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How to build a 'Skin on Frame' boat - and why
Ernie's 'Skin on Frame' advice.
Why Skin on Frame:
Skin on frame boat building can be one of the fastest, most satisfying, skills-rich projects you can do.
It teaches you essential skills to build almost any wooden boat. It's not difficult, and you don't need to already have skills to do the work.
You get a boat you can be proud of, and use.
This is the most basic list of things you need to build a skin on frame kayak.
What It Takes:
something to drill holes with.
materials for frame (wood).
skin material (nylon canvas, nowadays).
Kinda different than the tool list most projects require.
With these things, and a little care, you can construct a boat that will be light, strong, and seaworthy (at least, as seaworthy as its plan and owner).
The down-side of working with the most basic tools is: You pay in time ('til you have become skilled enough to use each of the tools to its full potential).
A Good Book
For your very first boat, a good book is a handy thing.
Even if you are taking a workshop, it's good to have a book. You can read the whole description and know what's coming, so you can do each step with more confidence.
This is perhaps my favorite book on the subject:
by Robert Morris. Building Skin-on-Frame Boats
Most books on building skin-on-frame boats have plans, tools, and methods the author worked out for a specific kind of kayak or boat. Mr. Morris's book has plans, tool lists, and easy to follow, step-by-step instructions. He also does not stop at kayaks; he goes on to European-style boats (all skin-on-frame) so you can learn from each type.
Any book will teach you things that apply to most of the different kinds of skin-on-frame boats. There are many kinds, from the small 'sneak koryaks' to big umiaks.
For those who want to branch out, or don't like the look of certain kayaks, you can try Mr. Zimmerlie's web site. He has drawings of several types of kayaks: http://www.arctickayaks.com/plans.htm (Explore this site, it has more information than one would think.)
There is also a nice juicy site with museum-replica pictures by Harvey Golden here: http://www.traditionalkayaks.com/index.html, which I mention only for purposes of inspiration. You don't need bone rub-strips or baleen spacers to make a workable boat, and you probably won't want to try it out for the first time in that much surf!
To get answers to almost every question you might ever ask about skin on frame building, this is probably the most comprehensive group of kayak builders and users around: http://www.qajaqusa.org
For even more information, and step-by-step instruction, you can go here: https://www.skinboats.org/
Skinboats.org is also where most of us Skin-on-Frame builders get our stuff. Fabric, sinew, dyes, other materials, plans, books, and whatnot at your finger tips. (Unless you can hunt down a walrus, or have a local manufacturer who gives you insider deals.)
You can expect to spend about $300 on the skin materials (fabric, stitching sinew, and urethane, optional dye) for an average-sized kayak. The frame materials can usually be found locally, either from lumber yard or building supply (see notes below), or by harvesting your own green oak or willow shoots for ribs. Choice wood for boats includes white oak, cedar, spruce, and tropical hardwoods. Avoid quick-rotting wood like ash, cottonwood, or fir, if you can. They will work better than nothing, but your boat may need careful dry storage to avoid rot.
Seriously, that's all there is to it. Get a plan, get the materials, and start. But it speaks well for your stick-to-it-ive-ness that you are finishing this first, so I'll give you a few more tips.
Choosing a Plan: The shape of the boat determines how it handles, and what you can do with it. Wide, flat boats are stable in the water, but may be slow or harder to steer. (The wind can push them around easily, too). Long, narrow boats are good for long distances and speed, but require more skill to balance. Shorter boats, and boats that are kind of plump and football shaped, are useful for quick-turn situations and popular for river kayaking. Folks that like to go out for a purpose (hunting, bird watching, cargo/camping/touring) may need larger volumes or extra storage gear on their boats. Some designs have a flat deck behind the paddler, where gear can be strapped on.
It is better to use a proven plan (many of the styles have thousands of years of testing already); avoid mixing and matching features. A classic beginner mistake is to think "if I make the sides of the boat taller, it will be safer!" but instead, taller sides make it harder to reach the water for paddling, and more likely to tip over.
