cork/ply layered rail mechanical properties

General Discussion
1

I was thinking about doing a board with the cork/ply layers ala paul jensen’s boards, thinking about the steps involved, and then my mind went off into the potential performance differences. A thought occurred to me that I would like to bounce off of the more technical-minded here. I think it is something worth examining, especially since there are now folks out there who are using this type of rail on boards meant to be ridden in “high-performance” situations.

The jist of it is this: How will a rail made of alternating layers of ply and cork perform under high-load situations?

Now, I may well be far off base on this, but if you all will indulge me, I’m interested in hearing some input.

In my thought-experiement, I am comparing the cork/ply rail to a compsand-style balsa rail, since that’s what I’m most familiar with.

There are two types of flex that I was thinking would be different with this kind of rail. The first one is the kind of flex that effects the rocker of the board, the kind you see when nev stands on a firewire for a photo op. Longitudinal flex, is that right? At any rate, when a balsa rail flexes, it flexes as a unit. The separate layers of balsa are bonded with epoxy, and there is no shear between the layers. When the board is flexed into a turn, it flexes back out with all the energy that has been stored in the wood (I’m sure there are many physics caveats to that statement, but this is just a thought experiment).

What happens in that case with a ply/cork rail? It seems to me that you would get full flex at the ply layer closest to the deck/bottom skins, but then since cork isn’t a structural material, it isn’t going to respond the same, or transfer the same forces to the next layer of ply, and so on until you’ve reached the outer layers. Won’t cork act as a dampener to any flex/flex return in this case? Is a rail like this going to have any “spring” in it? Or will that quality be dependent on the construction of the hull that its attached to?

The other kind of flex is the kind of flex that happens when you place your knee on the stringer and pull the rails towards yourself, again don’t know what the term is for this kind of flex, but just for the purpose of the discussion let me call it upward flex. This is very important to consider because in a high-performance situation, during fully committed rail-based turns, the forces acting at the very edges of the board, pushing up while the surfer’s feet push down, are enormous.

Ok, I know how that works in a balsa rail. The rail is 1/2" thick, firmly attached to the deck and bottom skins. Any flex that is going to happen as a result of a deeply comitted rail turn is going to happen inboard of the rail. The board might taco a little in the middle, but the rail simply won’t flex that way.

But what about the ply/cork rail? If it is 1.5" wide and has four layers of 1/8 cork in it, won’t the mild compressibility and shear-ability of cork cause the rail to curve upward in a hard turn, changing the rail-line curve and railshape?

With the differences in flex I’m hypothosizing along these two planes, I’m thinking that, all else being equal, a cork/ply rail would feel less lively at best, and performance-robbing at worst.

Now, I know that many boards have been made this way, with no mention of any of the stuff I’ve just brought up. I’m thinking that since so many of them are mid-sized or longboards, they simply aren’t being ridden close enough to those performance extremes to really put major force on the rails.

I also know that the glass/resin that encases these rails does bond them, and will help to transfer the forces from layer to layer. But the glass, as well all know, has most of its strength in tension, so when the layers are compressing or shearing, it would seem that the glass won’t be helping a whole lot.

So, what do you all think? Am I way off base here? I have no experimental evidence, or even direct experience, but it just doesn’t “feel” right, from a instinctive mechanical point of view, to have soft layers in a structure that is expected in the longitudinal plane to repeatedly load and unload, store and release energy, while not flexing vertically. You don’t see layers of cork in a recurve bow, right?

2

I think you will have very little flex. HWB don’t flex much anyway. And after you glue up the rail layers, and wrap them in epoxy, I’m thinking it will be very stiff. Unless you go very very thin. my 2 cents. Build it, ride it, report it. (I’m interested.)

I’m still looking for the magic rail material for compsands with EPS and balsa or bamboo. Just can’t seem to get outside of the box.

good luck with it.

3

I agree with Greg, mine have been all cork using contact cement to join. Wrap that with 4 layers of glass and nothing’s going to move.

There is the possibility of flex in the skin between the ribs, but not much.

Roy’s boards flex full length, maybe because of the lack of glass and he doesn’t build up the rails like on other HWS.

