Building a Child Sized Kayak from a Single Sheet of Plywood

What I got back from the CNC shop was a set of six panels for the boat proper, pre-drilled with stitch holes and a few other parts to make the coaming. The Okoume is a pretty enough wood, but maybe a little boring for a kid. I’m not that in touch with what 5-year-olds like in colors, but I figured a purplish-red “Cordovan Mahogany” stain that I had on hand would at least provide a bit of pop.

The Okoume comes with a pretty smoothly sanded surface, but I gave it a light sanding with 120 grit sandpaper to improve the finish. I went with an alcohol based stain that is also formulated to be light-fast with minimal fading in the sun. An oil based stains could have caused epoxy adhesion problems later.

Unlike a slow drying oil based stain, the alcohol based stain dries extremely quickly. This means I can’t just wipe it on and wipe off the excess later. Whatever I apply to the wood is what will be there forever. I worked with a very wet rag and spread the stain quickly. As the stain-soaked rag dries out, I poured more stain directly into the rag and keep moving the wet stain across the surface of the wood, striving to keep the coverage uniform. A second coat can help even out the surface, or sometimes I’ll wipe it down with denatured alcohol to spread the stain around.

I’ve found that any rough handling during the making of the kayak can mess up the thin stain layer. A quick coat of epoxy over the stain protects it from the vagaries of my carelessness. The sanding, staining, and epoxy coating took about half an hour, and then I let the epoxy cure overnight.

The first thing to assemble was the hull. I started by placing the two side panels face-to-face and pushed lengths of copper wire through the pre-drilled holes at the end of the side panels. A quick twist of the wires serves to stitch the two panels’ ends together. With the ends stitched together I open up the two panels so I could drop the bottom in between. All of a sudden you can see the makings of a boat.

By snaking wire through the holes in one side panel into the corresponding hole in the bottom panel I started to secure the seam between the two panels. The copper wire is soft and easy to bend into a quick twist to lock the panels together. Some people make the mistake of thinking they can twist the wires to tighten the seam, but this usually just results in broken wires. Instead I pull the seam tight by pulling the wire. A quick twist serves to lock the tightened seam in place.

To start the deck, I booked together the two front pieces and stitched them up along their common centerline. With the wires twisted together I was now able to open up the book and nearly flatten out the two panels. These panels were then stitched to the back deck piece.

I now have the two basic pieces of the little kayak, the hull, from 3 panels and the deck, from another 3 pieces. I notice that when I put the deck on the hull, the hull is a bit narrow and needs to be spread out a bit in the middle to match the deck. I cut a couple scraps of wood to brace the hull out to mimic the deck width. The process of wiring together all these pieces took about 30 minutes. So the total working time so far, not including epoxy cure time, is about 1 hour.

Stitch and glue boat construction like this depends on the geometry of the panels fitting together to define the 3 dimensional shape of the finished boat. Assuming the panels are cut accurately, as long as all the seams fit together snugly, the boat should end up being the desired shape.

I chose to run a few quick stitches between the deck and hull at the cockpit to hold everything in alignment while I start the gluing part of “Stitch and Glue” construction.

Originally, I picked up a bottle of cyanoacrylate (CA) glue and matching accelerant at a woodworking store just on a whim. I didn’t know what it was good for, but it sounded like it could be useful. It stayed on my shelf for awhile until I was building an earlier stitch and glue design and wanted a quick joint. A dot of glue and a quick spritz with accelerant, and I was off to the next thing. Since then it has become a staple in my shop.

CA glue is not the strongest or most durable glue available. I would not use it for many structural uses, but for a quick “spot weld” while creating the structural attachment with other glue to cure, CA glue makes an extremely useful adhesive.

I had done the “stitching” part of stitch and glue, I now started the “gluing” part with CA glue along the seams where the wires were. With several small dots of CA glue between each wire stitch, I quickly bonded the seam together. A light mist of the glue accelerant hardened the glue within a few seconds. Since the boat is so small, I was able to reach almost all the long seams on the hull through the cockpit.

