the neutral axis

General Discussion
21

I can see the importance of what your saying now, having the center of gravity and neutral axis aligned.

no offense Turbo, but this is precisely why i’ve nearly given up on these types of discussions…i dont know where to begin to shread apart these arguments…while i dig reading Bert’s postings and his excellent insight into certain things, i fail to see the revelance as it pertains to SURFING…

its not gospel. i read an old post where Bert swore up and down that balsa rails is what makes a better board, no matter the foam material…I tried the balsa rails and didnt like it (maybe i made it wrong)…now we are supposed to believe the NA just because he’s saying so?

no offense Bert, i guess im just a SURFER who just hasnt got the time to climb that tall fruit tree for those high hangers…every extra hour in the garage chasing rainbows means less fitness…less fitness means lower surfing ability and performance…owning a sunova means

sh!t if youve got a big beer belly and low stamina…so while others look at NA as a potential advantange or equalizer, i prefer to look at the lap pool…

interesting…Sunova is ten years ahead…five years ago Loehr was too…and yet the majority of guys are still riding poopees…at last observation about 70% of them riding Merricks…suckers

here’s the real key question:

when can i buy a Surfburger?

22

yes the neutral axis is/was hard to pinpoint , its related to shape and top and bottom skin arangement…

if we run any internal style booster , its at its freeist in the area where its not being stressed or having to deal with shear forces , of coarse there will always be some kind of stress or shear , but if its placed in the zone of least shear , the range of flex is increased , so your board is not locked up but can still take advantage of the full force when it springs back …

core compression also takes up some load , to allow the bottom to change shape under load …

its all a dynamic mix …

because just changing one variable on the extremity of our board will effect the posistion of the neutral axis , anything we can do to physically get closer to the NA the more control we have …

i dont have any magic formulas , its something you get a feel for , one team rider recently had 6 identical shaped boards , all with different internal arrangements , it was a serious eye opener for both of us …

because we were then faced with the prospect of past shapes actually being good , but we had written them off because we realised the flex was all wrong …

the NA starts to take a leading role when were designing with flex in mind …

it sounds crazy , but the outside shape starts to lose importance to the internal structure …

the NA needs to be figured into the design if we are putting stuff in there to change the ride …

its like changing the suspension on your car , you dont see it , the car looks the same from the outside , but what a difference to handling …

ok just a short stop in …

on the work trail …

got stuff to do , such a bummer when the crew you work with are lurkers and can see when your slacking …

regards

BERT

23

i dont have any magic formulas , its something you get a feel for , one team rider recently had 6 identical shaped boards , all with different internal arrangements , it was a serious eye opener for both of us …

because we were then faced with the prospect of past shapes actually being good , but we had written them off because we realised the flex was all wrong …

so your riding a diving board with an external skeleton…i guess your buddy Dan was right…and hence we stand before that beautiful fruit tree with all those high hangers…sounds like your saying that dealing with the NA/internal springboard was a necessary evil due to this type of construction… you didnt really comment on surflight but you and i know your dealing with shear stress in the similar fashion…just different materials…

now its making some sense…except you still have balsa on the skin which doesnt like to flex/elongate whatsoever…that has always been the chigger in the wood pile…

ok back to the lap pool…

24

an alternative beam , with a horizontal axis ?

This should get the desired results …

25

Lot going on in here…

I went back yesterday & read an old thread of Bert’s called “Latest Creation’s” (if you search exactly that, it’ll come up). Tons of information in there - a lot of which makes way more sense to me now, than it did a year ago :slight_smile:

A few things from that…& to mix into this discussion.

I understand from Bert that the ‘springer’ is only a substitute structural member that allows thinner, lighter boards. In other words, if you use a 2# EPS core, or thicker balsa rails, you don’t get anything out of adding a springer as well. So it may not be the only solution. (Which is complementary to Meecrafty’s ‘more than one way to skin a cat’ contentions.)

