Harrow said..
There's an assumption in his video which no one appears to have questioned. He says the carbon adds no strength after it breaks. But who says the carbon is going to break? The carbon does add stiffness to the total construction, and if the design does not subject the carbon to forces that will break it, then it will continue to do so.

The takeaway point is that the carbon will fail first under excessive loading, so the additional carbon layer is giving you extra stiffness, but not additional strength or toughness. Be aware of it.

Maybe the phrase 'Carbon Reinforced' is misleading...perhaps 'Carbon Stiffened' would be more appropriate, not leading you to thing the carbon is making it stronger.

I guess the video is best interpreted as follows:
If, under the given circumstances (more glass than carbon, so the glass is able to take a lot more load than the carbon) the carbon doesn't break, you're carrying useless weight in the shape of those glass layers.
So you would want to add carbon layers and take glass layers out, reversing the ratio to a combo in which the carbon can take more load than the glass: lighter, stiffer, more bristle and way more expensive.
If I've got this wrong, don't hesitate to correct me.
If you prefer more flex, but want to reduce weight of the board, flax fibre might come in handy.
I haven't worked with it yet, but according to all I've read, it would combine with glass nicely as elongation is similar to glass, so it won't break before the glass does, but it's obviously rather light and not as expensive as carbon.
O.k., a see-through surface would make your board resemble an old mailbag, but isn't this true understatement?
Cheers,
Sven

Imax1 said..
A lot is going on here.
This is what im getting out of all this.
Stiffness i get , as with full carbon , ( weight for weight its stronger or at least stiffer than glass , use less = lighter )
As to part of my first question,
So to use a layer of carbon over a fiberglass board under the footstraps for extra strength ( like many do ) is a total waste of time and weight.
Or is there still something im not getting ?

Well I can't talk for anybody else, but I put carbon under the footpads for STIFFNESS, so the load is spread reducing fatigue of the core foam. in these areas I use more carbon than glass so for a small area there is unnecessary glass, but I think it's better to have continuity of fibres, rather than cutting holes in cloth.
For the cloth underneath the impact area, I run 6oz carbon from the tail to the mast track at 45deg, tapering to the front of the mast track from the rail at the front pads. overlapping the carbon by about 100mm I then run 4oz cloth to the nose.

A point the vid didn't take any consideration of is, fibre direction. A carbon weave at 45deg to the glass weave will have very similar stiffness to the glass in the direction of the glass fibres.
And it's also only talking about an elongating force.
So yes if you are trying to stiffen a board, throwing a few layers of carbon in, will probably make the board more likely to snap, as there's not enough carbon to take the load, and the glass isn't taking any. The carbon gives way at the weakest point. This throws all the load on the glass at that point, if it doesn't give way immediately, constant flexing there will weaken it until it does.

Zee German,
yes you've got it wrong, because the glass is umpteen times stiffer than glass the carbon takes ALL the load, the glass does nothing unless there's umpteen times more of it and reaches the same stiffness as the carbon due to it's extra thickness.

I think what it's saying is you have to really understand what's happening before you mix the two. Most of the time it isn't a good idea.

One thing not addressed in that video, and the point I think decrepit and harrow is making is how the foam core interacts in all of this. Carbon does not stretch as much before it breaks, but isn't that only relevant if we attempt to stretch the two fibres the same amount? For example with your heals on the deck, if the greater stiffness in combination the foam, allows the deflection to spread out of larger area, then you are no longer comparing like for like?

So the original video would only be relevant with a hollow board, without a pvc sandwich?

But that will only comply to stretching without breaking,
we don't want any stretch,
so why is my brain now bleeding,
i got to start drinking again !

decrepit said..

Imax1 said..
Id like to pick your brain decrepit if I may.
>>>
If I am correct in thinking laying carbon at 45 degrees ( to get the stretch factor similar to glass) , then be relying on the sheer force of the carbon fibers working against the tensile glass fibers for its strength .

Sorry I don't understand what you are saying here. I don't think sheer is a problem unless there's an impact

>>>>
If carbon is absolutely perfectly stretch matched by angle to glass would carbon in its sheer mode be still better than another layer of glass ?

