A theory of how composite boards work

General Discussion
1

I’ve been meaning to post this for a while (especially every time a compsand breaks), but have always held off for one reason or another. Some of these ideas have come from my many rereadings of Bert’s posts and others from making boards with as many different techniques as I can think of. Many of the discussions on Sways regard the boards as a giant single panel (which is true in traditionaly made boards) instead of a deck skin, bottom skin, rails, and core (which I believe composite boards are). One of Bert’s biggest ideas he pushed was the benefit of the custom board and I believe these were the variables he tweaked to make fine tuned boards.

Skin Design

The skins play the most significant role in how the board works.  They are what controls the flex, springback, resistance to breaking, but they are also important for transferring load to the rails (I'll talk about this later).  There are three main parts of the skin: glass in tension, glass in compression, and a core that determines how the skin behaves.  Whatever is going on in one skin is reversed in the other skin (the deck is in the opposite state of the bottom).  GL said that fiberglass is roughly twice as strong in tension as it is in compression so an equally glassed skin 4Balsa4 is really unbalanced.    The side of the skin that is normally in compression needs twice as much glass to be balanced.  Bert talks about the deck skin being built to withstand compression and the bottom skin to withstand tension so (inside)4-Balsa-8(outside) would really be more balanced for a deck skin.  There are two important considerations in regards to which core material you use in the skins. The core you pick for the skin determines the liveliness of the skin and the thickness of the core determines the stiffness.  If the core is inert (think divinycle) you only get stiffening, but if the core is lively (wood) you get springback.  Since the deck receives the most compression force the wood should be thicker than it is on the bottom of the board (wood is good in compression). 

I think the two skins need to be designed differently in regards to resin penetration.  The deck skin needs resin penetration to fortify it while the bottom skin doesn't.  The deck skin is primarily designed to handle compression and impact (which is nothing more than downward compression) while the bottom skin mostly handles tension.  Resin is very good in resisting compression and so is wood.  This leads me to think that a resin impregnated wood would be outstanding in compression but would be weaker (snappier) in tension so..............resin impregnation in the deck and none in the bottom.

Skin/Rail Interaction

No matter which technique you use to assemble the skins to the rails you'll get stiffness in the rails (good for turns) and impact resistance, but they can do more.  I started out attaching both the top and bottom skin to the rails.  This turned out to be the worse thing you can do.  I've noticed that overlapping even one rail band had a significant effect on how the board surfed, but I couldn't figure out why (until now :) ).  The benefit of the 1 lb. eps (no, I'm not changing the subject to foam, yet :) ) is that it can withstand shear (turning a rectangle into a parallelogram).  When a board flexes the skins want to slide past each other.  The easier and further they slide the easier/more the board flexes.  When you lock the bottom skin to the rail you are making it impossible for the skins to slide past each other since the deck-rails-bottom are glued into one piece.  However, if the bottom skin is left free floating then it can easily move in relationship to the deck.  The durability problems I experienced occurred because when the rail flexes the top of the rail stays where it is (its glued to the deck), but the bottom of the rail wants to pucker (pull away from the bottom skin) because there's nothing to hold it in place.  This means that you can't skimp on the glass that covers the join between the bottom skin and the rail or you'll get the failures I experienced on the twins.  There are additional benefits to not locking the bottom rails.  The first benefit is that the energy you press into the deck gets transferred to the rails - pushing down on the deck is pushing down on the rails.  This stiffens the rail during a turn so you loose less energy.  If the bottom skin is glued to the rails it ends up fighting the rail digging in.   The other benefit of not attaching the bottom skin is that chop on the wave does not get transmitted to the surfer.  When chop hits the bottom of the board the energy wants to make its way to the surfer.  The first path it tries to go is directly through the foam to the deck.  However, if you use lightweight foam the chop's energy is dissipated/spread out so you don't feel it as much.  The next path it tries to travel the bottom skin, up the rails, and then onto the deck.  By disconnecting the bottom skin the energy can't be transferred from the bottom skin to the rails.