If you look at Bert’s boards in the abovementioned thread, you’ll see that the balsa rails are most definitely thin strips, built up & pressed around the perimeter of the board, not chunky pieces, cut to fit. And absolutely not the skins bent around the rails. Bending skins around the rails - I did it on the d-cell board & on the 12’'er with a 1# core - takes out all the flex. The 12’er should be a freaking noodle, with that length, veneer skins, 1# EPS, and only 3" thick, but its not. Its like surfing…well…another homemade Surftech.

So I went home last night & tried out using the offcuts from templating to press on some rail pieces, and goddamn but if it isn’t about a year faster than taping on one layer at a time. I mixed up 8 oz of epoxy, laid out 8 rail pieces (4 per side - just doing the middle 7’ of a 10’ board), brushed on the epoxy (8 oz resin + 2 oz hardener was perfect), and clamped them on with the offcuts, some back-up strips of wood outside the offcuts, and some pipe clamps. No sweat, I can’t believe I used to tape those things.

I also believe there is a lot more going on than EPS, epoxy/glass, and balsa with Bert’s boards. In that same thread, someone asks what are they. He says something to the effect of, “Lightweight foam, balsa skins, epoxy resin & fiberglass, some other fabrics and bits of foam…”

I wouldn’t even be surprised if the deck were a double sandwich - say glass/corecell/glass/balsa/glass. Bert also states a preference for corecell over divinycell because its not as cross-linked & therefore not as stiff (but still good & compression-resistant as a sandwich core). I don’t think he would have bothered with that if there wasn’t any in his boards.

The photo of the broken board shows the springer imbedded in the blank, not routed into the top. The foam above looks a little different - that could be the corecell there so the springer can push downwards into the EPS below easier than upwards into the deck…I dunno (sorry Meecrafty, I’m conjecturing again :slight_smile: )

Anyway, go back & read that thread again & look at the dozens of photos of boards. My analysis, based on this thread, the rail one, the vac bag one, and the creation’s one, is that a) there’s more than one way to skin a cat; and b) there’s more to these things than EPS, epoxy/glass, and balsa…

26

I agree - the Latest Creation thread is one of the best threads I’ve read at Swaylocks. I ran 5 or 6 of the boards through aps3000 and that really helped to bring out some of the details of Bert’s shapes.

I’m going to try the foam cuttoff approach next time. Last time a preformed the rails and I was really happy with how it turned out, but it did take too long to tweak the rocker table. It makes sense to use what you already have. I love the balsa rails and wouldn’t consider making a board without them, but it does take a little time to dial in the correct technique. This sounds like a step in the right direction.

I’ve got some theories about why the 1 lb eps works better with the horizontal stringer (it has to do with transfering energy), but …

Recently Bert mentioned that the horizontal stringer is used to dampen input that you don’t want increase desireable input. This is exactly what we were talking about in SanO in regards to recreating your first balsa (but lighter and cheaper). The question is how to transfer the characteristics you gained from the outer skin into the horizontal stringer without making 1000 boards

What do you think about the idea of creating a Frankenboard where there’s a precut section on the bottom of the board that could be cut out (like a giant ding repair), pop out the old stringer and foam (it should be easy since its free floating), pop in the new stringer, then glass over it with a strip of 4 oz tape. If you made the removable strip about 4" wide you could really play around with different stringers (and lack of stringer) and figure out whats going on.

27

Well, I’m going the 2# core route instead of the springer route. Its a much easier variable to control for me - thickness of the blank, really - and since I’m focused on the 10’0" shape at this point, I want to change as few variable at a time as I can…

I’ve laminated a 1/16th" balsa skin with 4oz each side onto a 2# EPS blank. I got my resin:glass ratio very close to 50/50 by vacc’ing on the skin with both inside & outside glass (& peel ply & absorbent) all in one hit. I taped up the balsa skin & flipped it over. Folded up the glass & wet it out. Spread it on the balsa. Flipped the blank onto the glass & wood & re-flipped it. Peeled the tape. Wet out the second folded-up piece of glass & laid it out onto the balsa. Wrapped it with peel ply, absorbent, and a strip of shade cloth. Pulled to 13-16 "Hg for 12 hours.