A few things it is OK to modify:
- You may wish to size the cockpit (ring around the opening of the kayak) to fit a standard spray skirt, if it is not already the proper size.
-It can be OK to add on 'skegs' or little rub-strips, attached to the outside with stitching or aquarium cement. These could include a long external keel if you want a straighter-steering boat.
- Gear straps can be attached on the top deck, whether or not your original design had them. Gear straps or bungies allow you to put the paddle somewhere if you are handling other gear, and can help with self-rescue (stick the paddle through the strap like an outrigger, with a float on the end if available, to help hold the boat upright while you climb back in).
Pick nice straight-grained lumber, cedar if you can get it. Grain stacked perpendicular, or flat, is best. (Not diagonal like for other woodworking projects.) See how it bends in the yard, asking "does it 'want' to be a boat?" Allow 10% extra for scrap. We have built some nice kayaks with two long 2x4s and a table saw.
Soak your ribs overnight to get decent pliability, whether or not you are steaming them. Especially cedar. Oak will also get brittle after it cools down from steaming, so allow plenty of extra material for ribs that might break during fitting.
Fair your boat often (stand back and check that its curves are smooth, symmetrical, and pretty)
Follow the step-by-step instructions from your book or workshop. (Example photos show how to tie different knots, etc.)
If your instructor and the book, or any two sources, say two different things: They're probably both right. Pick one and stick with it.
A little note on learning:
You don't learn much from never making a mistake. Go ahead and make some while building these boats.
You can test them before you do the most expensive part (the skin). Wrap the frame in palette-wrap or Syran-wrap. Take it out and float it. If you test in the shallows, all you have to worry about is getting wet.
Steps for Boat Builders
If your plan only show outside dimensions, not instructions:
1) Collect the materials, allowing 10% to 25% extra for scrap and breakage.
2) Prepare your wooden parts. For example a baidarka has several parts that make up the 'beak;' these can be cut from marine or ACX plywood, planks, or driftwood. Cut the stem and stern (front and back) parts to size, and all the deck boards/upper frames. Split or rip your long pieces as straight with the grain as possible. Sand or plane the edges. Do not bend the ribs yet. You may wish to pre-fit the parts before drilling.
3) Drill any parts that need to be drilled for lashing - usually the stem (front), stern (back), keel (bottom center), and gunnels (top/sides - also spelled "gunwale"), and the upper deck cross-frames (masiq, etc).
The gunnels will need to be drilled to accept ribs. Depending on your ribs this will either be a slot mortise (plunge router, or bit and chisel), or a lot of round holes if using round saplings/shoots. Older traditional designs may show a shallow groove: Before the use of steel tools, the rib ends would be pierced and lashed in place, rather than mortise-and-tennon fit.
The holes need to be exactly opposite each other for every rib. It's easiest to pre-drill before you assemble the boat.
Clamp the gunnels together side by side (tops and bottoms showing) as if they were a single, long plank. Mark the deck board locations (avoid drilling twice here if possible, especially near the center of the boat). Mark your pattern of ribs down the bottom side of both gunnels, using a square to place them exactly opposite. Stout green-oak ribs might be spaced about 6" apart, 3/8" round ribs might be every 3" or 4". Drill both gunnels before un-clamping. (Careful to plunge straight, don't drill diagonally out of the wood). Leave plenty of meat on each side (1/4" minimum) and drill no more than halfway into the gunnel, to prevent the gunnel from bending or crushing near a hole. (You may wish to mark the depth of the drill holes as a line on the side of the gunnel, for later.)
4) Lash the gunnels, keel, stem, and stern. These parts establish the main shape of the boat. Each of them forces the others to bend, creating a stiff 'truss' that gives the boat strength. The traditional lashing for Aleutian-style boats generally uses either a V-to-Y shape, or a II-to-X shape. You start with several straight passes, then tighten them by wrapping a "French Spiral" or "Dragon Staircase" of many half-hitches around the main lashing.