I then clipped the stitches holding the deck to the hull. With the deck upside down, I put CA glue spot welds along the foredeck center seam and on the V shaped seam between the aft deck and fore deck. I also finished off CA-gluing the stem seams at the very bow and stern of the hull. A spritz with accelerant kicked off the glue and allowed me to remove all the wire stitches.

I used a pair of wire cutters to clip the wires on the inside and then pull the twisted end out from the outside. The wires only served as temporary clamps to hold everything together until I could get some glue on it. At this point all that is holding everything together is the small CA glue spot-welds. The boat can be safely lifted and moved around, but is still a bit fragile. A sharp knock could break the spot-welds.

The sharp angle joints I have between the panels at this point are inherently a bit weak. Forces on materials don’t like going around sharp corners, they create stress risers that concentrate loads around the corner. This makes them more susceptible to breakage. What I need to do is somehow make a rounded transition between the panels.

This is accomplished with “dookie schmutz.” This mixture of epoxy and wood “flour” is the consistency of peanut butter and allows me to do liquid joinery. By squeezing a bead of schmutz into a seam and shaping it with a rounded object like a plastic spoon, I make a smooth structural fillet between the two pieces that transitions forces smoothly between the pieces.

Wood flour is fine grade of sawdust similar to sanding dust, but you could actually use bread flour. I first mix up a batch of epoxy before adding the wood flour. I add enough powder to make a putty that holds its form, but is not so stiff that it can’t be squeezed into place.

The best way I have found to distribute this schmutz into the seam is to scoop a batch into a 1 gallon ziplock bag, then cut one corner off the bag. This can then be used like a cake piping bag to inject a bead of the gunk along the seam.

After dispensing a ¼” bead into the seam I pressed and dragged a plastic picnic spoon down the seam. The radius of the back of the spoon does a nice job of forming a smooth fillet. I try to drag the full length of the seam in one go, starting and stopping only makes a mess. I clean up any excess that got squeezed out with a plastic squeegee and recycled it back into my mixing container to be added back into the ziplock bag as needed. The fine ends of the hull are hard to reach with the spoon, so I just dragged my glove covered finger up the seam. I repeated the dispensing and spooning process on the deck seams.

While the fillet was still wet I draped 4oz fiberglass cloth into the interior. It is desirable to lay the glass in while the dookie schmutz in the seams is still wet as this avoids the need to do any sanding after the fillet cures, but since the glop is still soft, I need to be careful not to poke at it and mess up the fillets.

The secret to getting smooth layup on the interior is to get the cloth lying smoothly before adding any epoxy. Any bunching or wrinkles existing in the dry cloth are even harder to get out when there is sticky epoxy everywhere. I used a dry chip brush to smooth out the wrinkles.

I mixed up about 8oz of epoxy. Epoxy needs to be thoroughly mixed in order to cure properly. Once it is mixed it starts to cure. Since it is an exothermic reaction, it will put off heat. If mixed epoxy is filling up a large container the heat it generates will accelerate the cure, which means more heat, which means more acceleration. By mixing small batches and using them quickly, I eliminate the chance of a run away reaction. Once the epoxy is spread out into a thin film, the cure slows down substantially so I had time to work.

I just pour the mixed resin directly in the bottom of the kayak. I then use a plastic applicator/squeegee to spread the puddle around. I first bring some up the sides on both sides and then start working down the length. I attempt to maintain a straight, uniform front of epoxy moving from the middle of the boat towards the ends. Any remaining wrinkles get pulled from the wet epoxy towards the dry areas of cloth where they are easily disbursed.

When the first batch of epoxy had been fully distributed I slit the glass near the ends so it would conform into the stems. I trimmed it so there was a bit of overlap on either side of the stems. While I could have made these cuts before starting with the epoxy, by leaving it until I got some of the epoxy down I didn’t have to worry about the glass shifting and messing up the alignment with the stems.

I mixed up another small batch of epoxy and instead of just doing the dump-and-spread distribution, I used a chip brush to apply dollops of resin onto the fiberglass, then used the squeegee to further spread the resin. I needed to be a bit careful brushing the epoxy on to the trimmed glass at the end. Overworking the raw edge may dislodge strands and cause a tangle.

