Composite Sandwich Wakesurf build - 2010 :)

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Towards the end of 2008 I posted a build thread on a composite sandwich (compsand) and I'm at it again. I thought I would document some of the theories for gearheads like myself on this build.

That 2008-2009 build is located here: http://forums.wakeboarder.com/viewtopic.php?t=79787

A surfboard is probably one of the most simple composite structures. A foam core, possibly with a vertical wood stringer and fiberglass reinforced plastic facings or skins. The FRP in this instance is typically E-Glass and Polyester resin. Wakesurf boards are evolving some and so we are seeing the FRP more frequently as S-Glass and Epoxy resin. Still the underlying structure is foam and FRP.

It's a formula that works and most manufacturers, pressed for profitability, typically won't change a formula that works for them.

During WWII, faced with steel shortages, the DeHaviland designed a plane that could withstand the rigors of combat and twin engines using wood. The wings and fuselage of the Mosquito were comprised of a balsa wood core and plywood facings. During this time, much of the formulas for determining the resultant stiffness of composite structures were documented and standardized.

One of the things that was most notable was that the stiffness of a composite sandwich becomes exponetially stiffer with the distance between the center of the facings. That is the thicker the sandwich, the stiffer the sandwich. All things being equal, a 4 inch thick board will be stiffer than a 1 inch thick board.

For those of you that have difficulty falling asleep, a great discussion of sandwich panel stiffness formulae are contained here:

[/url]http://composite.about.com/library/docs/mil-hdbk-23/bl1-3.htm[/url]

Quick images that represent the formula and the components.
[/url]

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The most important part of the formula for this introductory discussion is the h squared in the numerator of the equation. As you can see in the diagram h = the distance between the centroid of the facing. Increasing that distance has an exponetial (squared) impact on the resultant stiffness.

A thicker board can actually be lighter and stiffer than a thinner board, because to compensate for the reduction in thickness a buider must use higher density foam or more of the FRP plastic facings - either in the form of more layers or more dense fabric.

If you've ever ridden a fairly thin board, they tend to be more responsive than thicker boards. The reason being that turning involves sinking the rails in the wake, thicker rails are more bouyant and tend to resist sinking. Again, all things being equal, thinner will tend to be more responsive than thicker - however that is qualitative and only applicable within a relative range. It would be easy to grasp that a 1" thick board would be more responsive rail-to-rail than a 4" thick board. However, a 1" thick board might prove negligiblely more responsive than a 1 1/8" board. I know that I would have difficulty feeling the difference. Also, at some point, responsive becomes "twitchy" at some point for most folks.

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In the previous build, my sandwich was comprised of high density foam skins over a low density foam core. I used Divinycell for the skins. Divinycell or Dcell is a cross linked PVC foam. It's most notable attributes for composite construction are ease of acquisition, ease of hand forming - without heat and realtively speaking lower cost. It is NOT a great foam though. Typically it's used where lower cost is the primary consideration. A better choice, IMO, for board building is Corecell. Corecell is expensive, hard to find (I ordered mine out of Canada) and is substantially stiffer then Dcell. Corecell is an SAN foam, which stands for Styrene acrylonitrile. It's hard to sift through all the hype by foam manufacturers and board builders alike, but Corcell claims to be the leading foam core material.

In my experience, Dcell is very flexible whereas Corecell is stiffer. In wakesurf applications, this translates into more pop when using Corecell as the high density foam. Again, in a manufacturing environment, Dcell is cheaper and easier to work with. Most consumers, at this point, are uninformed about the quality of foam and resultant ride, so manufacturers are free to use lower quality materials - injection molded boards from the big manufacturers being a prime example. No one buying a HL Broadcast is saying - I wish this board had more pop!

Two pictures, corecell is the yellow'ish foam that seems almost unaffected by gravity, whereas the dcell sags under it's own weight.

[surfdad]

Two more pictures and that will be all for today. This is a composite sandwich I had built by Eric Roush of Roush Custom Creations. This will be what I'll be modeling in this build thread.