5) Step back and look to make sure the boat is fair (symmetrical, smooth curves).
If one gunnel bends more than the other, try to see why. Flip the boat over, take it outside... Is the whole boat crooked, or is one gunnel just not bending? You may need to shorten the gunnel that is bending more, or more likely, you might need to plane down or steam the stiff one to let it bend more. Do your best to fair it, but don't remove more than about 1/8" or 3/16" (2 mm) of material.
6) Wedge the upper deck frames in place (masiq, etc) and check for fair. If you have "crooked" places, or the gunnel makes a slight zig-zag, check to make sure you added the deck boards in the right order, and the right distance apart. You might need to move a deck board very slightly (up to 1/2 inch / 1 cm), or plane down the deck frame's angle where it meets the gunnel slightly (up to 1/8 inch), to get a fair curve. Check the gunnels and keel again - they should still be fair (smooth curves, symmetrical, the keel straight in the forward-back direction and a smooth curve if you look from the side).
7) Prepare to steam the cockpit and ribs.
Cockpit: Some boat designs use a laminated wood cockpit, others steam-bend a hoop and then add a rope for the lip where the spray-skirt clips on. We often steam-bend several wooden hoops and stitch them together, to make a tall rim and a built-out lip. Regardless of the method, you should end up with the right size lip for your spray skirt (borrow a standard one to check if you like). And it should be sturdy without being too heavy. Your cockpit rim will not likely be less than 1/4" (.5 cm) thick, even at the thinnest part. It will need to bear your weight and take a beating, so if it looks too delicate, beef it up.
Ribs: Estimate the rib length by bending a very thin piece of wood (or a piece of string) around where each rib goes. If this is your first time steaming boat ribs, cut the pieces generously - about 4" to 6" longer than your longest ribs (middle of the boat). Experienced builders may find it more convenient to cut each rib to its exact length before steaming, however don't get them too short!
Start with the center ribs for steaming and fitting. Pieces that break in fitting might still be useful for the shortest ribs at stem and stern.
Soak your wood, get the steam box hot. (You can build a steam box with 4 1x6" fence planks, a tea kettle, a camp stove, some towels and some heat-resistant hose, or borrow/rent one long enough for your ribs). Transfer 1 or 2 ribs to the steam box. Let them steam until supple (depending on the wood and humidity, might take about 15 minutes for 1/4 inch ribs).
Most people will want gloves, and some will want a friend's help, when bringing the ribs out of the steam box. Old sailors and callused carpenters may prefer to handle them with bare hands, to feel the pliability of the wood. Spring clamps (a LOT of them) are handy for the next step.
With the boat very close by the steam box, bring out the first rib and immediately bend it into the inside of the boat. Clamp the middle to the keel (inside), and the ends to the inside of the gunnels. Make sure it has a nice smooth curve - if it looks too straight on one side, shove some more material down from that gunnel, and re-clamp. It is better to be a little too long than too short, unless you have invented a board-stretcher.
If your rib is cut to length, wedge it into the holes now. To trim off, mark both sides to length (middle of the gunnel), then trim to length, and wedge it into your holes. If it doesn't fit right, you can plane down, or wedge with a scrap piece. You will drill and lash these ribs, make the wedge long enough to drill and lash and it will stay put.
Continue the same way with the other ribs, working the center first, then a few at a time forward and back. Don't steam more ribs at a time than you can place in about 15 minutes; if the wood cooks for over 30 minutes, it can lose flexibility and be brittle when it dries.
It is common to break a few ribs - sometimes more than a few - while fitting. Some of the scraps can be used for shorter ribs at the ends. For very sharp angles, like the front of a Greenland kayak, it may be acceptable to use two pieces for the last 1 or 2 ribs instead of bending a single piece. Drill and lash your ribs, if called for in your design.