After fully saturating the fiberglass, there is almost always a bit more resin in the cloth than I need. This excess just adds weight without improving strength. I scraped off the excess into a grunge cup made from a paper cup with a slit on the top edge. Holding the squeegee at about a 45° angle to the surface and moderate pressure I scraped the glass, picking up excess resin. I then ran the blade of the squeegee through the cup to remove the resin.

This scraping shifted the cloth very slightly, which is OK, but in the “chine” between the bottom and the sides, it created a little bridge of cloth. This looks like a bubble of trapped air over the fillet. Since this is just a sign that there is insufficient fabric to lay down on the fillet, the solution is to just push more cloth down the side with a brush until the bridging cloth conforms to the fillet again. If I had tried jamming a squeegee down into the bridge, I would have only messed up the still soft fillet and the bridge would quickly reappear. Bridges like this are almost inevitable on the interior of a kayak, and by basically ignoring them until near the end of the epoxying process, I am able to move quickly and then rectify the problem just once instead of fighting them every time they appear. When I finished the hull, I propped a few sticks across the top to hold it at the right width while it cured.

As soon as I finished the hull, I moved onto the deck. Since it doesn’t have the deep contours, the fiberglassing goes easily with minimal bridging to contend with. The whole process of filleting through fiberglassing took about an hour. I left it to cure over night.

The next day I attached the deck to the hull. I started by just strapping the deck down with filament style packing tape. I stuck the tape to the deck, then pulled it tight down onto the hull, aligning the edge of the deck with the top of the hull. I end up with tape pieces every 6 inches or so. Once the deck was securely positioned, I ran a strip of masking tape down the length of the seam to seal it up. I was careful not to allow too many wrinkles in this strip as they could cause a mess later.

I then tipped the kayak on one side and measured a length of pre-woven 3” wide fiberglass cloth tape down the length of the deck-hull seam, cutting it a couple inches short of each end, making a little reference Sharpie mark next to the knuckle in the sheer line for reference. I laid this strip of tape down on sheet of waxed paper and slathered on some mixed epoxy with a chip brush to completely saturate the strip. I also brushed some epoxy on the lower seam between the deck and hull on the inside of the boat.

By rolling up the ends of the pre-saturated tape towards the Sharpie mark, I was able to easily move the tape to the inside of the kayak, laying it in place overlapping the deck/hull seam with the reference mark aligned with the sheer knuckle. Then it was just a matter of unrolling the tape on to the seam. Using the chip-brush I worked the tape down into the corner between the deck and hull, smoothing out any wrinkles or bubbles. I left this to cure for a few hours so it wouldn’t fall off when I flipped the kayak to the other side to similarly tape the other seam. Each side took about 20 minutes with a few hours break in between.

The next day when the epoxy had cured, I stripped off the masking and packing tape. At this point the long seams between all the panels were fairly sharp. Fiberglass does not wrap around sharp angles, and sharp angles are delicate and weak in the finished kayak, so they need to be rounded over. I started easing the edges by making a 1/4” wide chamfer with a block plane. Once I have a consistent chamfer down the full length, I knock the corners of the chamfer to create the beginnings of a round-over. A couple more passes with the plane completes the radius. When I can run my hand over the corner and don’t feel any sharp edges, it is good.

The epoxy seal coat over the stain has had a chance to set up hard by now so it really should be scuffed up with 120 grit sandpaper to take the gloss off. At the same time as I sanded all the flat surfaces, I tuned up the round overs at the corners.

Because the planing and sanding cut into fresh wood and removed the stain, I used a cotton swab as a brush to touch up the bare wood with stain. Again, I keep the swab wet and move quickly to get a nice even coverage.

The next step was glassing the outside of the hull. To keep drips under control I ran some masking tape on the deck following the sheer line and then turned the boat upside down. Laying glass on the outside is easier than the inside, you can just roll it out and then brush down the wrinkles. I trim of the excess on the sides, but leave about 4″ hanging to help sop up any excess resin that may want to drip on the floor.