This was all done to a rockered & thicknessed rectangle. When it came out of the bag, I stripped off the disposables & cut the tamplate. That way my offcuts have a skin on the bottom just like the blank.

This thing is lighter & stronger than anything I’ve done before. The balsa hardly absorbed any resin at all, but peel & break tests on the offcuts show good adhesion. Wetting out the glass ahead of time was amazing. Since these were rectangles, I had to wet out pieces 10’ x 2’ and I used 8oz of resin & 2oz of hardener per side. Since it was 4 oz glass, that means 2.22 yards - 8.88 oz - of glass got 10oz of resin…and then quite a bit of resin came through the peel ply into the absorbent. Not heavy bleeds, but definite spotting. And the skin came out soooo smooth.

So the plan is, now laminate on the rail balsa, 1" thick. Shape it. Glass over that with 4" tape to catch the bottom skin. Then bag on the top skin to catch the rail glass from above, doing the top like I did the bottom, pre-wetting both pieces of glass & bagging inner & outer all at once.

The only hang up is I could only find 8oz 4" tape. But that’s like lapping a 4 from above & below, so its not that bad. And extra rail protection too. I don’t want to inhibit flex too much, but with such thin balsa I think it’ll be ok.

I’m also running a couple strips of CF cloth along the sides under the deck. Toughen up the deck, mainly for pop-up & knee-paddle protection. That will be pre-wet as well, and applied in the same bagging operation as the deck skin.

I’m taking lots of photos…

Its also going a lot faster than it used to. I’m in about 5.5 hours so far & I’m nearly halfway done. It used to take me 30 +/- hours to do one of these!

28

“Bert also states a preference for corecell over divinycell because its not as cross-pinked & therefore not as stiff (but still good & compression-resistant as a sandwich core). I don’t think he would have bothered with that if there wasn’t any in his boards.”

That’s right…

SAN foams (Corecell) behave in a similar way to toughened cross-linked PVC foams (Divinycell). They have most of the static properties of cross-linked PVC cores, yet have much higher enlongations and toughness. They’re able to absorb impact levels that would fracture PVC foams. PVC will fail in a brittle manner, SAN in a ductile (or plastic) manner.

(SAN) styreneacrylonitrile

(PVC) polyvinyl chloride

29

Very cool sounding project! Can’t wait to see the pictures.

30

Bert,

While I appreciate your prelude I won’t quote it or dwell on it. Suffice to say practical mechanics often amount to feel and instinct for what you are doing. I’ll jump to what is the meat, for me.

“it sounds crazy , but the outside shape starts to lose importance to the internal structure …”

Doesn’t sound crazy at all! This is sounding more and more plausible to me. On an almost daily basis. In a composite structure the exterior doesnt really matter (OK, within reason). Its what (material type, volume and dimensions) is at what place (in 3 dims) and how its aligned and structured that becomes important. As if I know :slight_smile:

"the NA needs to be figured into the design if we are putting stuff in there to change the ride …

“its like changing the suspension on your car , you dont see it , the car looks the same from the outside , but what a difference to handling …”

Good example! I daresay you could make a great board (compared to traditional materials and construction) without explicitly considering the NA (or NP, as I think of it). But you won’t get the best out of your material.

Isn’t it important to get the NA fairly evenly distributed, with variances that are gradual changes? E.g., Stiffening the front half of a board very suddenly will effectively move the NA towards the deck for the front section. But it is closer to the bottom in the rear section. Where the two sections meet there is a sudden change in the alignment of the NA – sort of sheering the NA. Will this not create a more highly stressed section of structure? And sections with comparatively higher stress usually act as a hinge and promote failure at that point/area. I think its safe to say that we intuitvely understand that stiffening, etc shouldnt be sudden (at least, thats muy intuition). But IF thats correct we benefit a lot from understanding why. And if it is incorrect we still benefit form understanding why.