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In composite construction, the 5# density foam such as Divinycell H80 and Corecell A500 (pictured above) are considered low density foams. In surfboard construction 5# density is significantly high density. The old school Polyurethane stringered blanks were in the 2 to 3 pound range. Current EPS and Epoxy boards tend to have 2 pound density foam. For my purposes, the 5# foam is high denisty.

The theory of sandwich construction is that an external layer of high density material, in this case foam, sandwiches a core of low density material. For this board that will be 1# density EPS foam. For those reading that have familiarity with construction you probably recognize the concept from SIP's - Structural Insulation Panels that are typically comprised of OSB on either side of EPS foam.

Technically speaking, anything less than 5# density foam isn't structural. In this application, the board will have a skin of 5# foam, and as I mentioned the 1# density core. This creates an I Beam where the flanges (5# skins) distribute the load across the entire surface (or at least distribute so that the load isn't localized) and the web (vertical part of the I beam or the 1# density core of this board) can absorb the load via shear (movement or sliding). In our construction the skins and the rails comprise the strength of the board and the core tends to mostly keep the skins and rails fixed realtive to each other.

In the I beam context, the board is an infinite I beam, that is the entire construction is comprised of flanges and webs.

For those that want to understand the theory more, Diab (the manufacturer of Divinycell) have an excellent published report, although a bit spam'ish for their products.

http://www.diabgroup.com/europe/literature/e_pdf_files/man_pdf/sandwich_hb.pdf

Another great source that is easier to read is contained on One Ocean Kayaks:

http://oneoceankayaks.com/Sandcore.htm

These pictures, visually show the simple I beam and Infinite I beam concepts.

[surfdad]

As described earlier, I use 1# EPS foam in the core. Principally it's mostly air. The foam is comprised of tiny beads that are expanded with heat from steam while being compressed. The beads are filled with what is called a blowing agent, most typically Pentane which expands when heated. In the early days of EPS manufacturing, it was used as insulation, but wasn't compressed well and typically would wick water. You can still find the crappy EPS that seems porous, but the EDRO process that most surfboard EPS is manufactured under solves that problem and the EPS manufactured today, for boards, doesn't wick water (much ).

I start with a billet of 1# EPS, about 4" thick and 2' x 5' width x length. I have a rocker template (the curve of the board when looked at from the rails) that are made of masonite that I will attach to the sides of the billet and then I melt the foam using the hotwire to create the rockered foam core.

Not all foam can be safely hotwired, Dcell and most high density foams will emit toxic fumes when being melted. Manufacturers have specific foams that don't give off toxic fumes when melting.

When attaching the hotwire templates I need them to be plumb on each side, so I mark the locations to ensure that the cut is square.

Pictures of the billet being marked and with the templates attached. I use 4" long wood screws to hold the template in place. Care should be taken to make sure the screws won't get in the way of the hotwire during the cut and also that the screw holes left over won't be left in the board after the outline is cut - I place the screws towards the ends of the template where they end up the in the offcut portion of the foam where the nose and tail are shaped.

[surfdad]

As described earlier, I use 1# EPS foam in the core. Principally it's mostly air. The foam is comprised of tiny beads that are expanded with heat from steam while being compressed. The beads are filled with what is called a blowing agent, most typically Pentane which expands when heated. In the early days of EPS manufacturing, it was used as insulation, but wasn't compressed well and typically would wick water. You can still find the crappy EPS that seems porous, but the EDRO process that most surfboard EPS is manufactured under solves that problem and the EPS manufactured today, for boards, doesn't wick water (much ).

I start with a billet of 1# EPS, about 4" thick and 2' x 5' width x length. I have a rocker template (the curve of the board when looked at from the rails) that are made of masonite that I will attach to the sides of the billet and then I melt the foam using the hotwire to create the rockered foam core.

Not all foam can be safely hotwired, Dcell and most high density foams will emit toxic fumes when being melted. Manufacturers have specific foams that don't give off toxic fumes when melting.

When attaching the hotwire templates I need them to be plumb on each side, so I mark the locations to ensure that the cut is square.