4

Jarrod, I think the tailward rail is the main element subject to flexion, so I think if such a rail is flexing at all, the board overall (and the inside rail, at that) is twisting, ie torque, torsion.

It seems like the vertical layup of the cork/balsa rails I’ve seen, wrapped in glass and resin or epoxy, would prevent much torsion at all–the caveat being that the number of plies/thickness would combine/multiply with the tailward outline/width to generate the product of torsion.

A wide tailed, thinnish fish, with not too many plies in the rail would tend to flex more torsionally.

It makes me think of how to lay up those plies in a large horizontal format, and then cut the rails to the planshape, if you want some tail torsion

(And I think you’re very much right about the damping characteristics of cork’s shear movement if the rail was flexing.)

5
Quote:

Now, I know that many boards have been made this way, with no mention of any of the stuff I’ve just brought up. I’m thinking that since so many of them are mid-sized or longboards, they simply aren’t being ridden close enough to those performance extremes to really put major force on the rails.

Wrong, a longboard has much more leverage (being longer) and there is thus much more force on the rails.

BTW, please stop this outdated "longboards are not performance boards’ nonsense, it is absolutely meaningless.

If you want a flexible wooden board, don’t use the kookbox method . . . if you get the board thin enough to flex it will break up

.

6
Quote:

HWB don’t flex much anyway.

Wrong. . . . hollow wooden surfboards are capable of flexing dramatically. . . . http://0000gyd.tnz.preview.hostingplatform.com/flexy.wmv

What you meant, perhaps, was that the Blake/Jensen kookbox style of HWS doesn’t flex much, but remember that they are only a subset of hollow woodens, not the whole story. The reason why the kookbox style HWS can’t flex much is that it can’t be made thin enough without breaking up.

:slight_smile:

7
Quote:

It makes me think of how to lay up those plies in a large horizontal format, and then cut the rails to the planshape, if you want some tail torsion

Which is exactly what we have been doing since 1995. . . . like “HELLO, IS ANYBODY HOME???”

:slight_smile:

8
Quote:

Roy’s boards flex full length, maybe because of the lack of glass and he doesn’t build up the rails like on other HWS.

Hi Hicksy, not so much because of no glassing (some of our flexiest were glassed balsa) . . . it is mainly because we are able to make boards thin enough to flex but strong enough to handle it. . … . and also because we use horizontal layers, some of which lie across the board. . . . the main point is the thickness to length ratio. . … . most hollow woodens are way to thick to flex. For a flexy board of any given volume, spread the volume out using a parallel profile, this gives the thinnest possible board in the riding position ( which is vital, no use having them stiff and thick in the middle, rather make them of even thickness)

:slight_smile:

9

Roy that little vid is an eye opener, from the pictures of your boards I would never have guessed they flexed like that. And that board does look thin compared to the boxframe woodies, I’ll have to scoure the archives for your detailed how to on your construction methods…

One question though, don’t you get some “floppyness” when riding as fast your boards go? How do you keep them stable?

Have you ever tried your construction method on a more “conventional” outline like say a mini-mal?

Yebo Bru! Keep it comin’!

10

My $0.02

The wood/cork layers is a composite. So it is worth considering the composite properties. Cork is a little high density for a sandwich material, but has EXCELLENT shear strength, so the sandwich will have great sandwith properties. This means the stiffness will be a strong function of thickness. As already pointed out, you can get fine flex with a thin enough sandwich. I don’t think cork-dampening is horribly relevant. With the number of layers used, the rail will have a neutral-force curvature, and will strongly return to that neutral-force curvature because it is a sandwich composite. That is to say, you have to build the rail at the right rocker, it is going to want to stay there.

I think the argument that the cork will dampen flex could be equally applied to EPS in Firewire boards. The cork doesn’t do a lot to support loads - it mainly acts as a sandwich between wood layers which handle the loads.

As to building it, I would make personal contact with Paul or Danny Hess. Paul brought this rail style to broad attention of swaylockians, and I know Danny has spent a lot of time and intellect in building them in a cost-effective manner. Although Paul is a master craftsman, Danny is building them for his livelihood. I’ve spoken with Danny at length about it, and know he has done a lot to innovate how to produce these rails efficiently.