Foam/Core

As you can gather from the previous paragraph I think foam is important to a board.  Its main job is to support the skins.  Its possible to design the board so that the load/forces that the board encounters are transfered to the rails (they do this with bridges all the time).  The problem is that it takes so much glass/kevlar/etc... that the board ends up being extremely stiff, and stiff remains stiff all the way up to when it snaps (there's no give).  The foam provides the support in a lighter package.  The amount of flex the foam has is influenced by the amount of air in the foam so if you fill the air holes up with resin or cram more foam beads (high density) into a given area the amount of air is reduced so the amount of flex is reduced.  I do think that you want some resin penetration into the foam since it provides a skin like you'd find on a Clark blank, but you don't want it to travel in very far.  You might say that boards need stiffness so what difference does it make whether its from the foam or the skins.  I say a lot of difference.  EPS is basically inert so all it can give you is stiffness (just like divynicle) till it snaps while a wood cored skin gives you a lot of benifits. 

I put the stringer in the core section because I think its basically an attempt to make a lively core.  I've ruled out the springer for myself, but I understand how Bert uses them.  On nonstringered boards the focus is the rails interaction with the wave – basically you push on the deck and it transfers energy to the rails which help it dig into the water.  This works great for traditional and power surfings.  However, areal surfing requires you push into the deck of the board to have it bounce off the water and without a center stringer you can't get maximum bounceback.  Normally you want the board very strong in compression, but on a springered board you need to dial this back so that the board flexes into the neutral axis and activates the springer. When the springer unloads it “pops” the board off the water.  I've noticed with the few board I've seen broken that the tend to break on one skin or the other (I haven't seen a clean snap).  This supports the idea that the neutral axis can move throughout the core since the force was able to accumulate on one of the skins.  As far as I'm concerned, the problem with the springer is that it locks the neutral axis into one place and downplays the roll of the rails.
2

Good show,

I wish you had posted this before, It took me days of reading to find some of this in the archives.

I have to agree to most of it.The boards I break seem to have a pretty clean snap, comp & poope.

Any body broke a compsand? how?

And after some playing around I think I prefer a stiffer board for my front foot style and juicy surf.

Ian

3

Nice article DanB

I’ve broken two compsands. The first was balsa railed in overhead surf, broke in two places. The second was redwood railed in overhead surf, it just bukled. Interesting to note… all breaks were in the middle 1/3rd of the board.

Just finished a SUP that I added a deck patch to reinforce the compression AND is in following with your suggestion of balance deck to bottom.

Another step I’m looking into to improve flex without breaking is different rail material. The redwood is a nice contrast to the balsa and stronger than balsa rails but I know that there are other woods that bend/flex better. I’ve used oak around the nose for protection and it bends nice. For contrast with the balsa I’m checking into Red Oak. After a little internet digging I’ve found that cherry wood might be good also, I’ll have to get some and experiment with it… unless someone already knows?

Les

First Board Break:

4

amazing how far everyone’s come in just a couple years straight from being a novice to the level of understanding you’re relaying here…

Two components I think that are totally mis-understood as to the role they play except to the old timers is the effect of the last 24-18 inches of your board where the fins are located and the impact of Bert’s use of an exaggerated “reverse curve” just in front of the front fins. There was some discussion early about the overall impact of this part of the board but for the most part we seem to envision the entire board nose to tail as governing performance when different parts are actually contributing differently.

I very rarely ride a board off the flat surface of the bottom unless you’re going straight in perpendicular to the wave and in fact most of the time we are moving rail to rail and applying force off the back on the tail to carve or pivot. Personally I really don’t want any flex response off the first 50% of my board unless it’s assisting in paddling out or helping with the chop. But my experience is that any flex in the front part of your board will cause it to bog unless the actual length of your body is no more than shoulder width like a mat or sponge. Maybe someone like taj whose super wide cheyne like stance one foot on the nose and one foot on the tail wouldn’t mind it. But my preference is to has the punch or snap effect to come off the back 1/3 of the board as I engage and disengage that part of my board in my turning.