8) Place the longs (chines) outside the ribs, and begin lashing them together. Ernie prefers a traditional running box lashing (the lashing cord is not cut between ribs, it runs along the inside of the long piece from rib to rib). Other builders teach other lashings, such as a square knot, which is straight forward but can loosen slightly over time. (I wonder if part of the difference is the supplier's preference for using less cord, vs. the sailor's reluctance to cut a cord that he might need to scavenge for emergency repairs some day. )
Box lashing: As you start the string at the intersection of a rib and a long, take a half-hitch around the rib. Go around the outside of the outer piece and the inside of the inner piece in a simple over-under pattern, 3x. Then tighten this slightly by going around the lashing 3x - under the outer piece, over the inner piece. (If the initial lashing were not there, this wrap would just slip between the wood pieces and come apart). You can make this tightening-wrap with half-hitches, (a little like the stem of a button), for extra security if you like. Don't pull it so tight it separates the wood, or cuts into the wood - you just want to draw the two pieces snug together. Finish with a half-hitch around the rib, then continue to the next junction.
9) Check for fair. You want a nice plump boat, no hollow areas where a rib seems caved in. The gunnels should still be symmetrical, and the keel straight. This is the point where people start taking photos and sticking their head inside the boat to go "ooh!"
10) Seat-test the boat at this point, to check performance and cockpit position. Tie the cockpit roughly in place - lash to the nearest deck beams, and tie with some tension to each gunnel. Wrap the whole boat in pallette-wrap or syran-wrap, at least 2 to 3 layers thick, and take it for a test paddle. (Be prepared to swim if it leaks - PFD etc.)
Do your legs and feet fit inside? Does it steer straight? Turn well? Can you paddle it up onto the beach? Do you like how it floats?
You can still make adjustments at this stage, before committing to your permanent skin.
For example, if the boat is too tender (flippy) for your skill level, you might be able to broaden the boat slightly by moving the deck beams an inch or two toward the front and back, and re-cutting the main deckbeam in front of the cockpit (masiq) and right behind the cockpit. You may also need to re-fit the ribs to make an adjustment this significant.
(This is a lot of work - unless you have a medical balance problem, you might be surprised how fast you will gain the skill to stay balanced in a narrow, fast boat. Especially as you build the confidence to use it at its designed speed. Long-distance kayaks can easily be paddled faster than most people run.)
A smaller adjustment might be if you want a foot-rest at a convenient rib, some seat boards lashed inside the ribs, or a back rest, you could add those during the fit-test.
You can adjust the trim of the boat, side-to-side and front-to-back, by where your weight goes in the boat. Adjust the placement of the cockpit, to center your own weight (or any cargo you would typically carry) with respect to the boat's center of buoyancy. (Don't worry what those terms mean, just wiggle around until it floats right, then adjust where the cockpit sits if needed.)
Most kayakers paddle seated, one person to the boat. However if you are a canoe person who wants to kneel, or you might lend the boat to a smaller person, you can mess around and see what works. You can even add a gear hatch, or a second cockpit for junior passenger or water-loving pup. Test with all these variations, to make sure you can balance the boat the way you want to use it.
When you are happy with the boat's performance, it's time for the permanent skin.
We use a ballistic nylon canvas, anything from 6 to 12 weight for most kayaks, heavier for bigger boats like umiaks. Heavier nylon canvas may give more durability and hard-wearing performance, but also requires more urethane to fill, and makes the boat heavier to carry. If you want to use something other than ballistic nylon canvas, see notes at the end.
11) Wrap the skin around the boat. Start in the middle, aligning the cross-grain of the fabric with the straight lines of your central ribs and masiq. Tack (take a few loose stitches) to the lower part of your cockpit, and check the fit at both ends. If you followed your design carefully, you should have enough fabric to lap a few inches beyond each end.
Starting from the center, do a giant zig-zag 'fitting stitch' along the upper deck. Pick up a stitch in the fabric - 1/4 to 1/2" stitch, inches apart - on one side of the gunnel, then go across and pick up a similar stitch on the opposite side, and come back to grab another stitch on the first side a few inches later. You will tighten the fabric by grabbing just below the gunnel, and pulling tight, like lacing a shoe. You will also do a fitting stich (straight seam) at the stem and stern.
