When you don’t wrap the skins over the rail they are separate structures. So your board has three components - deck, rail and bottom.
Not enough epoxy soaks into the joint to cause them to be truly bonded, when you have pre-wet the glass 1:1.
Remember we are talking about shear here. Are you saying shear forces are not transfered from bottom skin tru the overlapping glass to the rails? If so there should be alot of shear forces on the seam between the bottom skin and the rails.
I agree with a lot of whats being said. Fostachild - you seem to have a reasonable glass schedule, but the deck is out of scale with the bottom depending on what sized board you’re making
Sharkcountry- people who surf powerful waves like stiffer boards. I think its because the boards would get overloaded. However, stiff doesn’t mean lifeless. I think it means being able to stand up to whats being thrown at it.
This is a worthy topic and thanks for taking to time to explain some of your observations to the rest of us.
I think everyone who makes one of these does it for different reasons.
Durability
Looks
Surf Like Taj
The Challenge
Bert said he did it for durability and performance
I really feel if you are looking for performance you will not disapointed if you follow Berts recipie. What he said about strenghth was Flex is strong so the more you restrict the boards ability to absorb and flex the more you weaken it.
The details do make a difference and if you go back and reread all of Berts posts you will find the design changes needed to take advantage of what this tech has to offer.
As for the durability of the boards it is a little to early to tell. After 8 months of everyday use one of mine a 6’8" shorty that weighs5.5 lbs has yet to be pressure dented and has not had any rail separation problems and that is with one layer of 4ozE around the rails. The 7’2" which has seen probably 15 1.5 to 2x overhead days has had similar lack of wear.
I think minimal shear forces are transferred through the glass itself. When the skin and rail are monocoque that whole structure resists the shear forces… Absorbing and diffusing it, making for a stiffer board.
One thing is for sure - with unwrapped skins there are more shear forces at work, because the rail and skin aren’t a monocoque structure. Shear is only a bad thing when it’s not intentional and controlled. For example, controlled shear is the secret to making a bow that stores energy in the neutral plane (archery again).
Yeah, I’d say there significant shear force in the seams. But I also think that the glass doesn’t absorb all of it (even though it deals with it well if built with that in mind).
I reckon that’s why rail splits happen
Hope I answered your question. Wasn’t exactly sure exactly what was being asked.
These are just my personal opinions and theories based on bugger all real experience.
Christian, you’re right thats there’s so much in the archives. Things that didn’t mean anything to me when I started now are very significant. I think the challenge with the shortboards is making them flexy enough and with the longboards its controlling the abundant flex (thus the rail rip issues)
Doug, I agree that the effect of joining the rails is significant and thats why a talk about two panels. The top skin is joined to the rails so I think of them as one. In regards to shear, “SHEAR HAPPENS”. Its a biproduct of flex. The less your board flexes the more it becomes dependent on its built in shape. Its why I keep revisiting the 2 lb foam on my single fins. As Ben keeps teaching me more I’m beginning to understand why he likes the two pound stuff. Good single finning/noseriding is fairly static. You handle chop/close outs/ people by smashing your way through them. You’re not asking the board to adapt or conform as much, but instead use the shape you’ve given it. I think one reason my first 2 pounder was such a dog is because I didn’t understand what I was doing. However, on the performance longboards you’re asking them to pretend they’re shortboards which you can also stand on their noses. They need to adapt to many different situations so its easier to have the board bend into the shape you need it than it is to pre-put the shape in. Bending=flex=shear so light foam becomes a must.
Please forgive me if this has already been discussed, but I don’t recall seeing this facet of construction called into question and I’m curious;
So, a compsand constuct follows standard glass/foam construction in that during normal use, the bottom glass is in tension and the top glass is in compression. Clear. Now, what about the inside (foam side) of each of the outer skins? Does the inside of the bottom skin experience a degree of compression? Does the inside of the top skin experience tension? If so, are the forces so slight as to be ignored or does this aspect offer yet another round of materials choice to optimize composite construction?
It would seem to me that the more shear allowed by the core material would accentuate this behavior, but I dunno…
Yep, I agree 100% with “SHEAR HAPPENS” I reckon understand, accept, control and design for shear and all should be well… Nice theory Good points re the single fins too.
SevenTenths,
I wouldn’t say it’s never been discussed. I’m just not aware of it being discussed openly
I think minimal shear forces are transferred through the glass itself. When the skin and rail are monocoque that whole structure resists the shear forces… Absorbing and diffusing it, making for a stiffer board.
One thing is for sure - with unwrapped skins there are more shear forces at work, because the rail and skin aren’t a monocoque structure. Shear is only a bad thing when it’s not intentional and controlled. For example, controlled shear is the secret to making a bow that stores energy in the neutral plane (archery again).
Yeah, I’d say there significant shear force in the seams. But I also think that the glass doesn’t absorb all of it (even though it deals with it well if built with that in mind).
Isn’t monocoque alot about using curved surfaces to stiffen up a structure? Otherwise it would require all the strength to be in the material(skins) itself. I don’t think surfboards are a monocoque structure by design, they just happen to be shaped like that for other practical reasons. Monocoque theory is more relevant for traditional board imho, the hollow carbon boards and the HWS. Remember Monocoque is poor in buckling, add a stringer to non sandwich boards and the problem goes away, now it’s semi-monocoque. I think you want to design a compsand board to be as little of a monocoque design as possible, keep it thin so you don’t have to dome the deck too much, bottom flat, etc. to get as much flex as possible. However, as the density of EPS is decreased you could argue that you are approching pure monocoque design, using the formula composite*foam density + monocoque/(foam density + 1) = reality
If you don’t put the skin out onto the rails you say the shear forces increase, also indicating this increases flex. But the bottom glass is attached to nose and tail and rails and it’s strong in tension. The bottom glass being fused to the bottom skin cannot possibly stretch much. If the flex and shear is happening something has to move. What? Where?
