G’day Havard!
sorry if I came out a bit unclear, the comments about the angled rails was intended for josh, not you.
Didn’t have to even read between the lines to understand what you thought about it.
NP 
Sorry for the crossed wires, mate.
I’ve never made a bow myself, I’ve used a couple though and got the medals and national records to
show for it ;-).
LOL! I am not that great a shot - good enough to put dinner on the table, that’s about it. I prefer shooting long wooden bows. Which I also love making… Though this boardmaking thing is rapidly taking over 
Gotta get a hold of that bow bible, that would be interesting. A couple of your comments got me
thinking since the board has to have rocker, the stringer can be straight before it is preloaded. It’s like
stringing a straight bow, when you do it, that’s also kind of a preload. It will have less effect, true, but
it’s still better than no preload or in other words a stringer that easily conforms to the shape of the board
when it’s installed.
Spot on! There’s something called a deflex bow. It’s the opposite of the standard modern recurve - the tips of the unstrung bow point back towards the archer holding it. Those designs shoot like a straight stave bow 10+ pounds of draw lighter. The more tensioned wood is the more vigorously it wants to spring back to it’s original shape. Too much “snap back” in a surfboard wouldn’t be good. But too little makes it feel unresponsive. There’s three volumes of the TBB (Traditional Bowyers Bible) and one or two chapters in each is incredibly enlightening regarding the way wood behaves under stress and tension.
Another important factor in how easily wood bends while retaining tension is thickness and, to a lesser degree, width. Wood twice as wide is twice as hard to bend. Wood twice as thick is EIGHT times harder to bend. So shaving a small percentage off thickness may drastically increase the ability to bend. This should mean that wooden rails can be fairly highly pre-stressed (snap back more sharply) but not be too difficult to bend by tuning their thickness and width.
I’d suggest taking a more or less straight piece of wood for the stringer. Attach the bottom skin and rails.
Route a slot wide enough for the stringer and deep enogh for the stringer, plus a layer of EPS foam.
Now vaccum bag the stringer into the slot with an oversized backing of foam while the board is in the
rocker table. Remove the excess foam then bag on top skin, and laminate the deck on the rocker table as
well. I doubt the springback will be very significant. Bdw. the skins are also preloaded as they are
formed to tge board.
I reckon that’ll work for a centre stringer. I am going to pursue the wooden rails-as-stringers approach tho Yep - the skins are pretensioned, but the effect will be minimal - because they are so thin. Take a wood shaving and bend it - bends easily and doesn’t recover. A 4x4 won’t bend without great force, even then bends very little before it breaks, but spans back incredibly fast (if it doesn’t break). A 2 inch wide, 0.25 inch thick slat bends easily and snaps back readily.
But I have changed my mind about something I said that the whole board will recover away from rocker an inch or so after the skin is applied. Well, I have changed my mind - I realised last night that the mechanical bond interface (where the skins are “glued” to the stringers) will counteract this a lot. We see this in composite bows and Perry bows. That might be less pronounced in your “buried” stringer (because it’s only bonded to the skin on one side, to foam on the other and foam gives easily compared to wood).
Also, you could o two or 3 stringers to load up against the fins, do a V or Y shaped stringer spring on
the fins to help against torque, the variataions are endless.
Yeah! You could do virtually any combination of pre-tensioning strips under the skins. That should mean any area can be given almost any combination of flex, resistance and snapback desired.
You could obviously do the same with a stringer that already have a recurve or preload of some sort.
But even so, I think the preload of a straight stringer would be a benefit. Secondly, no matter how much
preload we put into it, it’s going to wear out since we cannot unpreload it like you do when you don’t
ride the snowboard or you unstring the bow, however, this advocates for a more heavily preloaded
stringer since some of the tension will wear out.