Seem like that’s where I would want to board to flex and fit in to the curvy part of the wave and not to slide out but power through the arc and then pop itself up and out into the next opposite direction tail/rail engagement to assist with that function.

I guess I’ve gotten more confused and frustrated over time as I’ve become aware through discussions with shapers that there’s some much more going on with how a board and the rider actually works within the parameters of the moving wave than one would believe, especially with all the complex stuff happening with the tail area regarding flex, water release over the tail/fin area and rails and the action of the fins in comparison to the forward 2/3’s of the board.

The problem is that when you address one thing you have the potential to really mess up something else…

Thank god for all the folks like you who are doing their best trying to figure all this out for us.

5

I remember seeing a picture of Bert’s 11’ yellow board and it had western red cedar strips in the balsa rails.

I think other guys use it also.Patagonia did some R&D and claim that it is the best stringer material,light and strong.

Bass has good flex and would look nice with red cedar.

Ian

Put it in the bucket and mix it up.


6

hi, tirides/les, sorry to hear you have broken a couple of your compsand boards, i counld/nt help but notice there does/nt seem to be much rail wood in the picture, how did you do your rails ,most people including me use 3 strips of 5mm balsa and then shape to suit, also if you lay up one side of the skin with the weave of the glass at an angle that should greatly help with breakage ,you probabally know all this allready but it just seems unusual to have broken two , pete

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Quote:

First Board Break:

Is that a “springer”? You know, the red thing about eight miles north of the Neutral Plane.

If so, what’s it made of? How wide? How thick? etc etc

If not, what IS it?

:smiley: :jester

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okay

this relates to build scheduals and it is set in basic sandwhich thoery

this is for compsands(not veneer boards)

3/16 is good thickness for a longboard deck

2/16 is good for a shortboard deck

if your using thin skins (1/16 or less)

the boards will break

if you dont use glass under

the boards will break

if your perimeter stringers are less than 1/2 inch

the board will possibly break

read berts original post!

he recomends 2/16 as a good thickness for a balsa shortboard

hes made a lot of compsands

bert recomends a 1# core or less

these are sound guidelines for a qaulity and strong compsand board

id be very surpzied if you could break one in any surf

if you wanna use light skins go ahead

you will need to compensate in other areas so it defeats the porpoise

just use a thicker skin

you can build a flexible 4 to 5 pound shorty with 3/16 skins

9
Quote:

okay

this relates to build scheduals and it is set in basic sandwhich thoery

this is for compsands(not veneer boards)

3/16 is good thickness for a longboard deck

2/16 is good for a shortboard deck

if your using thin skins (1/16 or less)

the boards will break

if you dont use glass under

the boards will break

if your perimeter stringers are less than 1/2 inch

the board will possibly break

Or to mix & match:

I balance everything. Same glass (4oz) under & over, same glass deck & bottom. Same balsa thickness deck & bottom (only 1/16"). 3/4" of rail material. Done both 1# & 2# cores - I prefer the 2# but that’s just me.

Haven’t broken any. Nor buckled, nor cracked, nor split. Well, ok I have one 1" long cracked dent in the deck of the first one I made from knee paddling, but that’s it.

I’m not taking these things in huge surf every day, but they’ve definitely been through some good knocks. You can’t see the board here, but its under me all right. And its been back & forth to Scorpion Bay & points in between. This one’s been in pretty solid use (3-4 days a week between me & several friends - its the first I’m really subjecting to big-time durability testing) for 8 or 9 months now… Its a little yellowed and seems more flexible now, but other than that, no problems…