I view the bottom glass kind of like bowstrings being attached to nose and tail, rail to rail for every thread of the weave in the glass, kind of like a tennis racket… If nose and tail is lifted, the tread in the warp direction is in tension and if movement occure the threads must straighten out, providing they don’t stretch much(just like a bow string) and the nose and tailblock doesn’t twist alot. This would indicate the bottom skin is compressing the foam core. Depending on the deck being convex or concave, the rails will want to move up or down relative to the deck level, with a convex deck it will flex the rails up creating tension in the sideways thread in the bottom, if the deck is concave the rails will want to move down creating compression over the same thread. In addition the threads in the warp direction is trying to pull it into a concave anyway. Regardless if the deck is concave or convex, the bottom skin is pressing against the foam core. That might be part of why flex benefits from a lighter core, the 2lbs core has four times the compression strength of the 1lbs core.
I say it’s more than likely that the foam core absorbs quite abit of the shear forces through compression.
Monocoque is separate to curved surfaces - the interaction is a critical key element.
The literal translation of monocoque is “single shell”. It is a construction technique in which an object’s structural load is borne using it’s external string (ref. http://dictionary.reference.com/browse/monocoque%20 and http://en.wikipedia.org/wiki/Monocoque). This skin is generally considered to be metal, but might be made of another (in our case composite) material.
IMHO monocoque is critical and fundamental to composite board construction. You see, all of the strength IS in the skins!
You see, two compsand boards identical in ALL RESPECTS, except one has overlapping deck and bottom skins and one does not will have very noticably (and measurable) flex characteristics. It’ll be obvious even in static flex.
And that, my friend, is why forming a single monocoque structure out of the deck, rails and bottom kills flex so very easily.
Oh yeah - What is the difference between a 3/4 pound core compsand and a HWS?
Compsands and ultra-modern HWS have far more in common with each other than either does with traditional surfcraft.
Shear occurs when flex occurs and I think the movement happens inside the board (Shear: 9. Mechanics, Geology. to become fractured along a plane as a result of forces acting parallel to the plane. See http://dictionary.reference.com/browse/shear and http://en.wikipedia.org/wiki/Shear). More specifically in the foam core and, also, between unbonbded panels.
I say it’s more than likely that the foam core absorbs quite abit of the shear forces through compression.
Bingo! We agree. That is shear… But I think it’s not only in the core.
I agree that for a regular composite board (PU/PE) monocoque is critical. However as the wikipedia article points out, monocoque is week in buckling. Therefore the stringer is added. So now it’s semi-monoque. Take away the stringer and the foam to make it truly a monocoque structure and you have to make the skins incredibly strong to avoid buckling. The only surfboards that are doing this sucsessfully are the hollow carbon bords(aviso, kolstof) and to a lesser degree the S-core by salomon where they add some internal foam ribs, again making it a semi-monocoque. The article in wikipedia also notes: “…With a sufficient thickness, one could do away with all of the internal structure. However this would be even heavier than the framing would have been.” In other words, a different design (the case in point in the article is semi-monocoque) is more efficent considering strength to weight. Bdw. the old vikingboats are propably monocoque, the use no ribs. This allows the boats to twist and flex in high seas.
For a sandwich board I would argue that alot of the strength is actually in the foam core separating the skins. Double the thickness of the core and you quadrouple the stiffness. If you take monocoque board and make it twice as thick you will not gain the same strength, you need to double the thickness of the skin to achive the same or change the curvature of the skins. You can only do so much with curvature within the limitations of the shape of a surfboard. Skins are usually much heavier than foamcore which is a huge penalty in strength to weight, so the sandwich core comes out with a much higher strength to weight. After all it acts as an I beam that is continous in two dimensions, the core will ditribute compression on one side to the other side where it creates tension. Every surfboard need to deal with these forces, but they deal with it differently. I’d say the HWS boards I’ve seen deal with this differently than sandwich board, being a ribbed structure. On both (or rather all composite) types of surfboards there will be a monocoque component though.
Take two pieces of cardboard, making one longer than the other. Glue the end of the two pieces together kind of forming a surfboard rocker profile. If you bend more rocker into it, the distance between the skins change. Something inbetween the two pieces and you stiffen it up quite effectivly. If you put foam in between, the foam has to compess to achive the same rocker.
Monocoque is certainly important to understanding the design of sandwich board, but it’s only a component. And Bert and Greg has already pointed out how to take advantage of it.
I never tried to have some kind of “floating” bottom skin, if that’s what you are meaning?
What I have found is that firmly attaching (overlapping) the bottom skin doesn’t affect flex, at least compared to the thickness of the board (foil) and the thickness of the sandwich skins, glass on the rails, EPS density and the like.
I don’t mean to say that there isn’t potential - anything is possible with this construction.
I just figured out how to get these boards where I want them without any complicated bottom skin tricks.
However, figuring out a way of doing something different can open up a whole new lot of possibilities…
by smashing your way through them. You’re not asking the board to adapt or conform as much, but instead use the shape you’ve given it. I think one reason my first 2 pounder was such a dog is because I didn’t understand what I was doing. However, on the performance longboards you’re asking them to pretend they’re shortboards which you can also stand on their noses. They need to adapt to many different situations so its easier to have the board bend into the shape you need it than it is to pre-put the shape in. Bending=flex=shear so light foam becomes a must.
Good points re the single fins too.