Hhhmm… Given timber’s greater strength in compression building our skins the same thickness for deck and bottom should actually result in an NA which is closer to the deck than the bottom. All this assuming no spingers/t-hats/etc. So if we want the NA close to centre the bottom skin should probably be 3-4mm if we use a 5mm top-skin. That’s assuming we want NA near centre. Do we? I think so, but… Opinions?

As usual all this is just my opinion and could be light years divorced from reality. All shared thoughts, feedback and correction greatly appreciate. No comment, no insult :slight_smile:

-doug

31

Hi Guys

Ive just been sitting on the edge watching this thread,Bert and I have some big differences in the way we look at and approach things,however i notice that we tend to agree on one major point and i would guess our end results are not hugely far apart,the only true way to see would be to ride each others equipment.

The shape outside compared to the internal structure.

You see to put my point accross, I had been thinking of lighter boards for a while and after four and a half years building a light weight balsa compsite sandwich yacht, then seven years sailing round the world where the time and space to build boards was very limited, in fact appart from one for the kids nothing was built.

I started again building boards first a few pu/epoxys which went ok but were still heavier than i wanted I was heading towards a hollow dcell type structure.

Then I saw Berts boards which appealed straight away.

But in my usual style, I dont like following others ways of doing things I embarked on an evolution trail

This has lead me to boards which are light and perform

However to get there I first decided that you need to find out how the struture works from the inside

I keep saying keep it simple,what i mean is that to keep it light,strong and still perform you just cannot afford to put too much of anything in to it. What you do put inside your board must do its job 100 % if it doesnt than there is waste happening,either in weight,structural strength or performance of the board

So it does not matter what you go for be it thin bottom ,thick deck,springer,perimeter stringers,top hat,concave deck,domed deck,flat deck,dcell,corecell,balsa etc etc whatever,there must not be more than there is required to do the job or something important, be it, overall weight or flex related performance etc will suffer.

I would suggest to every one wishing to go the compsand route,put the shape importance on the back burner and put all your effort into understanding the struture first then the correct shape will follow with some ease

If you try to do it the other way round it will be a very long and hard road with many disapointments along the way

Evolution takes time and small steps its not just a cosmetic new look!!

Mike

32

agree sabs , and to add , for every construction style , composite combination etc , there will be a corresponding shape that works best in that construction …

pick your construction , , be happy that its the one that works for you in regards to simplicity or whatever your circumstances are …

and then search for the curves that suit it …

or do it the other way , same shape different constructions …

doug !! you made 2 assumptions which were right on …

also sabs , agree on the weight , you can go so light , with fabrics so fine you cant even work with them easily , lighter and lighter skins , you then start to get a board that gets over flexy , so now your looking for a way to stiffen it up but not pay the penalty with weight , but its not even the flex that is the real problem , its the ability to spring back with enough force to drive you out of a turn …

i think one of my first comments when i mentioned sandwich construction, was the amount of variables and possible valid combinations were endless …

regards

BERT

33

I was speaking to one of my materials lecturers today about surfboard design and tried to ask him a few questions on the neutral axis.

He was pretty shallow on it and just didn’t really give me any good information.

One bit of good information that I did get from him though was about fibre reinforcements like fibreglass.

Bert, have you experimented with percentage composition fibreglasses?

All he was explaining was that you increase the fibre concentration say longitudinally, and decrease the fibre concentration transversely. So you have more fibres where you need more strength and less where you dont.

I’m thinking this is a way to achieve flex or stiffness in certain parts of the board that need it, not necessarily over the whole board, but maybe you could have zones of less fibres ?..weight loss would be marginal, but still there i guess…

What do you guys think?