Pictures of the billet being marked and with the templates attached. I use 4" long wood screws to hold the template in place. Care should be taken to make sure the screws won't get in the way of the hotwire during the cut and also that the screw holes left over won't be left in the board after the outline is cut - I place the screws towards the ends of the template where they end up the in the offcut portion of the foam where the nose and tail are shaped.

[surfdad]

Oops, sorry for the double post, not sure what I did!

[surfdad]

I use a Tekoa hotwire bow, but a wood or PVC pipe bow melts foam just as easily. With the templates attached, it's a simple task to run the bow along the templates and melt the rocker.

Pictures of the bow and templated foam, mid cut and finally the rocker cut into the foam.

[surfdad]

I want to talk about a moment about rocker and how it's measured. In surfboard terms, rocker is defined in nose rocker and tail rocker numbers. It is measured by finding the exact midpoint of the board both lengthwise and widthwise, then laying a long straight edge lengthwise and adjusting the surfboard so that this straight edge is level. Once that is achieved, measurements are taken from the inside surfaces of the board to the straight edge, both at the nose and tail.

This isn't too much of a problem with continuous rockered surfboards that are typically 6'2" in length, but try it on your Inland Surfer Blue with principally no rocker and just a nose flip or any skim style board. It gives us misleading results.

For my purposes, I have found that the apex of the board (and realize that this can change dependent upon how the board is oriented to level) is extremely important. The apex, or the highest point along the rocker curve is where the board will want to load into a turn up the wake. If the apex is too far forward, the board will not be responsive. It's like you are turning with both feet on the tail. If the apex is too far rearward the board will be slow, basically plowing through the water.

With longer surfboards manufactured for the ocean, you can move around somewhat on the board to place that apex very close to your front foot, which I have found to be optimal.

Many of the small independent surf style manufacturers have this apex placement wired, some don't. Taller folks with wide stances want an apex moved forward - typically a slightly longer board if you're buying off the rack.

I say all of that, only to describe the measurements and lines that I am placing on the core. Also to demonstrate that I take rocker measurements off the apex and NOT off the midpoint of the board, as I am of the opinion that the apex is more relavent on our small boards, than is midpoint.

Since this core doesn't have a stringer or a glue line, I need to draw in a centerline. This doesn't need to be eaxctly down the center - it can be off up to about an inch (I'll be cutting the outline which will leave about 1" on a side), but it must run exactly square to the cut of the rocker. Any diagonal will produce a crappy board.

Pictures showing the centerline, a line that signifies my apex and then the way that I measure rocker. In that rocker stick picture that is also how I determine apex. I place the squared deck side of the core on a level surface and then slide the rocker stick along until it is parallel/level.

[Rhawn]

Neat stuff Jeff. Keep us posted on the progress!

[surfdad]

Hey Rhawn, thanks and thanks for following along. I'm further along on the build than I've posted up up, so hopefully I'll catch up soon.

[surfdad]

Professional shapers keep a supply of templates handy in their shaping bays. Typically one for each tail shape and one for each nose shape. The templates are typically just 1/2 of the shape. The shaper aligns the template on one side (left or right), then flips the template over and does the opposite side. In this way, the same outline is duplicated on both sides. I tend to lose templates so I just have all the curves on one piece of masonite.

The template as alingment marks for centerline and apex.

Pictures of the template, half cut and the other side marked up then finally the outline cut. I clean up outline cut with a sanding block to bring it down to the mark up line.

[surfdad]

The above steps comprise the shaping of the core. It's also principally what's involved in shaping a board from an EPS billet by hand. You can find acceptable EPS at insulation contractors and also archetictural foam dealers nation wide.

For this build I am going to be building a full perimeter frame using corecell and carbon fiber. To make the rails, I use the rocker hotwire template as a guide and then trace the outline with a single edge razor. I'm using 12 mm Corecell for the rails and my outline template, above, takes this into consideration.

I cut the rail material at 2" high. This will cover the thickest point and give me some slop - just in case. I have to do a little shaping of the rail material - a 45 degree angle at the nose where the two rail pieces meet and then at the tail where the rail material is taller than the core. If I leave the rail material proud, it tends to act as a lever pulling the bottom away from the core and ruining the glue up. I also keep the rail material cut parallel so that the bottom is already cut when I go to cut the next piece. Each rail piece is then always a single cut.