Adjust your fitting stitch until the sides of the boat look nice, tight, and plump, with no wrinkles or loose/sagging parts. When you are satisfied with the skin, tie off the fitting sinew, and begin stitching the top seam. Keep things as tight as you can manage - the nylon canvas will not shrink any tighter as you paint it, and may even sag slightly. It's OK if the seams bulge a little bit here and there, it just adds to the 'natural' and 'handmade' look. The critical part for performance is the smooth stretched skin below the water.
We generally do a straight seam front-to-back, stem, and stern. Trim the nylon fabric using a heat-knife about 1 1/2" (3 cm) from the seam, and then roll the edge over and stitch it down next to the seam for a tidy, double-seam look. Where the fabric goes on the diagonal, you may have to re-trim to avoid bunching, or just tolerate a little bit of wiggle in your seam. (If you are feeling very fancy, you can roll one edge inside the other like the old waterproof leather seams, but this is harder to get tight, and doesn't really matter for waterproofing when using using woven fabric and urethane.)
You will also need to bring the skin right up against the cockpit, where you should have holes pre-drilled in the lower part of the rim. Make sure the canvas doesn't bunch up where it could break the seal on your spray skirt - you will want about 1/2" of clear space as the "trough" where the spray skirt cinches on. You can roll the edge of the canvas over before stitching to the cockpit, so you are sewing through double or tripled fabric, to avoid any messy edges right where you handle the boat the most.
If you have additional cockpits or gear hatches, stitch up to them the same way.
12) Dye the fabric, if desired, with a compatible dye. (Without dye, you will have a white to off-white, translucent boat, and you might get bleed-through if the wood or lashing cord stains the canvas.) Some people love to go for special effects like dappled or two-tone or handprints. Just remember dyes bleed so you won't get sharp lines; and don't use fabric paints because they would interfere with the sealer.
Solid colors look classy, show the pattern of the ribs through the translucent skin, and are easiest to do. Let the dye dry completely.
13) Apply your sealer over the outside of the clean, dry, completely finished fabric surface of the boat. Most builders use a 2-part urethane (Verithane) that needs to be mixed in small batches right before applying. Don't mix more than you can use at a time. If you want to waterproof anything else with the leftovers, have those items ready. (A little gear bag, or cockpit cover, or something.) You can urethane the wooden cockpit rim, right over the rope if needed. Other deck gear/ropes should not be in place yet, and won't get urethaned.
Urethane only sticks to itself when not fully cured, so ideally you will do two to three coats, about 4 to 12 hours apart. Apply the first coat lightly, just enough to saturate the canvas and maybe bead up inside, but not so much that you glue the canvas to the frames. Let the first coat cure just until it stops being gloopy/tacky, and paint the second coat on immediately. The first coat fills the weave, the second coat should finish filling the weave and lie on top to make a smooth surface that feels like plastic or leather, not woven canvas texture. A third coat can remedy any 'thirsty' spots, and allows extra thickness against future wear, or sanding and painting.
14) Deck gear, skegs, etc: Most traditional kayaks had leather thongs with bone or ivory sliders, and little bone 'skegs' that were stiched on over the longs, usually wherever two seal skins met, to protect the seam from wear.
Modern kayaks may have rope or bungee deck gear, and some have an external wooden keel or small skegs to protect the fabric from excessive wear on rocky beaches. The deck gear should be fixed to the gunnels or deck beams by drilling through the skin and wood, and knotting the rope (or using some bungee stopper hardware) inside the beam. A figure eight knot makes a quick stopper knot if you get the hole sized right. Skegs can be drilled, stitched, and filled/waterproofed with aquarium cement, or just cemented on as a temporary/fitting experiment.
