To me a board whether it is a kiteboard or a surfboard isn’t just some piece of hardware that goes under your feet that you will then just thrash around on but is an embodiment of stoke that contains ones hopes and dreams of good times that have been and good times to come. Yes there are the engineering aspects of it, are the hardware points in the right place, is it strong enough, etc. but there are also the aesthetics of it that to me are just as important. The thing needs to look sexy too!
That said I really have a deep distain for marketing crap… but perhaps we’ll leave that for another day.
Design Constraints
We have a few constraints that we need to work around on this build. As I mentioned in the introduction post I am going to use a rocker table and I am going to use prefabricated fiberglass/epoxy sheets for the skins. Both of these are going to limit me on what I can do for bottom and top contours. Because I am using a rocker table the bottom is going to need to be flat as there is no matching mold to press the laminate into any contours. There may be some ways around that limitation with the rocker table but for this first board at least I’m going to keep things simple and stick with a flat bottom. Again on the top I want to keep the contours soft and gradual especially staying away from any hard steps or edges in the design as I have concerns about being able to get the premade fiberglass sheets to bend around hard steps or take to doubly curved surfaces.
Materials
Oh here is where the marketers usually take mundane things and transform them with gobbledegook into amazing features that one must have to make their world a better place. If you like that kind of thing than you probably don’t want to read the next part because I am going to break down those materials into what they really are.
A twin tip kiteboard has 4 main components, the skins, the core, the edge and the inserts. In a commercially built twin tip the skins usually are made up of 2 layers, a top sheet which usually has the graphics on it, backed by a composite layer that consists of fiberglass and/or carbon saturated with an epoxy resin that bonds the skin to the board. Triaxial fiberglass is probably the most common type used although other styles can be used as well. Generally if carbon is used it is used in small quantities to stiffen the board to some degree or stiffen specific areas. Using 100% carbon often results in a board that is too stiff and passes the energy of pounding through the water into the ankles and knees of the rider. On this build we are going to use a 20oz triaxial e-glass for the skin with no top sheet. Triaxial e-glass has the fibers oriented in 3 directions, down the length of the board to give the board the proper stiffness and strength and at +/- 45 degrees to impart torsional stiffness. I am also going to add some vectornet (which is an aramid) mostly for cosmetic reasons. For a resin system we’ll be using an epoxy, in this case Pro-Set INF114/210. We will use a method called resin infusion where we place the fiberglass under vacuum and then pipe the resin in to saturate the fabric, this will give us a very high quality, void free skin.
For the core on this board we are going to use poplar. It is a commonly used core in skis, snowboards, and kiteboards. it grows pretty clear (knot free), has good flex characteristics, and is inexpensive. The downside is that there are other woods out there that are lighter. Paulonia is one that has kind of hit the market in the last couple of years but getting it in small quantities can be difficult and it can be pricey. Other core options are balsa core and high density structural foams like divinycell.
For the edges I am going to use casting urethane. This will give me a nice one piece edge that is well bonded to the wood. I am also going to use the urethane to cast inserts where the fins mount and in future boards I’ll also use it where the binding and grab handle inserts go to isolate those from the core. Casting these areas is a common technique in the industry and results in a high quality edge that is also easy to manufacture.
For inserts we are going to run stainless steel t-nuts. Unfortunately they will be 1/4″-20 inserts as I had not found a source for M6 by the time I got to installing them. Industry standard is a metric insert (M6). Not really a big deal unless someone tries to thread an M6 screw into it not knowing that it is different.
Challenges
Coming into this I have some experience with building boards, I have in the past built many surfboards and at one point build a plug for a wakeboard but when it comes down to taking thoughts and ideas out of my head there are many areas where I am just going to go with my best guess. Some of those are:
Rocker, the challenge on rocker is that we need to account for an unknown amount of springback from the poplar core. In other words we are going to take a flat core and bend it while we glue it up in the rocker table, I expect that when we remove it from the rocker table that it will relax back to some degree and we won’t have the full bend of the rocker table. I haven’t found good information on how much to expect and I expect that different woods and wood densities probably have different spring back rates.
Board outline, I have some ideas here but still when the rubber hits the road I am taking some educated guesses, I want a board that edges upwind well and can hold an edge to load for jumping, so I will probably go kind of flattish through the center/heels and gradually pull it in at the tips.