More glass on top or bottom?

[mattma]

I've been puttnig my first attempt at a twin tip through its paces in the surf and it has handled a real pounding very well. So well in fact that for my next attempt I'd like to use less glass to get a bit more flex into it as the current one has way to little flex in it.

The construction on this one is 3 layers of 6 oz e-glass top and bottom over 2x6mm Klegecell pvc foam stepped down towards the tips, epoxy and vacuum bagged it under about 22 inHg.

For the next board I have been thinking of using 2 and 3 layers of 6 oz but not sure whether its better to put less on the top or the bottom. Any one have any thoughts?

I've been thinking to take it off the bottom because it seems that glass is stronger in tension than compression which would mean more on top.

Another option if been thinking through it is to have 3 layers top and bottom in the midde section and just 2 top and bottom on the tips? Too weak??

Thanks in advance

Matt

[Bigdog]

I would thin out the core to get the flex you want and stick with 3 top and bottom.

Stan

[zfennell]

stan is always right.

my logic may be a bit circular, but...
typically the core thickness and length are the driving factors regarding flex.
Reducing the thickness will aslo increase stress in the laminate and require more glass.

so if you reduce the thickness of your board you will likely need the extra glass .

in other words, thick boards with less glass are not more flexible, they just break sooner.

just to keep things confusing.
i think you are correct about the relative stability of glass in tension vs compression.
its possible to buckle a laminate before it reaches max compressive load.
particularly where you have nasty point loads like your feet.
places like that usually deserve additional reinforcement

no worries,
-bill

[mattma]

Hi

Thanks for the ideas. I think I will experiment with the core thickness.
I did some googling around and found a great resource for sandwich board constructions.

http://www.fgi.com.au/files/images/stor ... ich_hb.pdf


A quick read of it (see equation 2.6) suggest that the flexural rigidity (ignoring concave and rocker) is close enough to

=(E.b.t.d^2)/2

where E is the modulus of elasticity of the glass/resin material
b= lenght of the section your looking at
t = thickness of the glass/resin layer
d = core thickness +/-

This seems useful as it suggests that flex is proportional to the core thickness squared but on only depends linearly on the amount of glass/resin. My take away is to go easy on the core thickness reduction.


Cheers

[sq225917]

You should really only use the thickness of the core, and choice of core material to control the flex. There's a minimum amount of glass you want on each side to stop the core splitting. 3 layers of 120-150gms top and bottom should do it.

You really do want to keep the layup of the board symmetrical, top-bottom, if you can. Things can get a little 'twisty' if you don't.

[gbleck]

Not true. You can use the fiber to controle flex with a uniform core thickness and material. True though just laying more fiberglass on one side or section is not the answer.