10

Bear with me through this long winded answer to some of your questions. First off, it makes me feel good to see this thread come alive. I started this writeup for myself (kind of like a mission statement of what I believe) because over the past 8 months I’ve watched the outside appearance of my boards improve but each design change sent the board’s surfability go down hill (at least for me). The design changes were a result of me buckling my favorite 9 footer and several boards getting the dreaded rail split. I shouldn’t have been shocked since it was only glassed with 2b2 on the bottom and 2B4 on the deck. I was going to junk it, but instead I revived it. I decided to be safe so I glassed both the top and bottom layers with 6 and wrapped the rails. I hated the board. It felt like it had lost everything I liked about it and was way too stiff. I gave the board to cj3 (Christian) and he ripped on it (I’ve included a picture - I hope you don’t mind Christian), However, he joked about how flexy it was. He was right (at least for him) since the board buckled soon after the photo. He’s rebuilt the board so strong that I couldn’t flex it on a triple overhead day, but its working for him. I’ve come away with the belief that compsands are best suited as a custom board based on your ability and the wave you surf. I think that this is the problem that Firewire is running into. The production of the boards is challenging so for now they can only make a board aimed at the average surfer. The problem is that there really is no such thing. What works for me probably won’t work for you. Give them another couple years to work out the production bugs and I bet they have a much more diversified product line that will blow people away.

The nose crease in tridge's board also happed to me.  My initial respone was that the nose of the board should be very stiff to stop the problem (just like Oneula says).  I think that this approach would work very well on a shortboard since are far as I can tell shortboarders don't surf the nose and nose stiffness transfers to better takeoffs  and paddling.  I think longboarders are more at risk because they have big, wide noses (the boards, not the surfers ;) ) that get pushed really hard when they go through the wave. Our hands focus all the wave's energy thats pushing the nose back right to the spot where our hands are.  You could just beef it up, but longboarder's surf the nose and ideally the nose would flatten out when we're on it (this lessens the tail hold requirements).  This makes me think that the longboards need the tension parts of the skin beefed up in the nose while the compression parts are weakened.

Lavarat, Benny1 has told me that you surf some really heavy waves so I bet your right about liking stiffer boards.  Flexy would be overpowered by the wave.  However, stiff and flex are relative terms that dependent on the surfers strength and the wave they surf.  I bet if you surf your favorite board at my break (Sano) you would hate it (toooooooooo stiff).  If I was building a board for your wave I'd start with double thick balsa deck and heavily glassed bottom.  When I breaks I would look which side of which skin failed and just beef up that side.  If it doesn't break I'd weaken the tension sides of the skins a bit.

As far as alternate woods go Bert said that he liked red cedar and balsa the best.  However, on a longboard the cedar on will come out almost 3 pounds heavier for the exact same board.  Ben, this is one of the things I'm looking into so that I can add weight to my single fins.  I just got through milling a bunch of cedar rail pieces and they are a really different wood than balsa.  Bert said that cherry makes one of his favorite looking boards, buts it makes an extremely heavy board (can anyone say noserider :) ).

Oneula,  I agree with you about the importance of the tail section of the board.  You mentioned that each time you change something it messes something else up.  You've just summarized my last 8 months.

In regards to skin thickness, I think that thick is good in some areas but not beneficial in others.  There's a reason the crafty likes heavily glassed bottoms (I know, it sounds kinky but he likes what he likes ;) ) - glass is good in tension and not good in compression.  However wood is just the opposite so I say put wood where its fights compression and glass where it fights tension.  The only time that I can see a lot of wood being good on a bottom of the board is if you're a longboarder who turtles his board a lot in big surf.  The one thing I don't like about glass is that its a limiter of movement so you don't want more/less than you need.

Ben,  I've been thinking a lot about your single fin theories and its starting to come together for me.  I'm starting to see why my hybrid approach might not the best.  I've got a two pounder pintail in the bag to test my new ideas.

11

You may find this article in Machine Design helpful. http://machinedesign.com/ASP/viewSelectedArticle.asp?strArticleId=60695&strSite=MDSite&catId=0 The basic point they make is one needs to be symetrical about the core. The article also mentions biasing the fiber directions.