15) Repairs and Service Life:
Urethaned ballistic nylon is pretty tough stuff. You should get years or decades of fun out of a well-built skin-on-frame boat. Keep the urethane out of direct sunlight when not in use, for longer life. Most paints don't stick well to urethane, but you could sand the urethane slightly if you want to try your luck with a compatible paint down the road. Colored paints will further protect the skin from UV damage.
If you do get bad wear spots, knife cuts, leaky seams, or need to patch over a damaged section, you can use aquarium cement (a type of silicone caulk for full immersion use). For minor cuts, you can do a baseball-stitch and daub with aquarium cement. For larger wear spots or actual holes with missing fabric, you can patch as you would sew a patch onto clothes, urethane the patch, and seal the edges if needed; or patch with a pre-urethaned section of canvas and use aquarium cement to seal the edges. Patches below the water line may affect the boat's performance a bit.
If the skin becomes damaged beyond repair, or the boat gets crushed (don't over-tighten ratchet straps when hauling!), or you just want to replace a dingy and sun-damaged skin with a hot new color, you can remove and replace the skin. Cut the skin free (saving any good sections for patches, if you like). If the urethane has stuck to the wood, trim it off with a knife or cabinet scraper, being careful not to cut any nearby lashings. Repair or replace any damaged wooden parts, re-lash where needed, and sew on a new skin as above.
You will get plenty of gawkers when you paddle your boat - especially if the sun is behind you, showing the rich colors and beautiful rib structure through the translucent skin! The flexibility of skin-on-frame designs allows them to ride more comfortably over waves and chop, which is a good thing if your admirers are doofing around in a motorboat.
Other Skin Options:
Hides: Traditional Arctic boats used seal-skin for kayaks, walrus hide for larger umiaks. (Walrus skin is thick, almost like plywood). Most folks can't legally get these materials nowadays. If you are a member of a tribe that still has license to hunt marine mammals, talk to your hunters about the availability and who might know the proper curing methods for seal skin.
Substitutes like cow hide or deer hide tend to rot badly in water, and all types of leather will shrink down when repeatedly wetted and dried. Many museum specimens and replicas have been crushed by their own skins when not properly maintained. If you want to use an authentic leather skin, you can plan on unlacing the skin each winter, examining and repairing the hides, and then re-lacing the skin onto the boat for the spring boating season. (This was traditional in the Arctic; "freeze-drying" conditions may also have helped preserve the skins and their boats from excessive shrinkage.)
Most leather objects handle water better if oiled. Natural/animal oils, or a good boot grease or mink oil may work for you, as long as it doesn't damage the stitching thread. Avoid petroleum-based oils with nylon or latex, and with leather generally.
Fish Skin: Salmon skin, halibut, sturgeon, and other thick-skinned bottom fish were sometimes used for waterproof gear in the Arctic. We have not tried using these skins for kayaks, but if you live near a cannery it could be an interesting project. Fish skins would need to be cured, stretched, and oiled much like animal skins, however they can be more delicate and time-sensitive to work with. Wonder if orienting the scales carefully would give you that 'shark-skin' effect, like those Olympic swim suits?
Cotton Canvas or other natural fabrics: You can use plant-based fabrics, and a heavy oil paint (such as asphalt or deck paint), if you want. They may be more prone to damage, including rot, sun damage, and physical impact and abrasion, and the paint may wear away and contaminate waterways. Natural fabrics can either shrink or loosen when painted (depending on the fabric, the paint, and the weather), so consider stretching a little sample over a frame to test before sewing on the canvas. A fabric that shrinks (like cotton) would need to be sewn on a bit loosely; a fabric that sags should be sewn super-tight.
Given the significant cost of any boat-sized piece of any canvas, we tend to order the stuff that lasts (ballistic nylon canvas).
Does Not Work: poly tarp (e.g. blue tarps), "water-resistant" fabrics that are not actually water proof; synthetic fabrics or stitching cord that are not compatible with your sealer (a few synthetics will actually melt away in contact with sealers or paints).
© 2009 Erica K Wisner