I guess a good rule of thumb is - if you have to bend it to glue it up you are pre-tensioning it (just keep in mind thickness, width, etc “rules”). The more you bend it the more you pre-tension it. So yes - a straight piece is going to pre-tension. I am going to disagree with your second point In a raw wood configuration you would be right - that’s why wooden bows are traditionally unstrung when not in use. But in the composite world things change! A huge number of modern composite material bows are never unstrung except when the string is changed. And they do not suffer any loss of cast or “noodling”. The secret to why is actually in the material interface and fibreglass layering outside the wood. But that interface layer becomes very important - you do NOT want a dry join 
I’ve been thinking about one thing. Bert always show us the board flexing while standing on the bottom
of it. Ok, so it’s easier and more practical to show it that way, but it’s not the way it flexes when you ride
it. So it got me thinking since he could so easily flatten it out, maybe it is heavily preloaded in the other
direction?
I reckon that’s probably right (if I read you correctly). While he’s simply demonstrating the flex present it suggests a certain level of snappy recovery (or you will surf a noodle). There is a relationship between flex and recovery, but it’s a fairly fluid one. And with composite construction we mitigate some of the insurmountable hurdles involved with all-wood construction. That’s why Bert can bend his boards back in the OPPOSITE direction to the way they flex without doing serious structural damage to the “flex systems”.
Like I said earlier - in theory we can put whatever flex and flex resistance we want in any vector or plane. Work out how you want your flex to work and introduce the appropriate design flex slats. Look at the board in any perspective you like (top/side/front/end) and work out what directions you want flex and how much. And how snappy you want it to recover. Then work out the slat system that implements it
I am visualising a skeletal frame of slats, stringers and rails implementing perfect flex and recovery :=D
Yeah, plywood is a probably bad idea. It’s normally the grains of each layer running perpendicular to
each other. That’s why it bends so easily. Also the wood quality is usually low. I also doubt the glue
used are made to withstand thousands of flex cycles. I would guess for a surfboard you would use balsa
to keep the weight down. I doubt the weight of an ash or yew stringer in 6’ length and 2" width with the
proper taper for a bow would be acceptable in a surfboard.
You have the right idea!
It’s an interesting area in it’s own right People often talk about grain without really understanding what it is. Grain is, in reality, a 3D lattice - wood fibre embedded in organic resins. Chop down a log. Look at the clean-cut end. The rings are the end of the wood “threads”. These threads are where all the integrity and strength of wood comes from (the “resin” is pretty week). Imagine making a composite “log”. Get a whole bunch of long fibreglass threads, resin them up, lay them on top of each other in a bundle, roll the cylinder into shape and wait for them to set. Next we’ll “make” a “plywood” log. Get the same number of fibreglass threads, chop them into itty-bitty bits, resin them up and roll them into a cylinder and wait for it to set. Now, which one is easier to break and why That’s the real secret to “grain” and wood integrity. That’s why splitting a log from end to end produces strong wood than cutting it form end to end with a saw.
This is very relevant to composite board construction using timber. And yes - lightweight woods will be a must. I have been wondering for some time whether anyone has tried using spruce? It has the highest strength/weight ratio of most light timbers. But too much strength may be a bad thing.
Bert? have you tried any spruce species??
Idea! We can use different timbers for our “slat skeleton”! This gives different strength/weight ratios for further flex tuning and weighting. And we can use heavier timbers throughout for big guns
How do you suggest the stringer is laminated and foiled knowing bow tech? Bdw. what did they use in
ancient times to glue laminated bows?
Old composite bows are constructed of horn, wood and sinew - held together with animal glue. Those animal glues (hide glue) are strong! Put a line of it on glass and as it dries at shrinks… And pulls pieces of glass off! The Achilles heal of animal glues is moisture (they aren’t water proof), so old composite bows were often wrapped in a moisture resistant layer of some kind. Composite bows have been around for at least 3,000 years. One was aged using radio carbon dating and by looking at the layers of earth it came out of. Simpler wood/sinew composite are almost certainly 500-1,000 years older again. In wooden composite bows the glue (interface) layer is critical.
Not quite sure of your question. Do you mean laminating the stringer as in layering multiple pieces of wood to form the stringer? Or as in how it’s layered to the foam? As for foiling… I am guessing you mean width and thickness tapering of the stringer for foil.
Unless my logic is fundamentally flawed I think we are heading in the right direction to unlock the “perfect board” aspects of flex. Course I have never been wrong before - not more than once a syllable ;D And if we are right there is a HUGE amount of experimentation and finesse that can be involved in this.
Damn, gotta get my vac bagging setup finished
Regards,
Doug
