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Designing a Traditional Skin On Frame


I recently took on the task of designing and building a skin-on-frame kayak in the Greenland Inuit style. "Designing" a SOF kayak before building isn't really the traditional way. Typically they are built to traditional measurements based on an anthropometric measuring system i.e. using hands, hips, arm spans etc. as measurement units. These basic measurements may be tweaked based on the builders experience, making a wider/narrower or longer/shorter as needed by the user. This system works quite well but may make it hard to create a really new design aiming to meet a specific set of design goals.

What I want to do is convert an existing design that was not originally built as a SOF into a SOF that can accurately reproduce the performance characteristics of the original design. For this exercise I chose my Petrel as the source design. My goal then is to do something some people may consider backwards: create a set of plans from which I can build a SOF reproduction of hard-shell boat.

To that end I need to determine the shape of the constituent frame parts. The backbone of a SOF kayak is the gunwales. Unlike traditional European wooden boats where the keel serves as the core structural member, skin on frame kayaks are built from the gunwales. The gunwales are initially held to their shape via deck beams, then the hull shape is determined by bending ribs and connecting them with longitudinal stringers.

Therefore, a set of plans for a SOF must include the gunwales, deck beams , and rib shapes, and the gunwales are the basis for everything else. So, I need to create drawings for each of these piece such that when they are all put together they will accurately reproduce (or at least approximate) the original design.

I have all my boats drawn up in MaxSurf so my starting point is the Petrel CAD model in MaxSurf. The first thing I had to tackle is the gunwales. The gunwales define the sweep of the sheer and from that the overall shape of the boat. In simple terms the shape of a SOF is determined by clamping the ends of two gunwales together then separating them in the middle. This makes long, narrow, pointy at both ends shape we are familiar with when we look down at the top of a kayak. Wood prefers to bend in just one direction. If you simply clamp the ends together and put some deck beams in, the boat will have a straight sheer when viewed from the side like this Chukchi kayak (PDF). But Greenland Inuit kayaks show some curve when viewed from the side. They achieve this by mounting the gunwales at an angle relative to each other. A small angle produces a little bit of sheer curve, a large angle more curvature in the sheer. This quite simple system allows the gunwale wood to curve in only one plane yet appear to curve when looked at from above and from the side. There is just an angle slightly below the boat where the gunwales will appear straight.

When viewed from the end this angle is visible in many boat designs. If you look and an end view of the boat you will see a diagonal line described by the sheer. The problem with my Petrel is the angle is different at the front of the boat when compared to the stern.

Skin on frameMy first solution to this was to attempt to draw in a twist of the gunwale planks. This would make the angle at the back of the boat different from the front, in this way I hoped that I could make the gunwales closely follow the sheer of the strip-built Petrel. On screen I was actually fairly successful, like many traditional boats, the gunwales would need a little added curvature at the tips, but I got a close match to the existing sheer line. This required a gunwale angle of 20 degrees for the stern and 30 degrees for the bow. After further investigation, I decided this wasn't going to work. The angles were too great, causing problems with the rib angles.

At this point I was a little flummoxed. I wasn't sure how I was going to get the shape. I continued to study my original Petrel design and noticed that the deck didn't really start to roll over at the sheer. Instead it rose vertically for a while before bending towards horizontal. I also thought more about traditional SOF kayaks. Their bow is higher than their stern (most kayaks follow this basic form), but it was achieved using the uniform gunwale angle by the simple expedience of lifting the bow... ...not exactly an earth shattering idea, but it hadn't occurred to me in my CAD process yet.

SOF gunwalesCombining these two ideas, I lowered the gunwale angle and then pitched up the gunwales and came up with a good representation of the shape of the Petrel. The gunwales did not exactly follow the shape of the Petrel sheer, but they did closely follow the over all shape of the boat. And, importantly, they were at an angle that allowed close representation of the rest of the hull shape.

Creating this hull shape required adding stringers along the chines and a keel. Again, this requires the stringers bend along natural lines. With the stringers in place to define the exterior shape of the boat, it is then time to see if it is possible to bend ribs that would support those ribs in their intended location.

Skin on frame frame