Pictures of this process: The corecell sheet with the rocker cut from the last project, template on the corecell ready for cutting, single edge razor mid cut, one rail piece cut, both pieces on the board, the tail shaped and the nose cut at 45 degrees.

[surfdad]

Just a quick word about rails. Surfboards have always been perimeter weighted and stiffened by virtue of the rail laps of the external glass. The laps are that area on the rails where the glass from the top and bottom overlap. A standard shortboard might have two layers of glass on the deck and one on the bottom, therefore the rails will have three layers of glass and as such will have extra weigh in that area.

Also, the rails are the area most prone to dings. A heavier rail offers a few advantages, one is the resitance to dings and the other is that a heavier rail makes it easier to set the rail into the wake face. The other thing I've found is that heavier and stiffer corecell affords greater resistance to rail "twist-off" under hard turns and aids with developing pop off the lip.

[surfdad]

I'm a big fan of perimeter weighting and stringers. They feel like the wakeside stringer is easier to load than a center stringer and give a more responsive ride. Also, when a board fails, say by creasing, that failure starts at the rail and progresses inward towards the center. Perimeter stringers seem to offer greater protection from this sort of failure.

For this build, I'm not going to use wood for the stringers, but instead biased 6 oz carbon fiber. In the first build of this type back in 08-09, I used a single layer of 6 oz CF, but I want to make this board super stiff and springy, so I am doubling that CF matrix to two layers of 6 oz, plus I have cut the CF on a bias to create a (close to) 45 degree angle of the weaves. I will lay the CF up such that the biases are opposed.

All boards flex out on the water. You could manufacture the board out of solid titanium and it would still flex, if it didn't sink. Flex isn't necessarily a bad thing, a too flex'y board would be floppy, but one that can be loaded up and that then unloads coming out of a turn or off the lip, will exhibit the "pop" I'm striving for.

Our tiny wakes, in comparison to an ocean wave, don't have much power and so flex cycles need to be much quicker than in the ocean to feel that responsiveness. With that in mind, I'm going out of my way to create a board that is super stiff and flexes, but also unloads or springs back quickly and with force. The first design change from last year was this double layer, biased 6 oz CF along the perimeter.

I used scrap CF for the perimeter, I had saved small sections of offcut that allowed me to cut the bias. I then cut 4 pieces - 2 for each side. I mixed up about 7.5 oz of epoxy, wetting out the CF first and reserving some of the resin to paint onto the corecell rail material. I will use a vacuum bag to apply pressure to the laminate stack while it cures, but to ensure I have everything in place, I tape the rail material into position. The procedure is double layer of CF laid up onto the EPS core, starting at the nose and working back to the tail. I ensure that the bias is running like I want and then I also make sure that the CF is alinged at the bottom, as this will be the most crucial area. Once the CF is on, I then align the rail material, again starting at the nose and working back towards the tail. Then I repeat that process on the other rail. When both rails are taped up, I place it on the rocker bed and then slide the rocker bed and board combo into the vacuum bag and pull the vaccum...not too much pressure, about 10 inches of mercury, just encough to consolidate and conform the rail material to the rocker bed.

Pictures of the process and also the board out of the bag. I think 5 pictures at a time:

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Pictures on the rocker bed and in the bag, then three views of the glued up core and rail material.

[Nooga678]

Looks good. I would like to here more about your rocker theory since I am having to design my wood boards from scratch in AKU shaper. I try to apply ocean board characteristics to the wave we surf, but I guess that may be wrong.

So you think it is best to have the apex directly under the front foot? And this is what you measure the rocker from. So using that method, what kind of nose and tail rocker do you shoot for? This board seems to have a lot more rocker then I would have guessed was desired. Is that more for sharper turns?

