 40cm [16"] apart. Ensure the jigs/molds are perfectly aligned with each other (centre lines) and at right-angles to the edge of your bench/'strong back'. I used pairs of 50mm [2"] brackets on both sides of each jig/mold. I'm also planning to 'clamp' the layers of lamination with compression straps (ratchet trailer straps), so I've attached chain plates to my bench beside each jig/mold (see photo).
Time to stick the frame together. Rather than laying down each lamination layer in its entirety, I found it easier to: begin by laying up layers 1 - 3 of the ribs (blue, above) + the 240cm longitudinal members (green, above); then go back and fill in the rest of layers 1 - 3 of the longitudinal members; and finish up by adding the whole of layer 4. If you're short of clamps, each of these three phases can be left to cure before moving onto the next (although this approach will hurt your timetable). I also found that using screws to temporarily secure the layers on either side of any join helped keep everything aligned (and saved a few clamps). Note that it is expected that the bow/stern ends of the gunwales will droop down, and that this is by design - they need to end up 50cm [20"] 'below' the keel strip. If the weight of the clamps causes too much droop, support the ends at the right level of droop until the glue has cured. Note that the keel strip should not droop - support it while the glue cures, as required.
Once the frame laminations are all layered up and curing, take a moment to locate the lengthwise centre point on each gunwale (corresponds to the midpoint on the middle rib). Measure 1995mm along each gunwale from this centre, towards the bow/stern and mark this point on all four gunwale 'arms'. Use your jigsaw to cut a taper in the end of each gunwale (blue in the figure on the right). Collect some ply offcuts and glue/screw the gunwale ends together with a piece of ply (red bit) sandwiched between the ends. Glue/screw a pair of ply offcuts to the underside on the gunwale join (green bits). We will trim the latter to shape once it's all cured.
Our last job for the day is to cut and glue the bow/stern timber. Use you circular/table saw to cut your 1.8m x 65mm x 32mm [6' x 3" x 1½"] at 45° lengthwise to give the green and (pale) blue pieces in the figure on the right. Flip the smaller piece and glue it to the larger piece as shown. Next weekend we will make the 22.5° cut (red) that will create the bow/stern pieces (bright coloured bit - the dark coloured bit will be discarded).
Today is done - time for a beer!
If you're looking for something to do before next weekend, I can highly recommend 'stealing a march' on scraping and sanding the frame. It is also a good time to check that the frame can be separated from the jigs/molds (even if this requires a hammer). Also, if you want your new canoe to last more than a season, now would be a good time to paint the frame and bow/stern pieces.
With regards to painting: any old exterior house paint will do (in fact, many DIY boat-builders will tell you that house paint outperforms the fanciest of marine paints!*); and leave the last 20cm [8"] of the gunwale ends and the inside face of the keel unpainted, as we have more gluing to do in these areas.
*John Welsford, a respected wooden boat designer/builder, had this to say about paint: "I do have some test panels of plywood painted in different systems and paint types nailed to stakes in an estuary where they are covered by the tide and exposed to the weather, and have been following the deterioration and weathering for 8 years now [in Feb, 2012]. Some of the top-end two-part paints have not done as well as you'd expect from the price tags, and the best at this stage is a very common oil-based house and porch paint."
Day 2: Bow, Stern & Skin
Our first job is to finish shaping the bow/stern pieces. Begin by cross-cutting the piece you glued up in 'Day 1: Step 6' into two 75cm [30"] pieces. Follow this with the lengthwise cut (red line in figure in 'Day 1: Step 6'). To make this 'inside' cut, the 45° edge needs to ride against the saw fence at 25mm above the saw table - I used a narrow length of scrap to hold the 45° edge at the correct height (see first photo on the right). These bow/stern pieces now need to be cut, as per the diagram on the right.
Locate the lengthwise centre point on the keel (corresponds to the midpoint on the middle rib). Measure 150cm along the keel from this centre, towards the bow/stern and truncate both keel 'arms' at this point (to give a total keel length of 300cm). Next we need to join the bow/stern pieces to the keel and gunwales. Glue/screw the keel-to-bow/stern piece (see photo). I used three long screws. The glue is not structural - its more of a bedding compound to stop things wiggling (and as such we don't need it to cure before proceeding). The gunwale-to-bow/stern piece join is made with three short lengths of metal strapping (TimberLok or similar). Again, glue acts as a bedding compound. Shape (taper) the keel to match the bow/stern piece.
The frame is now complete. Retouch any paint, as required.