I again mixed up a fairly small batch and poured it directly on the flat bottom and used the squeegee to distribute it across the bottom and down the sides by picking up a bit of resin with the blade of the squeegee held at about a 45° angle and pulling it across the cloth. As I turn the corner at the chine, I rotate my wrist to maintain the 45° angle down the side.

When I reached the ends of the kayak the cloth needed some trimming to fit the stems. I trimmed the cloth about 1″ longer than the stem and cut a dart up to the knuckle where the stem meets the bottom. I was then able to gently lift the cloth off one side, wrap the excess from the other side around the stem and the lay the lifted side back down before wrapping it around to the far side. This left a little tuft of strands sticking up at the knuckle, but there is really no way around it. I’ll clean it up later and wrap patch of glass over it. Once the fabric was fully wet out, I came back with a squeegee to scrape off the excess like I did on the interior. From stripping the strapping tape off to completely scraping the excess resin took about 1½ hours.

After a few hours I peeled off the sheer line masking tape so it wouldn’t be stuck in hard epoxy. When the epoxy had dried to the touch I came back and trimmed off the excess glass along the sheer using a utility knife. This left a bit of an edge that I sanded down and I touched up the stain that I messed up again.

With the kayak right side up, I ran a strip of masking tape 1″ down from the sheer and then rolled out fiberglass over the top. This glass got trimmed to overlap onto the tape about half an inch. This glass got wetted out in the same way the hull was done. I allowed the epoxy to cure for a few hours and then used a sharp utility knife to lightly score the new glass just above the masking tape. When I peeled up the tape, the excess fiberglass on the tape came up with it.

Properly saturated fiberglass cloth should end up with a distinct weave texture where there is only enough resin to completely wet the fabric, but not so much that it is left shiny. Too little resin and the layup will lose strength, too much and it will gain weight. The problem is the best finished surface is smooth and shiny. To achieve this, I brushed on another coat of epoxy over the whole boat. This doesn’t take long, but needs to be done in stages, top first, then the bottom. I applied the resin with a chip brush and spent time making sure there was a consistent film thickness over the whole kayak. I masked at the sheer line to keep drips from running into undesirable places.

The coaming always presents a problem. It is a bit of a complex shape, this ring of wood that is raised off the deck. On a full size kayak this is where you would attach a spray skirt to keep water out of the cockpit. It really isn’t necessary on a kids kayak, but it does serve to reinforce the hole in the deck that comprises the cockpit. It consists of two basic parts, a riser and a lip. The riser gets the lip up off the deck so the spray skirt can be secured around the lip.

The cockpit is sort of egg shaped with the coaming lip being a matching shaped ring of plywood about 1″ wide. The easiest riser solution is just to make a stack of ½” wide rings with the same inner diameter and place the lip on top of the stack. But these rings are all ovals that chew up a lot of plywood if you try to make them out of one piece. Another riser solution is to use one long strip of wood that is bent around into an egg shape. This system is more efficient with wood and is what I chose to do, but it is the PMLB (Piece Most Likely to Break) because it requires a pretty tight bend.

Since the PMLB needed to be longer than the width of a sheet of plywood, I had it cut in two pieces that I CA glued together. I stuck a layer of filament strapping to what was to be the outside diameter of the coaming. This helped prevent splitting and breaking as I bent the wood into a circle and joined the ends. I then distorted this circle into the cockpit egg shape and wedged it in the cockpit hole. Spot welds of CA glue around the perimeter held it in place while I wedged the coaming lip on to the top of the riser. This was similarly spot welded.

I reinforced the coaming with 4″ wide strips of fiberglass “cut on the bias”, or at a 45° angle to the weave of the cloth. The angle of the weave allows the cloth to distort easily around complex shapes. The first step was to brush some epoxy on the coaming. This allowed the strip of cloth to stick in place as I carefully wrapped it around the inner circumference of the riser. I could then brush the top out over the lip and the bottom down under the deck. With that saturated in place I then reinforced the underside of the lip by tucking a bias cut strip in between the lip and deck and carefully brushing it smooth with epoxy resin. The coaming is a bit tricky and constituted about 3 hours work, but it was very solid when complete.

