Well the weather here in Norcal has definately changed from sunny with highs pushing 80, to rail and highs around 60. Not much in the way of wakesurf tricks going on right now. Cold and dreary is the forecast!
Going back to our MOW specific wakesurf board build you may remember that we were attaching the tail section and then mowing foam to shape the bottom of the MOW specific wakesurf board. The next step in the process is to attach the bottom skin to the core of the wakesurf board and use the vacuum bag to hold that in place while the epoxy cures. We are using epoxy, in this instance, as the glue to attach the bottom skin to thecore, because the core itself is EPS foam, which severelly limits the available options for adhesive. EPS will melt with exposure to styrene, so rules out two other resins: polyester and vinylester. We have a bunch of epoxy available, and so opt for that.
We are going to be using Carbon Fiber for the sandwich layer of reinforcement between the EPS core and the high density skin. Here is a quick picture of what the Carbon Fiber rolled out on the core.
Now we should take a few minutes and talk about the general concept of how a composite sandwich works and how we like to glue things together. Intuitively, if we think about gluing a structure together comprised of different types of materials, we would think that any order or layering technique would be ok, but that isn’t the case. We’ve talked about teh concept of shear, wherein different parts of the structure are allowed to move relative to each other and that is what allows the composite sandwich to develop great strength. Remember when we were discussion epoxy and it’s elongation ability compared to polyester and vinlyester? How that elongation is what lead to the strength of epoxy? You can see that internally, a wakesurf board would be stronger if rather than BREAKING it was able to give way a little, through flex and that would allow the forces being applied to the wakesurf board to be distributed over a wider or larger area and absorbed within the structure of that larger area.
As an example, what is easier to break? A 2 x 4 piece of pine or a toothpick made from that same pine? The 2 x 4 with it’s greater area is much harder to break than the toothpick. Also, compare a toothpick and a rubberband. A force adequate to break a toothpick can normally be applied to the rubberband without causing failure because the rubberband can stretch and pull out of shape. So that elongation ability can allow the rubber band to withstand greater forces.
Ok, so normally when we construct a composite sandwich we attempt to layer components from the inside out, keeping like elongation properties next to each other. So Carbon Fiber which is the stiffest, would NOT normally be attached to EPS which has almost no stiffness whatsoever. We said – to hell with that! 
Actually, we are constructing a relatively stiff structure, but we still need some flex and shear within the core. Our expectation is that by using the carbon fiber as the sandwich layer, that we’ll actualy cause a delamination from the eps core after a few rides and that the EPS will only act as a support for the external skins, they won’t actually be attached. That is the EPS will break away. We don’t really know how that will work, but we’re going to give it a try! Now we also went with Carbon Fiber as the sandwich layer because of it’s stiffness. We want super fast rebound from any flexing and that dictated the stiffest reinforcement we could find.
Next up we show how we have cut the carbon fiber to the general shape of the core of the wakesurf board.
We didn’t get any good pictures, but the next step is to apply resin to the carbon fiber reinforcement and then lay the bottom skin on top of that wet out carbon fiber. In this instance we are using some A500 Corecell, not for any mechanical properties, merely because we had some left over. Once the bottom skin is in place, we use some blue painters tape to hold the skin in place while we slide the project into the vacuum bag.
Finally we pull a vacuum, which presses the bottom skin and core together and it is also being pushed against the rocker bed to insure that we have the revise rocker and the bottom concave.
That’s all for this post and in the next post we’ll pull the MOW specific wakesurf board from the bag and being trimming the flashing or excess material around the rails.
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