Cheers

L

34

Sabs, you said it well. That’s what I was trying to say with my ‘design means nothing’ thread on the peanut board. As weird as it looks, it surfs like…a surfboard.

Your sweet spot is the lightweight thruster Mine is the 10’ single fin. I’m done (for a while) building fish, eggs, 12’ers, and peanuts. 10’ single fins until I understand the materials.

And you & Bert are both so right in that there are many ways to achieve the same things. You’ll like my current project. :slight_smile:

35

I’m like chip…

All this engineer raw raw is giving me a headache…

you guy need some serious water time to mellow out and start talking human readable again.

Quote:

All he was explaining was that you increase the fibre concentration say longitudinally, and decrease the fibre concentration transversely. So you have more fibres where you need more strength and less where you dont.

I believe this is called Warp.

so 4ozE for normal 4oz Warp for extra longitudinal fibers and 4oz S for added strength.

Volan for old school weight

Carbon for light stiffness and

Kevlar for puncture resistance with flex (rails).

Like Sabs and Meecrafty said you can over engineer something to where it becomes something else.

kind of like me wanting a water driven scram jet built into my next board so I can hit mach 7 when I need it.

For it’s all about speed and controlling it that creates what’s now called high performance (airs and carves).

I’m like silly and meecrafty and am also very intrigued about the use of polypropelene foam as an insulator between the skin and the core as well as a flexible bonding agent. I’m gotten tired like Benny1 of light and stiiff (all mine are rail wraps) but don’t want to end up at the other extreme…light and mushy.

If the compskin can transfer it’s load directly to the rail which in turn can transfer it to the attached bottom nose and tail, then you can produce deformaties longitudinally to get some form of snap back out of the compression. I don’t see how transferring the load to the core and it’s insulating affect will give you maximum snap back you need to make the board feel “alive” under your feet. Probably why 1lb EPS works better cause it’s giving in to the skin alot more than 2lb transferring the load directly to the rails and altering the longitudinal profile during compression.

You can insert floating springers or reduce the thickness profile to get the same effect which is reducing the impact of a foam core which is causing most of the problem in the design of these things. What does foam add to the equation other than float? I think someone said an air or helium cored sandwich would probably be the most lively reacting board if not too light to maintain drive. But Like bert said drive can be increased through proper design and better placed flex.

A couple of observations of all my experiments so far…

It seems the shorter you go like real short, even smaller than fish short the better off you’ll be.

To mind surf you shouldn’t have move your feet at all

everything should be driven off of body/arm position and weighting (kind of like snow skiing or strapped surfing) I think that’s why Cheyne’s no-nose micro zap/nuggets where so far ahead of their time especially with the keel under foot. I presume going shorter real short 5’5"-5"10" and real thin will have a dramatic impact on the NA and it’s impact to how the board operates in a Compsand design. Like sponging or hand boarding, you can ride something small in places of extreme curvatures that you can’t fit something bigger. And usually this is where the greatest energy or greatest speed is located because of the physics of what’s happening the dynamics of a breaking wave.

The question and challenge is how to maintain decent floatation and the speed without getting stubbie or chunky.

Like some of the compsand crew’s thinking I agree we need to start pushing the envelope to go way way thinner, and way smaller using materials that will provide the best floatation and response if we want to create tow-board experience in a paddle board environment. Almost everything being done is typical Mcdonalds supersized for the technology we’re using. Meecrafty once said push the design limits till it breaks then slowly back up from there…

Size matters…

I think it’s time go get way smaller and way thinner than we’ve dared to so in the past.

