Last summer, my wife and I stayed at an island resort in Bocas del Toro, Panama, which furnished us kayaks to use in the island-strewn surrounding ocean. Since the region was a tropical bay, small waves were of local origin, wind-driven but not open ocean swells. Anyhow, we enjoyed the experience, and my wife said, "Can you build me a kayak?" Well, of course I can!
I built my first boat, a self-designed kayak, in 1975. Not a great design, but very appropriate for our circumstances at that time. Now I have the opportunity to do it again. My wife, Dawn, and I will never be hard core kayakers. What I have considered is something easy to store and transport, fairly stable, but also easy to paddle with a decent glide and directional stability.
Looking at the commercial roto-molded plastic kayaks, most are about 12 feet long and 2 1/2 feet wide. I used those dimensions as a starting point. My result, mathematically generated, will be about 12' 4" long and 28" wide with a waterline width of 23-24" depending on the load. Height at the midships sheer of 12" and at the cockpit center of about 14" (cambered deck). With a waterline width that narrow, stability must come from a centerline flat, gentle deadrise, and then progressive steeper topsides.
Concerning length, I have spaced the frames (or molds) 12" apart. A longer hull can be easily created by re-spacing the frames and redrawing the bow and stern entries. My 1975 kayak was almost 16 feet long, but it was for two persons.
Progress will be slow. We have other interests to deal with. (My wife wants to re-paint some of our home spaces.) Not a problem; we already have several boats. Actually, buying a light trailer is another task to research. I'll post some sketches soon.
This photo shows an overhead sketch view and profiles of some of the frames. There will be a one-inch rocker in the keel at both ends. I don't need exact drawings here because a table of accurate dimensions was the first thing created. Full-size frame patterns will be made from those dimensions.
How can this be a developable hull design when the frames are so smoothly curved? The answer is that I intend to sheath the hull by strip planking. Wider planks would result in a more faceted surface. In a hull of such narrow width, planks by necessity need to be narrow to hug the curves. I calculated many spaced points at each frame location and then connected them with a smooth curve. If I had connected them with straight lines, you would see a faceted surface on this sketch.
Should I explain in detail the steps in this design? I am thinking not; the design calculations are easy for me due to my experience, but it would get complicated to describe clearly in words. I start by describing the major chine, then create a midships cross-section, find mathematical curves to fit the overall dimensions, and then create further projections to describe additional minor chines. Finally, I describe the desired sheer location and the desired curve for a cambered deck (not shown).
X is the length from bow toward stern calculated to the inside of sheathing. Yc (width)and Zc (height) describe the major chine. Y1,2,3 and Z1,2,3 describe the topsides projections, additional "chines". Ys and Zs describe the sheer location.
This is a table of all values needed to make patterns for the eleven frames (or molds) excluding the cambered deck which will be on the next graph. Note that between X=75 and 87 all the values are constant. This is a parallel section and could be omitted for a smaller paddler.
First is the equation of the deck camber. Next are the dimensions defining the bow and stern profiles. The bow is a finer angle, 16.7 degrees, versus 20.5 for the stern profile. Finally, we have the outline of the plank keel. Total length is about 129.5 inches, 7.2 inches wide for about 77 inches of its center length. The included one inch of forward rocker extends the forward length of the keel with some curvature. All dimensions are to the inside of the sheathing except the plank keel, since it serves as both frame and sheathing. I have been using 0.5" lumber for the keel and consider 0.3" as exterior and any added thickness is part of the frame.
These two tables of dimensions will give me everything I need to build the kayak. I will build a pattern for the deck profile which I can match up to the sheer beam at each frame. After laying out the outline of each frame, I will design the frame elements and then can start a list of materials.
First thing purchased was the wood for the keel. Needing 11 feet by about 7.5 inches by about 0.5 inch thick, I found two fairly straight boards, about 8' x 7.75" x 0.5". By scarfing at a 5/1 angle (I usually use an 8/1 bevel) I ended up with about a 16' plank. Looking at the keel plank and envisioning the total hull built with it as the foundation, I decided that the hull looked too short, with too much wasted keel plank. So, I have decided to re-space the frames to a 15" interval resulting in a kayak about 14' 2.5" long. Still a handy size which will fit on most light duty trailers.
I will be posting altered dimensions soon. The twelve-footer would function okay, but by increasing the length and fineness, the bow half angle is reduced from 16.7 degrees to 13.5 degrees. Should make it easier to plank. Spanning 15" between frames with 1/4" planks, I will need to be careful in aligning the plank edges, but I have many clamps for that purpose.
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