To start out with I’m a college student studying civil engineering and I’m constantly trying to think of ways to apply what I learn to surfboards. In fact, it makes my classes much more enjoyable. Anyway, the other day we started learning about the process of prestressing concrete and I was wondering if it could be done with foam blanks also. Does anyone know if this could be possible or if it’s already being done? I don’t pour foam myself so I won’t be able to try it out but maybe someone could pass the idea to a blank manufacturer who could experiment with the idea? Let me know what you think about it.
The closest to prestressing foam that I’ve seen so far is bending and glueing XPS or EPS blanks to match the rocker.
Don’t think people do it for the same reason as prestressing concrete. Also the structural strength is not in the foam, but in the glass.
And glass is tensionally very strong as opposed to concrete, which is the reason for prestressing concrete if I’m not mistaken.
The only advantage I see is that the blank density is increased on the deck side and decreased on the bottom side, meaning a lighter blank for more dent resistance on the deck side. (Not sure if this is really the case because you damage the foam cells by bending it)
Good point that the strength is in the glass but depending on how you prestress it couldn’t you add flex characteristics that would help the board rebound to it’s natural form better? Also is there a reason why you can’t have strength in both the glass and foam for better durability/longevity? Trying to do a concrete process with foam seems kinda silly just thought it could possibly work.
The contribution to the forces parallel to the glass fibres added by the foam or even the stringer is neglectible. The main contribution to the breakage resistance and stiffness by denser foam and stringer are because of the increased glass bondage strength. (read increased buckling resistance)
When you order a custom blank with a custom rocker, that’s what you get.
Flatter rockered boards are bent flatter.
The opposite is true for increased rocker.
They are bent to fit that rocker.
What most don’t realize is as they shave down the blank, the stringer looses its ability to to hold that rocker.
Especially at the ends of the board.
I’ve sold my Rocker Bars to shapers trying to get that rocker just right.
Only to watch their boards flatten out and loose the curve.
That is why blank companies reccomend thicker stringer wood to hold that shape.
To keep a rocker exact, use of a rocker bed to hold its shape is best.
Or vacuumed to a form.
The stringer backbone in conjunction with the fiberglass skins return the board back to its original shape after flexing.
Linear, Parabolic, lots of flex going on.
Flex is your friemd.
http://www.barrysnyderdesigns.com/torsional-propulsion–torsion-drive-stringers.html
Pre stressing a blank, similar to concrete is a mistaken path. Concrete has great compression strength, and terrible tensile strength. The reinforcement in the concrete (rebar or cables) can be tightened like a guitar string, to a point that doesn’t overcome the concretes compressive strength, giving a stronger end product.
Surfboard foam has next to zero compressive strength, to any force applied to the blank is immediately destructive. Any tension to the fabric just brings it closer to its breaking point, so again not in your benefit.
Any “spring” gained by tweaking the blank with forcing stress into it is just bringing it closer to failure. If you want to modify the “spring” in the board’s flex, your best bet is a strategic application of both thickening/ bulking in the lamination, modifying the numbers of layers of fabric, and using different materials in the lamination.
“”“”" Does anyone know if this could be possible or if it’s already being done? I don’t pour foam myself so I won’t be able to try it out but maybe someone could pass the idea to a blank manufacturer who could experiment with the idea? Let me know what you think about it.“”“”“”…a most excellent concept — you could could call them …“stringers”…!!!
Like Barry said nost pu blancks are made like that, molded near flat then rockered when gluing stringer. Some eps buid are made from flat panels rockered while gluing sandwich skins on rocker bed.
Because of it flexibilty stressing foam like this is not a big problem. Foam lenghtwise flexing resistance contribution is really light compare to skin and other strenghener (stringer,springer,etc…) Foam have to keep shape = keep skin separate = avoid skin buckling,
My understanding is that most blanks are molded with the rocker shown in the catalog - if you order a custom rocker they bend the blank accordingly.