Time to skin this baby! Begin by marking up your sheet of polycarbonate: the lengthwise centreline (aligns with keel); the athwart centreline (aligns with middle rib); athwart 40cm on either side of the athwart centreline (aligns with outer ribs). I also marked fixing points at 10cm centres along and between the ribs and at 15cm centres outside the ribs. Drape the skin over the frame, ensuring each line marked of the skin is sitting at the correct point on the frame. Begin screwing (6G pan-head screw + nickel/brass cup washer) along the keel, from the middle rib outwards. Stop when you get to within 20cm of the bow/stern. Return to the middle of the hull and screw down each rib (do not screw to the gunwale). Use a staple gun to tack the skin along the gunwale. Begin by tacking the skin between the ribs and then shift to working in from the bow/stern. On my hull I needed to introduce a small 'nip & tuck' on each outermost rib to ensure the skin lay correctly (see photo). Cover each 'nip & tuck' with waterproof gaffer tape.
To complete the skinning we need to secure the skin to the bow/stern pieces. Unless you've got very lucky, there will be a bulge in the skin where the keel joins the bow/stern pieces - this is fine, and simply requires another (if more complex) 'nip & tuck' (see diagram and photos, right). Begin by trimming the skin to match the bow/stern pieces. I cut one flap oversized in order to lap the skin join. I'm not sure this was worth it, as it was very difficult to fold this flap. Screw the skin from the gunwales up to within about 15cm of the keel. Cut/drill the 'nip & tuck' as per the diagram. Rivet (or bolt) the skin flaps and then cover everything with waterproof gaffer tape. Skin is done!
Now we need to laminate on two more gunwale strips. The inside strip isn't glued (it won't stick to the polycarbonate), but the outside strip is glued to the inside strip. Screw (6G pan-head) the new strips to the frame. Trim the bow/stern (as per 'Day 1: Step 5').
We pause now whilst the gunwales cure for a bit. Grab some lunch. If you're painting your frame, now might be as good a time as any to do some touching up and/or paint the new gunwale strips.
While the gunwales finish curing we'll fabricate a pair of thwarts. Cut your 1.4m x 45mm x 45mm [4½' x 2" x 2"] dressed, clear, treated (H3) timber into two 64cm [25"] lengths (to match the span across the inside of the ribs). Cut these square across for now (to make them easier to work with) - each will be tapered to match the inward tilt of the gunwales in a minute. Drill a Ø6.5mm [¼"] hole into the centre of each thwart end. Use a spade bit to drill a Ø19mm [¾"] hole into each thwart end, perpendicular to your Ø6.5mm [¼"] holes. Square the outermost side of the Ø19mm [¾"] holes so the washer+nut set lies flush in these holes. Now taper the ends of each thwart to match the ribs. Finally, drill a Ø6.5mm [¼"] hole through the gunwales where they meet the outer ribs (the thwarts will be attached by passing a bolt through the gunwale and then into each thwart end and secured with a washer+nut set seated into the 19mm [¾"] hole).
Time to remove the hull from the jigs/molds and flip the hull the right way up. We're now going to add one more strip to the inside of the keel (glue + screw) and attach the thwarts. Attaching the thwarts is fiddly work as there is not much space to manouver - at least you can see what you're doing, though (the benefits of a transparent skin). I can recommend using a hex spanner to hold the nut, and keeping the nut in the spanner with a small square of masking tape whilst you screw the eye-bolts from the outside.
The last step involves making a seat. This needs to be as low as possible in the hull (to keep the centre-of-gravity as low as possible). The initial design specified a seat spanning only the rear pair of ribs, but I decided to span all three ribs so my daughter could sit in front of me (probably adds strength/stiffness, too). I used one of the MDF molds to trace and cut-out/rout three small support blocks (these sit between the flat of the seat and the curve of the ribs). The seat itself is a rectangle of 12mm [½"] treated ply, covered with a layer of thin, closed-cell foam (after the seat is installed). Thin, closed-cell foam plus gaffer tape wraps the rear thwart to create a back rest.
All done - time for another beer!
Sea Trials And Project Evaluation
As stated at the top of this article, this project was a mixture of success and failure. The materials and construction method are highly 'fit for purpose' and work very well. However, the hull shape used was not a success - it is not very stable and has too much buoyancy in the bow and stern.
The poor hull form is predominantly a mistake on my part - the 'twice-as-wide-as-it-is-deep' demi-curve is pretty close to the circular cross-section I was trying to avoid. In hindsight, this is an embarrassing error. The ratio of width-to-depth should be more like 2½ - or 3-to-1. Also the pure catenary curve puts too little buoyancy at the points furthest from the centre of the curve.
A second cause of the poor hull form is a result of how the skin constrains the design. Most canoes have a skin that can not be flattened out - their shape is not formed by curving a planar surface. The polycarbonate sheeting, by contrast, is a planar surface, and our hull must be formed with this constraint. Nips-and-tucks are the only way of modifying this, but need to be used sparingly to ensure a watertight hull. My hull form resulted in too much buoyancy in the hull and stern, which detracted from the hull's righting moment, further destabilising the canoe.
Version 2 will focus on a greatly improved hull form whilst using the same materials and construction method. Stay tuned...