[surfdad]

Hey Nooga,

Thanks and thanks too for following along. Typical ocean short boards will have the apex close to the front foot, but behind it. If you look at smallerish ocean wave, say waist to head high, the face has a much longer transition than what we can generate behind the boat. Our pockets are much smaller and tighter in comparison. What my "tinkering" has shown me is that we can't build to ocean spec's. Like you no doubt have discovered when you size down a computer template the whole thing shrinks in proportion. What I have found most useful is to remove most of the length from the front foot to the nose. We aren't dropping in, so don't need 4 inches of nose rocker. We still need the full width of the tail, but not all the rocker, which just sucks the tail down and creates that plowing effect. In the pictures above, it's a little deceiving, because I am going to flatten out the board from the apex back to the tail with the vacuum bag. When I am done, I will have a nose rocker in the 2.5" range and a tail rocker in the 1" range. The pictures don't show that through the middle the board is basically flat - think of a 3 stage rocker, rather than continuous.

You're right in your deduction, more rocker tends to allow more aggressive turns and vertical entry into the wake face. However, it also can slow a board down. I counter that with a deep concave that is almost flat between the feet. It seems to offer the best of both design elements.

For folks following along, flatter rockers tend to create faster down-the-line boards, but at the expense of turning responsiveness. Heavily rockered boards are more responsive in turns, tighter and quicker, but tend to be slower down-the-line.

Finding that balance between speed and responsiveness tends to be difficult.

[Nooga678]

I see now, your second to last picture looks alot more like what I would have expected. I sounds like I was somewhat on track in that I knew they didn't need a lot of continous rocker or tail rocker. I was taking board templates and lower the tail rocker to .5"-1" and the nose rocker to 2"-3" then flattening out the bottom rocker to make it more 3 stage like. I just need to slide the apex more forward than a reduced ocean board template places it, which makes total since from your explanation. Some specs on your convace would be appreciated as well.

So with a wood board where the rails are built up, they will add up to two inches of wood to shape the rail into. The corecel doesn't look that thick. Once the rail is shaped how much is actually left? Do you only need enough to strengthen the rail rather than the whole rail being corecell?

[Okie Boarder]

Looking good. This is a great and very informative thread. I have even more appeciation for the work this takes, than I did last year, since I've been working on the rebuild with my boat.

[surfdad]

@ Okie: Thanks for tuning in again and the props.

@ Nooga678: yeah, the corecell rail material is 1/2" / 12mm, I'll be adding another 1/8" / 3mm after I have the bottom skin attached, so a total of 5/8" / 15mm.

Many surf style boards still use a domed deck. The reality is that flat is a better arrangement - something the skim style folks will attest to. The dome in the deck of center stringered boards was done merely to increase break strength, that methodology was crappy to begin with and added nothing to the ride. However, without it, those board creased constantly. Flat gives better leverage, some have even gone concave, but that just feels weird to me. Also, a flat deck distributes the load more uniformly than a domed deck. Lastly, it slightly reduces the C of G bringing the feet closer to the water - less "foam" to manuver during tricks.

This construction methodology, the sandwich, doesn't need the thickness for strength or stiffness, but below 1" and the board is not forgiving, it tends to be grabby in turns. I'm going to have a max thickness around 1 3/8" and the deck will be flat out to about 4" off the rails where I'll start my first rail band. I'll have a solid 3/4" high, by 5/8" wide section that is rounded for the rails running the full length, it will be taller at the apex, and shallower at the nose, however.

With the flat deck though, the corecell will comprise the majority of the rail.

Your rocker measurements sound good, you'll experiment and get a better feel for what work in your situation to fine tune it. EXACTLY! as to the software, it just takes a 6'2" and smooshes it into a sub 5' . There will be some change in the tail - that last 6", most of us like that skatey feel of a broader tail, but the mid is the same and the nose doesn't really exist, it's basically just the tip. Jerry Price of Shred Stixx once describe wakesurfers as a tip with a tail.

If I were building cutom boards, I'd get the stance measurement from folks, or create a standard that was somewhat based on height, as well as, weight.

[surfdad]

I blew right past the concave question. I run a single, 1/4" max depthat the apex. Fades to flat past the fins.

[Nooga678]

By flat do you still mean concave, but with a flat section or do you mean the concave fades away?