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I agree with the article about the importance of the fiberglass orientation - its just that I’m not at that point yet. I’m still trying to come up with a workable construction technique and getting the board to flex. In regards to symmetrical around the core - which core would them mean. There’s the ones in the skin and the other being eps. I view the deck/rail as one panel and the bottom skin as the other. Each skin is primarily exposed/designed to withstand compression or tension (not both). If glass is twice is strong in tension compared to compression then by having the same glass on each side of the skin you’ve made it unbalanced. If you look at the headset on the bike (from the picture on the website) its not symmetrical - it has a reinforcing flange in the back to help absorb the forces it encounters. Look at traditional surfboards (which are single panels) - deck=double 6 oz and bottom=6 oz. The deck deals with compression so it has twice as much glass. I think composite boards CAN be two separate panels if you don’t lock the bottom skin to the rail.

13

I still have a hard time believing that there really is relative movement between the bottom skin and the rails.

If you butt joint the bottom skin to the rails, not overlap, it is still attached.

Epoxy as a glue is stronger than the Balsa itself, so that join is as solid as if it were overlapped, in terms of allowing movement.

The difference is that the butt join has a lower failure strength, in the direction of the plane of the skin especially, sometimes resulting in rail splits.

The only way to have the skin move next to the rail is to not have the skin glued to the rail at that join, a floating bottom skin.

It would still be connected by the glass over the top though!

This may allow very limited movement, but I feel that localising that movement to the hair-width unglued join between the skin and the rail is too much to ask of that glass.

Especially as that movement could be in any one of 3 dimensions, the rail pulling away from the skin (rail split), the skins compressing into the board or bulging out of the board, and the 3rd dimension, sliding longitudinally past each other.

My conclusion is that that join should be made robust, and flex controlled by tweaking other components, especially shape.

I’m working on that principle, and am hesitant to weaken a key area of the board in search of flex.

I’ve made boards with overlapped and non-overlapped bottoms, with no real discernable differnce in flex.

This could of course be that i’m doing other things incorrectly that mask the effect of that join, I completely accept that that may be the case.

But at the moment i’m reading it as support to the theory that both types of join offer very similar range of movement, but are very different in strength.

What I see in my head, what I see in my boards, and the preferable strength, all line up, whether misguided or not, so i’m chasing that!

Quicker to build as well!

With respect to the tension or compression loads at failure that the respective sandwiches experience, we need to distinguish if it is when being ridden or hit by the lip of the wave.

Ive never heard of someone snapping a board whilst bottom turning. Does that happen with Longboards? I don’t know.

So to me, talking about failure means when your board is getting flogged in the impact zone.

In which case, the compression/tension forces depend on whether you board is upside down or right way up.

So the deck and bottom should be identical… Not quite, as the deck has you jumping around on it, so I believe in localised reinforcement for your feet/knees.

And the deck is a different shape to the bottom, so forces are focussed differently.

It’s hard to figure out the best balance for springy performance and snap-strength at the same time!

Kit

14
Quote:

In Reply To

Quote:

Hi Drew,

If its any consolation, the Firewires that I’ve seen have that gap as well. Between the wooden rail and deck there’s this white line that could be the core. Looks good btw.

That means the load from the surfer has to go to the top skin, then to the core, then to the bottom skin, then to the rails…

You don’t get a true I-beam unless the skin is fused to the rails top and bottom…

You could even cut a relief into the rail for the skin to sit in…should

Kit, I understand your skeptism but I really think this is whats happening. This is a quote from blakestah from this tread http://www.swaylocks.com/forum/gforum.cgi?post=302595;#302595 that gives a little bit more credence to what I’m thinking. I agree with what you say about shape, but I think the amount of flex that is possible is just too limited when you join the bottom skin to the rail. The amount of foam you need to keep you afloat makes the panel to stiff. I originally moved to lapping the bottom skin because I kept getting rips in the glass where the bottom skin bumped into the rails with the free floating approach. A soon as I locked the rail up the problem, and the flex, went away.

15

thanks for starting this thread. Keep it going guys!

16

Well, i’m only sceptical of the case where the skin is simply butted up (not overlapped) and glued to the rail.

I definitely feel this is not really much different to overlapping.

