A Skin on Frame Greenland Kayak Model

History

When European explorers first visited the arctic regions of Alaska, Greenland and Russia they encountered natives in their light and nimble kayaks. These people lived in a world of ice and water and used their boats to get around and to hunt. Being far from forests they did not have the luxury of making a boat out of solid wood. Instead they made the most of the few pieces of driftwood they found by combining it with readily available materials of animal bone, hide and sinew. The skin on frame kayak consists of a lightweight frame made of thin wood sticks which is tied together with sinew. The Eskimo did not use nails or glue to hold the frame together. The entire frame is covered by walrus hide and stretched taught. The paddler sits with his legs facing forward and propels it with a two ended paddle. Paddlers most often wear a waterproof poncho that cinches around the cockpit. This keeps waves and rain out, and if he should capsize it keeps water out as he executes a sharp "Eskimo roll" back to the upright position.  The Greenland kayak is pointed and raised at both ends. In other arctic areas such as Alaska we see other designs such as the baidarka which is blunt at one end and has its characteristic split bow. The Greenland kayak is a marvelous example of a highly evolved design which has been changing for centuries. Today modern kayak makers build on the design principles of the native kayaks but use new materials like nylon, fiberglass and metal.

The Science Connection

The kayak is extremely strong, flexible, light and buoyant. These physical properties are critical to its performance. Behind each property are scientific principles which the native designers probably did not know as such. Instead they had a host of design traditions and rules which were discovered through years of trial and error problem solving. Here are some of the scientific ideas that are illustrated in this technology.

Strength – The kayak must resist the force of waves and maintain a buoyant shape against the weight of water. Most boats are built from the keel up. The strongest part of the boat is the heavy rigid keel timbers on which a frame and thinner hull is built. By comparison the strongest part of skin on frame kayak are the gunnels running down either side at the top of the deck. Into these thin curved ribs are inserted and on top of the ribs long stringers are tied. The thin wood is not notched for this would create a weakness in breaking the long fibers of the wood. The ribs are bent with steam so the grain of the wood follows the shape of the rib. This prevents splintering along the grain.

Flexibility- Tying with sinew is both secure and flexible. In fact the whole boat is somewhat flexible. Often the kayak builders deliberately put extra joints in the keel's stringer or even the gunnels to make it even more flexible. This means the wooden frame does not have to be hefty enough to remain rigid but can bend with the waves and with the paddlers force. Flexibility adds to strength and performance.

Bouyancy- Kayaks are quite buoyant because they trap a large airspace. Remember that a surfboard has enough buoyancy is to support a person. Compare the volume of a kayak with that of a surfboard. A typical kayak displaces about 5 cubic feet of water when fully loaded. 5 cuft x 64 lb cu ft for sea water comes out a load of 320 lbs. If a kayak were to be loaded down so it sank down to the gunnels it could support it would be holding 1000 lbs of weight. Paddlers load kayaks with that much stuff. Imagine the weight of kayak that is completely filled with water after capsizing. Usually paddlers keep inflatable bags in their boats so that the kayak will not completely fill with water if the capsize.

Stability- Another aspect of buoyancy is how the weight is distributed. Depending on the hull shape kayaks can be hard or easy to tip. Sitting on top of a kayak is stable if you sit straight up. If you lean over you reach a point where the weight is pull sideways and over you go. Once they tip over the heavy passenger is dangling below the buoyant hull. Unfortunately upside down is a very stable situation. During an Eskimo roll you can use the paddle to push sideways and tuck the body against the deck to keep it close to the center of gravity of the boat.

A Skin on Frame Model

Materials

Gunnels-(2) 18" x ¼" x1/4"

Masiks, bow and stem pieces 1"x 1/8 balsa or plywood.

Stringers-strips of hardwood, balsa or bamboo basket strips (from a crafts store)

covering- tissue or sheet

waterproof- varnish or latex paint

1. Gather Materials- Part of the fun of building this kind of model is looking for locally available materials and seeing how they perform. Balsa is easy to find and cut but it is not very strong.  Bamboo is easy to split with a knife and is very strong. Cut the materials according to the plans with a utility knife or coping saw.
2. Cut and drill Gunnels- After cutting the gunnel pieces mark the rib positions every inch. Hold the gunnels in a vice and drill a 1/8 hole ¼ deep. Its better if you don't go all the way through the gunnel to prevent the rib from shifting through.
3. Tie gunnels to masik and bow and stern pieces- Cut out the masik and drill a hole at either end near the slanted angle. Position this on the gunnels as shown on the plans and tie in place with dental floss or kite string. Tie the ends of the gunnels together. Now insert the two additional deck supports and tie them in place as well. You now can see the shape of your kayak.
4. Bend, measure and insert ribs- Cut the ribs and right the number of each on each so you will know what goes at each station hole. This thin wood can be soaked in water to make it more flexible. If you want to try steaming use the hottest setting of steam iron. Soak the wood for a few hours and then steam iron and while it is still almost too hot to touch bend it to fit the holes drilled in the gunnels. When it cools and dries the bent wood will keep the shape.
5. Tie on stringers- Place the stringers on the ribs in the position shown.
6. Tie on keel piece-
7. Bend and tie on cockpit
8. Cover- If covering with tissue paper you can apply glue to the keel and center the single sheet of tissue along it. Pull it taught and make cuts to take up the slack as needed and glue to the ribs. Wet the tissue with water spray and then dry it with a hairdryer to which will pull the skin tight.

If applying cloth stretch the cloth in a single piece across the bottom of the keel to the middle of the deck. Use clothespins to hold the cloth in place. Cut the excess cloth along the center line. Fold both side of cloth underneath to make a neat seam and then sew it with an overhand stitch. With each stitch reach further towards the slack cloth so that when you pull it tight it pulls the cloth tight. Cut a whole around the cockpit and tie the cloth around the cockpit the with a loop of string. You could also sew the cloth with an overhand stitch all the way around the cockpit.

9. Waterproof the model- Paint the tissue or cloth with clear acrylic varnish. This will look a little like the traditional skin and the ribs and frame works will show through a little

A Paper Cut-out Model

1. Print the cutout plans out on thick paper stock or and glue them to cardstock.
2. Cutout the shapes with scissors or a utility knife.
3. Pull the tabs through as shown and glue in place
4. Pre-Fold the tabs along the deck and then place the deck on the hull with glue.
5. Coat the boat with clear acrylic varnish to make it slightly transparent and stronger.