I would love some input on a good rail for my second build. It is heavy and therefore going to sit lower in the water so I don't know if I should go with a 50/50 for the whole rail or stick with my orginial plan to transition to hard tucked rail.

[surfdad]

My 2 cents - stick with your original plan. Water wraps curved surfaces and if the board is going to be sitting lower in the water, the water will wrap that 50/50 rail creating all sorts of drag. Give it something to release off of and you'll be pleased.

[surfdad]

Back to the build.

After it comes out of the bag, the rocker is pretty sloppy, I hit with a planer set as shallow as possible to clean it all up.

[surfdad]

The last part of the frame is the tail block. I just square up the tail, and then glue up a section of left over Corecell. I don't trim it too close as I will be shaping both the deck and bottom shortly.

I glue it up with some cheap 5 min epoxy, but I mix in some cabosil to make it harder.

[surfdad]

I glue it up on the rocker bed to make sure the bottom is close, then trim everything up with the planer.

[surfdad]

That's the application of the perimeter frame. The entire rail and tail will have the 5# density corecell.

[surfdad]

I missed the discussion on Cabosil. There are all sorts of things that can be mixed with resin to alter it's properties. If you can gain access to a compsoites shop, there will always be some old-timer that knows how to mix swine anti-biotics and Vinylester to create some super strength bond. The two most common additives for resins are the Cabosil I used above and Microballoons. Cab, does two things, the first is that it thickens the mixture and helps it to cling to vertical surfaces. As you can imagine in my gluing of the tail block, that the resin would have flowed downward with gravity, leaving the top resin starved and the bottom resin rich. The Cabosil prevents that running. The other thing that Cab does is make the resultant mixture harder. Epoxy is a medium soft plastic and, the addition of Cab makes it very hard and slightly brittle.

Microballoons increases the volume of the mixture and reduces the weight, mb's are hollow spheres and as such weigh much less than resin. Also, mb's help aid in sanding the resultant cured mixture.

[surfdad]

This method of construction places skins over the low density core, to create the infinite I beam concept. Most of the load is carried on the skins, but the fin boxes will sink into that core. I will be using Futures fin boxes, which are 3/4" deep for the rail fins. I've been careful to ensure that the thickness of the board is sufficient to house the rail boxes.

The only problem is that the 1# density EPS won't handle the side to side load of the fins when pumping or turning. To compensate for this inadequacy, the common practice is to sink high density foam into the low density foam. I like to make sure the rear edge of the insert is connected to the deck skin, to eliminate any side to side moment, while still allowing lengthwise flex of the board through the tail.

I use the soft malable divinycell H80 for the inserts. I always save all the offcuts for just this purpose.

[surfdad]

The process isn't all that hard, but there are lots of small steps. I first lay out the fin box location, allow about 1/2" all the way around. Next I cut enough strips of the Dcell to allow me to sink the boxes into the foam without poking through.

[surfdad]

In the last picture above, you can see that I have traced the outline of the inserts. I then use a router to route out that part of the foam so that I can sink the boxes in the foam. I leave the inserts a little proud, so that when I shape the bottom concave, the inserts will follow that shaping.

Then I glue the whole thing together with a slow cure epoxy so that the heat of the curing epoxy doesn't melt the surrounding EPS. I hold the insert in place with blue Irwin clamps while it cures.

[surfdad]

There are any number of bits and pieces that could be added at this stage. Wood, more carbon or whatever makes sense.

[surfdad]

Nooga,

I didn't mean to ignore your question about single to flat! Old age is NOT for wimps! I just forgot until I thought about posting up the next section.

The common normenclature in shaping is to describe the bottom contours from nose to tail. So: rolled to single to double to V would describe the countours, starting at the nose, as:

1) Rolled
2) Single Concave
3) Double Concave
4) V

Flat is just like it sounds a flat section. I use a single concave from in front of the apex through the fins and then that fades out to a flat section from the fins out the tail.

With that said, I shape a single concave in the bottom before affixing the skin, the external skin will conform to this shape. Most any shape that is curved, can be skinned, with the exception of 90 degree angels as would be found in channels. Those are built with separate sections of HD foam.