The case of not glueing the join, and relying on the glass is valid, but imho not structurally sound.

Now, having a pronounced gap between the bottom (or top) skin and the rail, allows the glass holding the two together much more freedom to flex with the movement/deformation of the skin.

I’d fully agree with you that it is an area to be pursued!

I think I should revise the comment in my previous post, about making that join robust:

“Either the join should be made robust, and flex found in different areas, or the skin should be made truely floating, and the outer glass holding it together should be designed to allow the movement to occur, whilst still preventing rail splits.”

At first glance, to me at least, this means having a fairly large gap.

Maybe this would give the flex benefits that you get when putting only glass on the bottom, but also take advantage of having that sandwich skin on the bottom for awesome “pop” ?

The finer the join between rail and skin, the more the stress is focussed on those few fibres that span the gap between them. The wider the gap, the more threads that take the load, and the larger the free range of movement.

Hmm next board might get that treatment!

Kit

17

i agree with you kit

ive built very flexible boards with 2/16 skins and fully overlapped rails

the increase in durability from 1/16 skins and unlapped rails was amazing.

Corecell doesnt come any thhiner than 2/16 as far as i know so i have to presume

that is what firewire use.

also i agree with you about making the deck and bottom pretty similar

the board getting flogged onto a reef could be upside down just as much as the right way up

also why add more compressive strength to a board that is undentable etc.

IMO A light core is more effective at allowing seperate skin movement than rail lapping

18

I’m not suggesting make the board one sided, but rather to design it for the way it will most likely get used. If your boards are often flogged around then a balanced approach makes the most sense. The only failures I’ve seen on longboards are compressive failures on the deck and rail splits. On my busted twin the damage was on the deck suggesting that it had failed in compression. If the bottom skin had a tension break I’d leave the deck alone and beef up the bottom. Look at where jjp’s board broke and look what Bert said.

Here’s something I came across today

Quote:

well try and look at it in reverse …start with something flexible and then stiffen it in the places you dont want as much flex , those places would be the areas of highest load in certain situations …coz if the flex is to much the curves change to the extent that they become unworkable …as an example ive been working on something new lately , got way more overall flex happening more from the center of the board but proportionatly less at the tail in comparison to my known flex ratios …took off on a juicy wave where the bottom fell out so the drop was real fast , when i hit the bottom all my weight was on the front foot , i felt the board flex and the rocker increased at the centre ,it slowed because of the new curve and threw me off the front …so now i know with this particular construction technique to handle a bigger range of condititions , i must decrease the flex in the middle 1/3rd of the board …

19

I have 4 stringerless .75 lb balsa compsand boards. 2 have the skins overlapping the rails, 2 have balsa rails and the bottom skin overlaps the first layer of the rail on one of those. The other doesn’t have any overlap because I cut the rails off after skinning the board.

The boards with the skins overlapping the rails are solid. I can feel the boards without lapping flex under my back foot when I make hard turns. I haven’t figured out if I’m doing damage or not, but I know it’s compressing. I also have a compsand with the overlapping top and bottom skins that is really thin under 2". This one has a stringer, but I can feel the top compress when I push down hard with my back foot.

Personally, I like the solid feel of the boards with the wrapped skins, but I can make a better rail edge with solid balsa.

I’m going to place the first layer of balsa on the rail before I add the skins on my next board. I think that will make a stronger more solid feeling board. I’ve also planned on having 2 thin layers of balsa for the skins (2 - 1/16" layers) that will have the grain structure running in different directions. Mainly for asthetics, but I think it will also add strength.

20

hey Dan

i agree with most of your mission statement actually

just details on construction and the fact that i make shortboards are the major differences.

im yet to damage aboard other than the rail splits and a tear through the thin rail material between the futures boxes

a 1/16 deck over 1# pound will fail in compression in a longboard on a heavy wave

especially if the rail material is quite thin

jjp s board was only 1/16 skin

and we dont know the full details of his construction

im betting if the board was flexible(1# foam)

and had 2/16 skins

the board prolly would have survived