Maybe then we’ll see the real effects of the compsand construction and the elusive neutral axis we’re all hunting…

36

Mike,

Couldn’t agree more. There is no place for components that do not do their job 100%. What ever you interpretation of that job might be. In fact, I am taking this to heart and simplifying my #001! See that thread soon, assuming you are interested at all :smiley:

Bert,

Thanks - I think I know which assumptions you mean, wont push it any further tho. My thoughts and comments are based on experience in other paradigms and my understanding of physics/practical mechanics. Not sure how much dumb luck comes into it ;D

Lavz,

I suggest you have a look at some of the modern fibres if you are interested in this area. Like uni-direction cloth, etc. I think a lot of the gain in this area will be the ability to add resistance in one plane and direction that is greater than the weight added. be interesting to see/hear.

Oneula,

Yes I need more water time. Desperately. Unfortunately work isn’t cooperating and I am being rationed to 2-3 session a week… Can’t wait for daylight savings to kick in!

In the meantime (and at work and at night) I see value in understanding how these structures work. Your comments on breaking the limits and then pulling the parameters back in are a VERY good way to zero in on the exact limits of the construction you are using. But it means you cant change your internal structure - you need to make internally consistent boards that are very short and thin. Then use EXACTLY the same structures and progressively change the dimensions. That means building a lot of boards. If someone takes this on they are going to have to invest significant time and money. But it’ll be a true investment if they value knowledge of this. It’s been done in other areas very successfully.

This comes back to what Bert said above about choosing your shape and finding the right structure for it or choosing your structure and finding the right shape. You can’t change too many variables at once and get an exact result.

All,

So far no disagreement on how I see the NA/NP. I don’t know if that’s agreement or not :slight_smile: Anyway, back to simplifying my #001 design!

-doug

PS. Vac setup is getting much closer to finished. Pump, done. vac switch 80% done. Reservoir and values 80% done. Experiment time soon! Yippee!

37

Through his surfcraft and wave riding, George Greenough demonstrated the same concepts, inspiring the shortboard revolution almost 40 years ago!

“…it’s all about speed and controlling it that creates what’s now called high performance…”

“…drive can be increased through proper design and better placed flex.”

“…It seems the shorter you go like real short, even smaller than fish short the better off you’ll be. To mind surf you shouldn’t have move your feet at all everything should be driven off of body/arm position and weighting…”

“…you can ride something small in places of extreme curvatures that you can’t fit something bigger. And usually this is where the greatest energy or greatest speed is located because of the physics of what’s happening the dynamics of a breaking wave.”

“…start pushing the envelope to go way way thinner, and way smaller…”

“…it’s time go get way smaller and way thinner than we’ve dared to so in the past.”

38

Doug

I will be interested to see your #1

To All

Ive just been reading the latest tracks mag (i live in hope that one day they or any other surf mag will finally expose what we are all up to so the so called normal surfers will see the light)

Any way there is a section on tips etc from some of the pros, now one of these was about the magic board and how it is a fluke of nature.It was said that even with computer shapes no two will surf the same.

I have to ask this,the computer may have shaped the board but i bet the good old glasser glassed it and as we all know here changing the lap ratio even just a little bit alters the whole flex relationship of a board.So maybe that is were the differing ride is coming from and not miniscule shape differences?

Ive long belived that shape in the fine degree is maybe not as important as flex control in the fine degree

One day when i 95 % happy with my flex im going to make a board very much like a door ie square in rail shape just to see what difference it really does make.

Mike

39

You can also bet the stringer wasn’t from the same tree if the same species. If you are using wood you have to expect variation.

40

im so stoked Dale has jumped in these conversations…Dale, that was a real nice convergance of thought…makes me stoked…

and to All…i’d have to say that hearing these recent posts have been a step in the right direction…i.e. there’s more than one way of getting similar results or better…bert has blazed his own trail but there are plenty others to start

finally wrt construction v design…they go hand in hand…cant ingore one and get the best out of the other…but i would say that approach would be educational…as rail surfer, rocker is super critical no matter the construction…others can have similar findings wrt other design features…

oneula, goin thin…like thick kiteboard thin just to see…with other mat’ls too