So we’ve had plenty of time out wakesurfing and posting about a wakesurf trick or two, now it’s time to get back to work on a wakesurf board build! We talked briefly about creating a perimeter framework for this wakesurf board, out of high density closed cell foam, that sell under the trade name Divinycell. It’s a cross-linked PVC foam, the base material shares it’s origin with your sprinkler pipe! Now we are going to talk about a concept that we use frequently and it’s sort of an unwritten rule in composites. As students of composites you know that thickness = stiffness and that additional thickness creates an exponential increase in stiffness. But there is one more thing that helps create stiffness and we like to call that the power of three. That is that any single object can be fairly floppy or NOT stiff, but if you afix three of that same thing together, it will exhibit a much improved stiffness. The perfect example is to take 3 ordinary playing cards for this example.
One card by itself is not very stiff.
Interestingly enough, when you glue two playing cards together there is an increase in the stiffness, but it’s not all that much. You can still bend it relatively easy and there is little resistance to those bending forces. The reason is that that there is a compressive force and a tension force that matches the two pieces and with that here is no shear resistance involved. When you add a third card to the stack, there is a decided increase in the stiffness. Not that you couldn’t bend it or cause the glue joint to fail, but the structure effective resists the bending forces enough that it’s noticeable. What we’ve found when doing things like building up the rails or creating this perimeter framework, if we can glue up 3 pieces of material, rather than using a single or even two pieces to achieve the desired thickness, the resulting 3 piece structure will be stuffer.
Now, that may or may not be what we want in a particular area, but when we DO, three pieces is a better choice than a thicker single piece. So as an example, we want a 3/4″ thick or wide perimeter framework. Rather than using a single piece of 3/4″ thick divinycell, we will glue up three pieces of 1/4″ thick divinycell. This is preferable even to gluing together a 1/2″ thick piece and a 1/4″ thick piece. Again it’s the “power of 3″ sort of rule of thumb.
Ok so with that said, let’s start delving into he creation of the frame. We ordered a single 4′ x 8′ x 1/4″ sheet of divinycell and had it cut down into a few 2′ x 5′ sections for shipping. Here is what it looks like out of the box.
We are going to build up both rails and then a tail block. So for the rails we’ll need six piece of the same basic shape and then for the tail block 3 sort of straight pieces that will wrap the tail.
We use a template to cut the rail pieces that has the basic shape of the rocker. This gets a little complicated because when we wrap the curve at the nose, the material curves inward, but also upward, as if increasing the rocker. To counter this we could have a separate template for the rail material or simply make the rail material a little wider so that it overlaps both the top and bottom of the sides of the board. That was probably clear as mud wasn’t it? We’ll show you a little later, but for now, remember that we are going to me making each piece of rail material 2 inches wide, which will introduce some waste and you’ll understand it’s for this bending and curving issue.
Here is a picture of the template laid out on the divinycell
This is a little tough to see, but what we’ve done is timmed away a section of the divinycell to the left of the picture at the bottom of the template. We use a straight razor for this, but a utility knife or xacto works just as well.
Now here is a picture where we have cut the bottom of the template off and that leaves us with a shape that is close to our rocker. We will then slide the template UPWARD so that it is 2″ above the first cut and run the razor down the template creating a piece of rail material that is 2 inches wide and has the general shape of the rocker.
Viola! Now by using the rocker as the template for cutting we’ll only need to make one cut for each additional piece and the bottom will always match our rocker, so while there is some waste material, it’s really efficeient labor-wise.
Then we lather, rinse and repeat until we have six pieces, three for each side.
Finally we do something similar for the tail block. The tail block is principally flat, so we don’t worry too much about the rocker, but again as it curves, the pieces will curve UPWARD at the ends so we need to make those pieces slight wide than the area to be covered. Here is a picture of the three pieces we cut out.
So that’s all of the material preparation and in the next post in this thread we’ll glue on the tail block pieces to this wakesurf board and describe our process there. Thanks so much for following along, we appreciate it!