Part 2 - 12.9 pound 17 footer
After some modest success with my first Cuben Fiber folding kayak, I wanted to try for a more radical design. Since the maximum length I could fit into the available space in my apartment was 17 feet, then that would be the length for the new boat. Also, I admit I was inspired by those sliver-like K1 racing kayaks, which just happen to be 17 feet as well.
I was happy with the width of the first boat, at 21 inches, so I decided to keep that dimension. I also wanted to stick with the plumb stems, for ease of construction, but also because I kind of liked the look. The shape of the 14 footer was entirely determined by the method of construction, because the sponsons were what gave the boat its shape. This time I wanted to come up with a construction method that would give me more freedom over the hull cross section. I wanted a more rounded shape midship and a finer shape at the ends. I would need to go with a complete frame this time. Somehow the sponsons would have to be relegated to only a portion of the hull, the frame determining more of the shape.
I concentrated on the cross sections to determine how to build the boat and how to incorporate the sponsons. The drawing below shows what I came up with.
I wanted to build the entire frame using the tent pole tubing I had used before, including the cross frames. I wanted to do this partly as a challenge, but also because I couldn't see another way to support pairs of sponsons. Frames are normally cut as curved shapes from some kind of sheet material like high-density polyethyene. To make the cross frames from tubes, I would have to start with triangles, then add posts to the sides of the triangles that would support stringers for the sponsons. Whew! Sounds complicated, and I guess it is a bit. I was also a bit worried that my legs would rest uncomfortably against the sides of the triangle of the forward cockpit frame, but this turned out not to be much of a problem.
To allow finer ends on the boat, I would use 2.5 inch diameter sponsons, a lot smaller than the 3.5 inch ones I had used before. And since the height of two sponsons no longer determined the height of the hull, the diameter wasn't critical anyway.
To create a more sliver-like appearance a la K1, I decided to drop the heights of the ends. The center height would be 8 inches, the bow 6 inches and the stern 4 inches. Another change from the first boat was to incorporate a raised area in the deck forward of the cockpit to provide more leg room when entering and exiting the boat. For this design I used Blender extensively, creating a full 3D mesh for the hull. This allowed me to extract dimensions for the patterns directly from Blender.
Materials and Construction
I quite liked the Cubic Tech material I had used on the 14 footer, but after discussions with the technical specialist at Cubic Tech, I decided to try a new material designated CT9HK.18/W6-2.0. This material is quite translucent and has a abrasion resistant coating on one side. The translucency would certainly give the boat a unique look. The weight was about the same at 3.4 oz/sq yd.
As before I wanted to use carbon tubing for the keel tube and tent poles for the rest of the frame. Again, I would use nylon tees for the tube connections, though this time they would be much more complicated.
Since I was not aware of anyone making 2.5 inch diameter sponsons (at least not 15 foot long ones), I would have to get some custom ones made. Fortunately, I live in the same town as Feathercraft, and was able to get them to make me two double tubes in one piece each to my specs.
The skin would be more complicated than the previous boat, but I was confident I could make it without much difficulty. What I wasn't sure about was the frame. This was a complete unknown, so I decided to start with it.
The first issue with the frame was how to use the nylon tee fittings to create the basic triangles at the heart of each cross sectional frame (there would be four). My first attempt was to use two tees for each corner of the triangle, and offset the tubes as shown below for the aft cockpit frame.
Towards the end of the frame construction I hit on a way to avoid offsetting the tubes. The bow and stern cross frames were eventually make planer by cutting away half of each keel tee so as to overlap them as shown below for the stern frame.
This method involved some extreme bending of the upper tees, which was difficult, so I left the larger frames as they were and I ended up with two frame types.
In the vein of pursuing a more nuanced design this time around, I wanted to take advantage of the fact that the bending force on the frame would be greatest in the center where I would be sitting on it, and least at the ends. This meant that continuing the large diameter of the keel tube all the way to each end would be wasteful. A tapered tube would be the most efficient, but impractical. A compromise would be to step the keel tube down to a smaller diameter at some point. I chose that point and had a 3D printed part made to step the tube down from 1.125 inch diameter to 0.75 inch. This also meant the stem fittings could now be made of the smaller diameter tubing, thus saving more weight. One small detail remained. How was I going to join the two gunwale tubes to the stem? This was a three-way joint at a shallow angle, which would not work with the 90 degree tees I had been using. The answer was found online (where else?) with nylon 'Y' connectors, in just the right diameter. Yes, the gods were smiling on me. And the frame was finished.
I had to write a short script for Blender to mathematically "uncurl", or flatten the curved sections of the hull mesh. This gave me the numbers I needed to loft the patterns. I would make the hull from a top panel, a bottom panel and two side panels. The pattern lofting, tracing and cutting was routine, if a bit tedious.
I used the same techniques for creating the cockpit coaming and for closing the stems I had developed from the first boat. The new CT material was a pleasure to work with, taking on a softer feel the longer it was handled. One minor confusion I encountered was that the skin ended up being about 0.2% shorter than expected. This might not seem like a lot, but it works out to about 1 centimeter over 17 feet, and meant the frame needed adjustments for it to fit into the skin. I had the same problem with the skin on the first boat. The startling conclusion I came to was that simply handling any creasable material creates tiny permanent undulations that can never be straightened out. The effect is a small amount of "shrinkage" after a significant amount of handling. At least, that's the best I could come up with. I did confirm this effect by crumpling up a piece of paper, flattening it out as much as I could, then re-measuring the width. It was noticeably less. This is something I will have to take into account with future projects using non-woven materials like Cuben Fiber.
The Finished Kayak
After making the frame adjustments, the moments of truth arrived. I pushed each of the frame halves into the skin, rolled the coaming over the coaming ring, slipped the partially inflated sponsons over the stringers one end at a time, muscled the frame halves apart to install the center sleeved sections of the keel, gunwales and sponson stringers, then inflated the sponsons. It looked good.
Not surprisingly, the full frame made a big difference to the longitudinal stiffness and resulted in a more pleasing shape than the first boat. The downside is much greater complexity than the 14 footer. Ah, life! It's so full of compromises.
I did the Powell Forest Canoe Route in the new boat summer 2015 and have no problems to report. After hitting more than a few trees during the portages and scraping along quite a few sandy river beds there is not a blemish on the skin that I can detect. The boat paddled beautifully, as far as my minimal paddling ability can attest. The weight, at just under 13 pounds was easy to carry over a long portage without stopping.