After the epoxy cured I trimmed off the excess glass around the perimeter of the coaming and gave the whole kayak another fill coat of epoxy resin.

The trick to creating a smooth finish is all in the prep work. This means sanding. I start out with very coarse 60 grit sandpaper with the goal of leveling the surface of the epoxy. The idea is to make a flat smooth surface where the gloss of the bare epoxy is gone, but without actually sanding into the fiberglass itself. With a couple fill coats of epoxy I was able to quickly get a smooth surface using a random orbital sander. Once I got it flat with 60 grit I moved on to 120, then 220 grit sandpaper.

Epoxy does not play well in the sun. Ultra Violet will eventually break down the epoxy exposing the fiberglass and eventually delaminating from the wood. This is prevented with paint or varnish. I like the appearance of the wood so I went with varnish. I built up three coats of varnish by first coating the deck, waiting for it to dry then flipping the kayak over and coating the hull. I did the deck in the morning and the hull in the evening. The next morning, I gave it all a light sanding and repeated the deck and hull coatings. Over the course of 3 days, I got 3 coats. The actual work went quick; it was the dry time that dragged out the process.

I cut some chunks of foam to make a seat and glued it in place with contact cement and shoved some more foam into the ends for flotation. I also made a quick little Greenland Inuit style kayak paddle out of a piece of 2-by-3.

All of the above happened from the raw kit to installing the seat happened over the course of a weekend and a few mornings and evenings.

Launching a new boat is always a momentous event, especially when there is a 5 year old boy involved, but — given the age of the future captain — champagne did not seem appropriate, so we just dunked the kayak in the lake. I christened it the “Io” after the inner most Galilean moon of Jupiter.

The design was meant more as a pool toy than a boat for expeditions, as such I put my nephew and waded out into waste deep water with him in the kayak and then showed him how to tip over. I wanted him to feel comfortable with the idea that he could get wet and that it was OK. I was able to pick the boat up to dump out the water and put him back in the seat. He immediately took to the capsizing thing, thinking that was great fun. I showed him the basics of how to use the paddle and he splashed around with that for a bit, with a few more capsizes thrown in for fun.

He then took off across the lake with me following in his wake in my own kayak. He adapted immediately to paddling the kayak, quickly figuring out how to steer. While he was initially a little wobbly he only capsized when he wanted to. I found that I could pick the kayak up while in my kayak, dump it out, and then lift him out of the water and put him right back in his kayak.

All said, it was a great success. I had fun building it and my nephew was able to paddle it for a few years before he outgrew it. His younger brother was then able to use it and other nephews who came to visit also had a fun time paddling it around. I would have been happy to let nieces paddle it but my brother and in-laws were woefully deficient at producing female progeny. I should talk as my wife and I don’t have kids at all.

I have made a few changes to the construction since I built this first Io for my nephew. I found that the kids were sliding sideways in the seat which threw off their balance, so I created a horseshoe shaped piece of foam that wrapped around the seat to keep their butts centered. I also ditched the PMLB coaming riser for a stacked ring system as well as some other minor tweaks just to make it a bit easier to build.

I’ve since scaled up the basic design to a full size kayak for adults that I call “Ganymede”, a stable, easy to paddle recreational kayak. While the bigger design requires several sheets of plywood to build, it is still a very straight-forward construction. The simple design lets a beginner builder concentrate on learning basic boatbuilding techniques without being overwhelmed by complexity. Both Io and Ganymede can be built in a weekend or two with fairly minimal tools.

I have made the free plans available for both the Io kid’s kayak and the Ganymede adult’s kayak on my website. With the drawings on my website you can plot out the plywood pieces full size and then cut them out of plywood with a jig saw. If you don’t have the tools or want to save even more time, I have made complete material kits for Io and Ganymede available through Chesapeake Light Craft.