As to the term ‘kookbox’, Danny uses the same techniques, but uses an EPS frame, which ultimately doesn’t stiffen the board much if at all, his boards have a righteous flex to them. If the kookbox is thin enough, flex will work out just fine. There is really no need to degrade the construction techniques of others with pejorative terms just to thump your chest a little.

11

Righteous post–

Blakestah, have you ridden Hess’s boards? more than one? I’m interested if what I thought is true about the wider tailward outline allowing more torsion–I’m sure the fish or a Flyer would, compared to, say, a gun.

Thing I theorized about cork’s dampening characteristic (given its compressability) didn’t really take into account the rigidity that would be imparted by resin getting into the material a mm or so. It also varies as to the cork layers’ thickness of course.

A kookbox with an EPS frame seems really almost a whole different animal from the wood-framed iteration, given the physical properties of EPS.

One thing about your boards, Roy, is that they’re so extreme in terms of length and rocker and rail and even outline, and ridden so mildly (beg pardon), with a fin or fins that vary pretty completely from our’s and so impart less (or different) torsion, and have as a pilot such an extreme proponent of their construction and riding advantages, per his definitions, it’s hard to know what part of any of it is accurate (per other definitions) and/or transferable to our surfboards.

Makes it worth taking another look at your construction method though.

Anything as to the question about floppiness at the V you’re doing?

12

very cool thread.

There are so many factors that go into making a cork/ ply/ timber rail flex properly. After years of tweaking the rails I feel that I’ve come to a point where I build the amount of flex I want into the rails. These types of rails can be very stiff. I use a combination of things to build that out of them and make them come alive. The deck shape adjacent to the rail and rail shape play a big role. The ply/ cork, timber sequence. The rail thickness. I allow alot of sheer movement between my core, skin and rail to allow the rail to really work. My rails are also hollow. The last three layers of ply are really the stringer. the rest is really just a skin.

The cork in my rails basically just acts as a skin, But used in a solid rail it certainly doesn’t dampen the flex once it is laminated. That cork soaks up a lot of epoxy.

wider tails do give you more torsion, if you are building the same flex into the rail on every board. The narrower the tail the hollower the rail. Different deck contour/ rail shape. Greater sheer movement through the rail at specific points.

I build flex into each board based on what a person weighs, how they surf, where they surf, and what they want the board to surf like.

http:hesssurfboards.com

13
Quote:
Righteous post--

Blakestah, have you ridden Hess’s boards? more than one? I’m interested if what I thought is true about the wider tailward outline allowing more torsion–I’m sure the fish or a Flyer would, compared to, say, a gun…

I’ve sampled more than one, but not recently, and I know the flex issues were developed somewhat later. I moved from the Bay Area almost a year ago, and Danny hadn’t started shaping professionally yet. He lived in my neighborhood, and had similar surfing hours to me, so I saw him in the water all the time, and we swapped boards etc. He was a green carpenter doing contract work and usually very busy.

The last I rode was one of the first EPS frame boards that was weight competitive with Pu/Pe boards. A friend of Danny’s had recently gotten a Pavel twin keel. Danny’s was a quad of the same length, and was more buoyant and lighter, and had better action on the fins (of course comparing a quad to a twin). I didn’t get much flex out of that board, but Danny is much bigger than I am, so I wasn’t surprised (I think he is ~40 pounds heavier than me and 3-4 inches taller).

The hold and turning power of the board was very nice, definitely beat out the Pavel twin. All of Danny’s finished boards looked amazing - rail, inlays, skins, just so beautiful, like a master woodworker/artist made the board. This particular boards was a keeper, if I had wanted a quad it would have been great for me.

I think Danny has probably done five times more boards since I left than he had done before.

14

hi Daniel !

thanks for that .

Is there any chance of you , or someone here , doing a diagram of what you are referring to , please ? … just to help me get a clearer picture of what is involved … [but if it is patented / copyrighted / company property , then I fully understand if it can’t be shared here]

cheers



 ben
15
Quote:
hi Daniel !

thanks for that .

