Crazy hollow carbon balsa kevlar composite fish!!

General Discussion
281

Good question Thrushter.

Indeed, why make the rails a big block (in effect) of wood when the wood at the center is doing NOTHING ?

Ok, which is more resistant to torsional loadings - A: a TUBE or B: a Solid Bar ?

Answer = A: a tube

I’ve rolled tubes made of balsa laminations to contain a rubber motor (for those who dont know,some model planes are powered by a wound up rubber motor) with the lams at 45deg eg 3 x 1/16 for a tube to carry 1pound in weight of rubber in a length of 20inches.

Peak torque for a motor like that would be about 30 - 40ft/lbs.

It would be possible to make a board Jensen style with moulded and laminated rails that lap over onto the deck. Of course, you’d have to use a laminated deck so you can tie the rail wood in, more hassle but several steps closer to the idea of a semi - monocoque construction (a-la S core)

For rails you could mold the two inner lams at 45 deg and the outer layer to run the length so as to retain the grain direction, same on the deck but leaving excess on the rails to lap INTO a deck of similar construction.

THEN cos you’ve used less material in the rails, some of it can be used to beef up the high compression stressed areas and the rest is weight loss.

In theory anyway.

282

i dont think that theory apllies to surfboards sippy

a surfboard should be tough and flexible

tubes dont bend very well at all

unless you fill them up with something

283

Delving deeper than just the strength factor, is a simple curve the best shape for a rail? I know it sounds like heresy, but cool your jets for just a minute guys, and have a guess at what might be possible…

Maybe like the 2nd pic theres room for a ‘capture’ style of rail something that holds water only when its on its side.

I know theyre not the standard, but a design forum should have the ability to look forward as well as ruminate over the past.

Truly different rails might give us a stronger board construction and maybe better hold / efficient use of hydrodynamic forces…

284

My opinion is the rails are critical to the strength of a hollow board. I’ve broken about thirty of them, mostly while holding them in my hands whilst getting pounded. In nearly every case, I could feel the rails compress and flatten out right before the board folded.

There are all kinds of forces working against a surfboard when it’s getting bounced around by a crashing wave. On traditional boards the foam keeps the board from deforming under pressure. Hollow boards have no core to keep the skin from reacting to the forces against it.

Flat sheets of carbon and epoxy aren’t nearly as strong as folded ones. The rails keep the sheets folded. If they get compromised, the board is just like a flat sheet and can easily fold.

An internal stringer can help keep the shape of the board, but not as effectively as the rails. If the board flexes - which is good - the deck and bottom will want to flex at different rates, and in opposite directions. The bottom of the deck skin will be pulling on the stringer connection, and the top of the bottom skin will be pushing on the stringer connection. Either the stringer or the adhesive (or both) should have some elasticity. This is why Salomon uses EPS. Hydro Epic strengthens their skins but depends on their rails for strength. Alviso uses seamless construction, so their boards are cohesive, singular units. The disadvantage is they can’t get inside them to add things that may benefit the the board’s performance.

285

30 Broken boards wow!!

I agree with your observations about the core and stringer. My theory about the rails comes from handling long lengths of guttering which, although strong, dont seem to have any rigidity about their longitudinal axis. They keep their shape but wobble all over the place, so it might be the same for rails too.

So Im saying that the 1/2 round rail shape seems inherently unstable.

286

except that by connecting the top and bottom, you’ve created a box, which is strong.

287

Eeeee gads,

thats an example i gave.

Saw the tube in half.

Close the half round with a flat plate and you have what we call in aero design a ‘D Box’, depending on how one orientates the grain of the wood, weave or whatever you can MAKE it flex.

NOW look at a conventional board and what do you see?

Stringer in the center with a nice ‘D Box’ either side made of glass with a foam filling.

What i was trying to point out is IF you know how to use grain or weave orientation it should be able to dial in or out the the things you need (flex) and dial out those things you don’t.

In the case of a Jensen style board the rail wood is NOT tied into the structure, just butt jointed onto the outside of the structure that is load bearing.

The glass laminated onto it ties it in, leaving material that does nowt but add weight.

Unlike the laps in glass in a conventional board or like the rails in a wegner style board.

Now heres a thought…

Instead of useing shaped bulkheads and one measily centered stringer in a Jensen style build, how about getting rid of them bulkheads and useing multiple stringers (a la S core)?

As the skins are tied together in a more efficent manner then the rails could be made lighter.

And how do you make a model plane (15oz weight, 2bhp, 145mph and 300 - 350 sq inch area) withstand a dead stop impact?

You desighn it to flex in the right direction.

How do you get the structure to cope with the 2bhp nitro burning 2stroke hauling it through the air AND cope with flight loading?

You desighn in rigidity in the right directions.

Not an easy thing to do.

When i snapped my Score i was slightly more than horrified to find that though the desighn in theory was awsome, in practice not so hot.

We could already have pushed the limits of these construction methods to the point of rapidly diminishing returns or even past their peak.

All good stuff to think about till i can next get online.

288

Kendall,I see that the multiple stringer idea creates a series of boxes or I beams that would give a board greater strength with each added stringer.

And most boards have simple foam filled glass rails, but the range of variations from hollow to multi ply/glass composites still doesnt create an unbreakable board. ( within the parameters of surfing)

So the construction methods are many and varied and seemingly at their limits,

And the materials are state of the art.

But is it the design thats letting us down?

We have made great advancements in other areas such as fin design, hull shape, foil, shaping accuracy and even surface finish.But no quantum leaps in rail design.

I’m questioning the future of the rail and why we havent dissected it like the other elements of design ??

Quote:

We could already have pushed the limits of these construction methods to the point of rapidly diminishing returns or even past their peak.

289

This all sounds extremely interesting (and extremely complicated)!  I can’t wait to see some pictures but have to say that this does not sound like you are looking for a hollow wood board.  I, too, was influenced by Paul Jensen’s boards. They are remarkable to say the least.

I think it’s important to note here that there is really something special about riding a board with as much wood in it as possible. Kevlar and carbon and all of these intricate elements going into this board kind of defeat the purpose of making a “wood board”. But again, I am totally interesting in seeing how it comes out.

I made a board based on Jensen’s methodology and have to say that while the weight is obviously going to be a bit greater because of the materials used, there is certainly no need for more ribs or tighter spacing. At nearly 12" between ribs and a skin that is just over 1/8th of an inch thick, the finished product, especially after the epoxy and fabric has been added, is ten times as strong and durable as the typical foam board. No denting, no cracking, no creasing, nothing! These boards last forever and literally surf as though they are an extension of the wave and the rider all at the same time. There really is nothing like surfing a truly hollow word board. It feels different - And until you ride a board that is truly wood, it’s hard to explain.

The only part of my boards that not “real” wood is the 1/8th inch luan that I use for the skeleton and of course the Epoxy and 4 oz e-cloth. I plane all of the strips of wood, cedar, pine, maple, that make up my fin and skins to 1/8th of an inch. It’s an amazing process and the finished product is unreal.

 

Can’t wait to see some pics. Good luck with your project. Please include some pictures of the process. Sounds neat!

 

Kindly,

Erik