OK, so what I'm doing here, is matching the stretch of carbon along the length of the board, so the glass and carbon work together to resist board creasing and snapping. But at 45 degrees to the board centre line the carbon is still it's normal stiffness, so in the heavy load areas that I use it, the sandwich is still very stiff and spreads the loads over a much wider area. It also gives the back of the board a fantastic resistance to twisting under the mast base, feet and fin loads.

well that didn't work very well getting too smart for my own good, should have switched to plain text mode.
I guess you all can hit the "select to expand quote" button

I'm getting closer to the way you think , however,
Are u only using carbon to stop the board twisting ?
If your using carbon to help stiffen the length of the board then your using carbon sheer strength ???, or at least 45 degree sheer ( probably half sheer to tensile ratio ) ,probably 10% sheer to 90% tensile.?
what about heel deformation ?

Decrepit,
thanks for your honest answer, but now I'm not getting what you mean when you say that glass is umpteen times stiffer than glass.
Do you mean, carbon is a lot stiffer than glass? I agree.
When you say that the carbon takes all the load, what's the use of the glass? I'm not quite sure if we're only talking on cross purposes.
I may have been too general in my statement, with the discussion now focusing on the heel section and me thinking about a section of the board being violently stretched, e.g. the bottom of the board in a flat landing: For this situation the video still seems to imply to me that you either go for more layers of carbon so it can indeed take all the load or leave it out as it might turn aout to be w waste of money once it' elongated too much.
Still wrong?
I'm really amezed too see such a vivid discussion on the issue and wouldn't know where else to get this kind of info.

My JP FSW has FWS (Full Wood Sandwich). Where does this fit into the argument? I remember reading somewhere that the timber laminate had a great combination of physical characteristics. (ie. combination of weight, durability, stiffness, strength, etc.)

Imax1 said..
Are u only using carbon to stop the board twisting ?

No, my main aim is to stiffen the sandwich to stop fatiguing of the core, the anti twisting thing is a side benefit .

If your using carbon to help stiffen the length of the board then your using carbon sheer strength ???, or at least 45 degree sheer ( probably half sheer to tensile ratio ) ,probably 10% sheer to 90% tensile.?

I'm not intentionally trying to stiffen the board lengthwise, my boards are quite thick, (I hate sinking when the wind drops), so even with only glass they are quite stiff, but having the extra carbon there does increase length wise stiffness)

Here's a pic of Ben's board with underneath sandwich layup before wetting out.

it's 6 oz carbon at the rear and 4oz glass at the front, with an overlap, so here I'm not using glass and carbon together.

what about heel deformation ?

I think the main cause is fatigue of the core with use, once it compresses too far then the sandwich also starts to break down.

So stiffening the sandwich enough prevents the core fatiguing and every thing stays together.

Interestingly the other day I started the "unusual repair" thread. This is possibly an example of core fatigue and sandwich break down.
www.seabreeze.com.au/forums/Windsurfing/General/unusual-repair?page=1#11




The board was in for repair, because a fine crack, (running across the board), had appeared in the deck behind the front starboard foot pad. When I sanded through the sandwich I noticed a lot more cracks in the sandwich underneath the surface glass, so I started following them with the router set to the depth a bit above the underneath glass.
The deck has sunk considerably in this area, about where the old back foot goes in a gybe.

The vertical white area is where the damage came all the way through to the deck, the rest was hidden.

I'm not sure if this was initially damaged during construction or it's longterm fatigue.
It's an old hypersonic with not so light owners.

decrepit said..
The one thing I have against timber decks, is the timber starts to rot when it gets salt water in it.

And I think the oils in the wood and / or UV doing something - even with no water ingress all the mid to late 2000's age Starboards with exposed wood turned to crap and the non slip fell off wood cracked or warped it was weird. Maybe best kept away from harsh sunlight

Imax1 said..
I would think heel depression is stretch on stretch over the radious,
if you were to measure the two radii it would be less than the compression measures , so both radii would be stretching ???
mabee ???

You've had me thinking here! In my head was what happens to a pipe when you bend it, you get compression crinkles on the inside radius. Same forces would apply to with a free floating sandwich, any bending force compresses the inside radius and stretches the outside radius. But as the sandwich is fixed at the rails, nose and tail, this could change the dynamics. How much by I've no idea.