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SKIN-ON FRAME

Skin-on-frame boats are lightweight, delicate but very rugged anyway. They can be built in a resource-efficient manner and offer some interesting crafting steps such as steam bending or skinning and sewing the skin.
Traditional construction methods do not require glue, screws or nails, as the boats are held together with dowels and artificial sinew. A modern variation on traditional skin-on-frame construction is the fuselage frame construction, which uses frames sawn out of sheet material instead of steam-bent ribs. This construction method is based on techniques used in ancient aircraft construction.

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WOOD

In traditional skin-on-frame boat construction, the gunwales define the sheer and provide the main structural strength of a skin-on-frame boat. The transverse ribs, which are steam-bent from narrow strips of ash or oak, are attached to the gunwales. The ribs hold the stringers, which determine the shape of the outer skin and are responsible for the typical hard-chined shape. Everything is connected with artificial sinew or dowels. Nails, screws or adhesives are not necessary.

The gunwales and stringers are made of the lightest possible wood. Red Cedar is particularly suitable for this purpose, as it is very homogeneous, knot-free, light and easy to process. In addition, its high resistance to rot and fungus should also be pointed out. Other woods such as spruce, pine or douglas fir can of course also be used.

The ribs are made of ash or oak. Ash wood has the advantage that even kiln-dried wood is suitable for steam bending.

THE GUNWALES

In a canoe, the gunwales are equivalent to the coaming; in a skin-on-frame kayak, the gunwales are located at the transition from the hull to the deck. They carry most of the forces acting in a skin-on-frame boat. In kayak construction, they are usually set at an angle of about 17° and thus naturally produce the typical sheer as well as the trapezoidal cross-section of the hull. In canoes, the gunwales are glued together from several strips to form the desired sheerline, since the shape of the hull requires vertically positioned gunwales. Placing the gunwales at an angle, as in the kayak, is not an option here.

The gunwales are provided with pockets on the underside, in which the ribs are attached.

Since the gunwales are important statically, a careful choice of wood is important. Red Cedar is the best choice, as the wood is very strong and resistant to rot in relation to its light weight. It is also very easy to work with hand tools and can usually be obtained knot-free. A board with lying annual rings results in gunwales with lying annual rings (see photo).

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STEAMBENDING

Finding wood suitable for steam bending is the first challenge in skin-on-frame canoe or kayak construction. Ash and oak are particularly suitable, but the wood should ideally be undried. On the market, almost exclusively kiln-dried wood is offered, which is why you should look at the beginning of the supply chain: in the sawmill. For all those who are now thinking that this all sounds very stressful: unlike oak wood, ash wood has the property that it can still be steambent well even when kiln-dried. A little more breakage must be accepted here, however, so to be on the safe side you should buy a little more wood than calculated. Ideally, you will find boards with standing annual rings, from which you can saw strips with lying annual rings on the table saw (see photo). It is important to always saw parallel to the annual rings. Strips in which as many annual rings as possible run from the beginning to the end are particularly strong and less likely to break.

LASHING THE FRAME

The lashing of wooden parts is an old joining technique, which was realized with animal sinew or plant fibers in ancient times. Today, in skin-on-frame boat construction, waxed artificial sinew made of polyamide (nyln) is used. Lashing has the advantage over doweling, nailing or screwing that the construction has a better tolerance to impact because the frame of a skin-on-frame boat remains flexible. Where a glued joint would break, the lashed joint remains intact.
The stiffness of a Skin-on-Frame canoe hull depends primarily on the strength of the lashing. It is therefore very important to keep the sinew under high tension at all times while lashing the frame. A waxed string has the advantage here that it already sticks to itself, making it easier for the boatbuilder to maintain the tension. A very tightly lashed frame is less likely to become a "banana" later due to the forces that occur while skinning.

 

YOUR BIGGEST SEWING PROJECT...

the wooden frame of the 'nanook' skin-on-frame kayak rests on trestles and is covered with nylon fabric, which is loosely laid on the frame. view through the frame and its cnc-milled components

SKINNING

The skin turns the frame into a boat. Polyamide (PA), also known as "nylon", polyester (PES) but also cotton are suitable. Polyamide is the most robust material and can be varnished well with polyurethane varnishes. A major disadvantage, however, is the characteristic that polyamide absorbs water and expands. Even careful varnishing cannot change this. Skin-on-frame canoes covered with nylon can look wavy in damp or cold weather because the skin loses its tension. This can be countered by soaking the fabric in water while skinning and sewing it under high tension. When it dries, it stretches even tighter around the frame. Caution is advised, however, as there have been reports of canoe frames breaking under the contracting, drying skin.

Polyester, on the other hand, does not absorb water, so it does not become slack. Compared to nylon, however, it's not quite as robust. Another advantage of polyester is that, unlike nylon, it can be shrunk with an iron. So it does not have to be skinned under extreme tension, but can be shrunk later and retains this tension.

When choosing a fabric, the weave type is also important: in relatively loosely woven fabrics such as "Ballistic Nylon", the seam quickly pulls large holes in the fabric that cannot be sealed with varnish. Other, more compactly woven fabrics are better suited here. This should be tested beforehand...

The easiest way to skin a canoe is to simply attach the skin to the gunwales with stainless steel staples. The staples are later covered by a wooden strip. Sewing is only necessary along the stern and bow of the canoe.

VARNISH & PIGMENTS

A nice way to customize a skin-on-frame canoe is a colorful varnish. Hereby, many tones can be realized, from cool blue or green to warm red or orange tones. Homogeneous to patinated looks can be created by thorough mixing or a streaky finish. Natural earth or mineral pigments are added to the hardener of the varnish to achieve a uniform color throughout the entire varnishing process. The color shade and, above all, the desired opacity should be tested on a test piece beforehand. For this purpose, it is advisable not to mix too small quantities of paint and pigment, since the weight proportions are difficult to transfer to larger quantities.
Polyamide and polyester are coated with polyurethane varnishes. Boats covered with cotton can also be coated with oil-based varnishes.

photo of a white nylon fabric onto which square color samples in shades of orange and red have been applied. a test of natural pigments for dyeing skin-on-frame canoes and kayaks
the 'SOFi' skin-on-frame canoe is lying on the shore of rolstorpasjön in southern sweden on a dense cover of blueberries. several pine trees are standing on the shore. in the background you can see the lake. the light shines through the skin of the lightweight canoe and makes the wooden frame inside the canoe visible

LIGHTNESS

The fascination of a skin-on-frame canoe, besides its light weight, is the transparent skin through which the sun shines, making the frame visible to the outside as well. When paddling, you can also see the waterline sloshing through the skin and can see lily pads passing under the boat.

For those who find the strip construction method too glossy and labor intensive, the skin-on-frame construction method is recommended. The more traditional steps, less likely to escalate into monotonous labor, reward with new experiences in steam bending, lashing and sewing.

The result is a delicate-looking, yet robust boat that is suitable for everyday use and, like my "SOFi" model here at Lake Rolstropasjön in southern Sweden, integrate perfectly into the landscape.