Is there any chance of you , or someone here , doing a diagram of what you are referring to , please ? … just to help me get a clearer picture of what is involved … [but if it is patented / copyrighted / company property , then I fully understand if it can’t be shared here]

cheers



 ben</blockquote></div>

Here is a closeup from Paul Jensen’s site

16
Quote:

As to the term ‘kookbox’, Danny uses the same techniques, but uses an EPS frame, which ultimately doesn’t stiffen the board much if at all, his boards have a righteous flex to them. If the kookbox is thin enough, flex will work out just fine. There is really no need to degrade the construction techniques of others with pejorative terms just to thump your chest a little.

Hi Dave,

No offence intended, . . . it’s just that kookbox is such a lovely word!

Not sure what other term to use to describe the Blake/Jensen/Hess/Hicksy/etc etc method, any suggestions? . . . . HWS won’t do though, because it applies to all hollow wooden surfboards . . . . and I know of at least three completely different HWS methods.

:slight_smile:

17

thats what they used to call em in the 50s and 60s

is that right!

im with roy on flex

it really is that simple

thickness ,rail and deck shape, length and planeshape

if you got frames and stringers it inhibits flex

thats why roys flex so well

if you applied roys construction process to a modern hyrid shape

im sure you could attain firewire type flex performance characteristics

in a albiet heavier board

(a lot of riders like that though)

if you got too short they would stiffen up considerably i imagine

re cork rails

yep that would work well

is it water proof?

and is it lighter than balsa

18
Quote:

One thing about your boards, Roy, is that they’re so extreme in terms of length and rocker and rail and even outline, and ridden so mildly (beg pardon), with a fin or fins that vary pretty completely from our’s and so impart less (or different) torsion, and have as a pilot such an extreme proponent of their construction and riding advantages, per his definitions, it’s hard to know what part of any of it is accurate (per other definitions) and/or transferable to our surfboards.

Makes it worth taking another look at your construction method though.

Anything as to the question about floppiness at the V you’re doing?

Janklow, in spite of my current interest in very long boards , I have built lots of flexible balsa singlefins in the 6 to 7 foot range, so my comments apply to shorter boards of more 'conventional planshape as well as to longboards.

As far as riding ‘mildly’ goes, let’s see some video of Paul’s boards in action and then compare !. . . . . I have seen a video clip of a Hess board being surfed, and it was very mild indeed, but you are not calling him out on it. … . … all is not what it seems. . . it might look mild in terms of style but those big heavy boards have a lot of power and take a lot of handling. . . . there are plenty of forces involved, I just choose to make it look easy !

Your comment regarding my fins imparting less torsion are way out of line. . . . the tunnel fin setups are very powerful and flex the board far more than conventional vertical fins because of their horizontal area. . . . just thought you might like to know.

As for floppiness at the V. . .nice try but no cigar. . . . there is no V in any of my boards. . . . and they have a stiff or gentle twang depending upon how I design them. … . the more flexible models twang like longbows. . . remember that the flex is always less pronounced in the water than on land too. . . . the board in the ‘flexy’ video is about as extreme in flex as they get, most of my boards have only an inch or two.

Nice talking to you

Cheers

Roy

19
Quote:

thats what they used to call em in the 50s and 60s

is that right!

im with roy on flex

it really is that simple

thickness ,rail and deck shape, length and planeshape

if you got frames and stringers it inhibits flex

thats why roys flex so well

if you applied roys construction process to a modern hyrid shape

im sure you could attain firewire type flex performance characteristics

in a albiet heavier board

(a lot of riders like that though)

if you got too short they would stiffen up considerably i imagine

You’re on to it Paul, with shorter boards it helps to use balsa as it is much softer and compresses more easily.

In fact my 4 layer balsa system is very similar to a balsa/eps core system.

If more flex for a board of any given thickness is needed, then find a way of running the grain in the rails (at least some of it ) ACROSS the board, rather than fore and aft or following the rail. … . unconventional but it works.

Here’s a mild video clip, good track for relaxation anyway: http://0000gyd.tnz.preview.hostingplatform.com/CycloneVaianu.wmv

20

Sorry, Roy, I meant Velocity.

No worries. Cheers.

Anyway, the point was that your current iterations are rather removed from much of the rest of surfcraftdom, for better or worse. It’s just hard to guess at what would work at smaller dims, with more usual rail contours, with regular old fins.

Again, no worries, no critique especially meant. Just differentials.