Overner said..

racerX said..

There are other substitutes for kevlar in this application, innegra for example.

Innegra and Kevlar are not good comparisons. Innegra is like an elastic noodle, kevlar is relatively stiff - Certainly the K49 composite version is. Both are awful in compression.

I personally would avoid innegra for all windsurf applications. It has a huge elongation on breaking, something like 20%. So it is super elastic compared to anything else in your boards. It also provide very little in terms of stiffness (other than taking up space like a sandwich might). I have heard of surfboard makers in the UK using it between glass layers like a micro sandwich, but I struggle to see its benefit over another layer of glass, certainly not when compared to other fibres out there like Dyneema. Here is a nice video of Innegra being used in a laminate:

Have a look at easy Composites video on the materials:

I have found that Kevlar needs to be hidden in the layup, preferably under lots of glass, so you never hit it when sanding. Kevlar is good under the sandwich and especially under the sandwich under the heals, where its low density means you can put more fibres in for the same fabric weight of glass for example.

Kevlar does come with down sides though - it doesn't like to stick very well to anything, it costs the earth and it is absorbent. I actually think under your sandwich and under your heals you would be better off with carbon.

Still trying to understand max tensile strength being relevant when it comes to fins. Are fins really under that much force? Foils I could understand, but not fins. Unless they are mega big ones perhaps.

One useful application for Innegra is impact resistance. Alex Morales (Tillo International) uses it on nose and rail areas to protect the carbon from mast impacts. Works extremely well for that purpose, with minimal cost in weight. It's touchy to lay up and I can't picture it in a fin.

Here some data about the most common resins (and also some laminates) in Europe/Germany. The list is german/english. (Harz = resin, H?rter = hardener)
www.r-g.de/wiki/Vergleichsdaten_Epoxydharzsysteme

As you can see, there is some significant difference in tensile strength between glass and carbon (an of course the young modulus).

Sepp

Imax1 said..
I was curious about Kevlar, but I don't want to play with it now .
Where would I use it on a board build , nose guard , on the top ridges of the rails next to a sunken mast track ? The way I read it , Kevlar is good for high speed impact and abrasion resistance . Not good at stiffening such as under heels ?

Excite Ride had kevlar around the nose.
First real repair i took on. Started sanding n ended up with all this fluff sticking up.
Think that was my first HELP! Pm to Mark Australia.

Basher said..

As a material, carbon is expensive but it's value is in its high tensile strength. You can make something lighter and stiffer by using carbon and, with the right layup, you can also get a better reflex response.
....
So when a manufacturer boasts of selling a 'carbon' board or fin they are letting you know that they didn't skimp on costs by using the cheapest materials. But they can still legitimately use the word even where the actual carbon content is quite low.

Doesnt tensile strength mean how it resists being pulled apart? Carbon fins can certainly be made lighter/stiffer/thinner. I dont know the tech term for measuring bendyness and how it resists breakage. Torsional strength maybe?

I think plenty of items 'made of carbon' where materials are skimped with. Fins, boards, booms. There are different grades and weaves of carbon.
I've got fins which just seem to have a few layers of carbon on the outside and it is thinner than its all glass predecessor, performs better. You can tell by the price. Same with the cheaper 60% carbon booms. The cheaper 70% carbon booms are 20% cheaper and probably just as good for me as a higher spec'ed one.

mob dog said..

PhilUK said..

Basher said..

As a material, carbon is expensive but it's value is in its high tensile strength. You can make something lighter and stiffer by using carbon and, with the right layup, you can also get a better reflex response.
....
So when a manufacturer boasts of selling a 'carbon' board or fin they are letting you know that they didn't skimp on costs by using the cheapest materials. But they can still legitimately use the word even where the actual carbon content is quite low.

Doesnt tensile strength mean how it resists being pulled apart? Carbon fins can certainly be made lighter/stiffer/thinner. I dont know the tech term for measuring bendyness and how it resists breakage. Torsional strength maybe?

I think plenty of items 'made of carbon' where materials are skimped with. Fins, boards, booms. There are different grades and weaves of carbon.
I've got fins which just seem to have a few layers of carbon on the outside and it is thinner than its all glass predecessor, performs better. You can tell by the price. Same with the cheaper 60% carbon booms. The cheaper 70% carbon booms are 20% cheaper and probably just as good for me as a higher spec'ed one.

What! Carbon booms aren't 100% carbon either? I didn't know that. I thought carbon boom meant full carbon as well.

Some are 100% carbon. They are the more expensive ones generally. If you see a cheap carbon boom which doesnt say the carbon content on the website I would wonder.

Unifiber have 70% carbon
www.unifiber.net/windsurf-gear/booms/carbon-hd-monocoque-v2-5e574cf33af76e648c39cd2a

?% carbon is 20% more expensive in the UK.
www.unifiber.net/windsurf-gear/booms/carbon-elite-monocoque-v2-5e574cf33af76e648c39cd34

I used rolls and rolls of Kevlar in the 80's building racing whitewater slalom Kayaks in moulds. This is where it was a godsend because of it's light weight and extreme resistance to being punctured or torn by rocks. It was kind of a revolution when it became available as Glass was super easy to tear holes in in rapids. It also enabled us to make much lighter boats. It was great for river touring for the same reasons before roto-moulded Kayaks superceded that construction.

Cutting kevlar is really difficult, but there are a few tricks you learn. One I found was using 'staysharp' scissors that came with their own sharpening case. The scissors ran over sharpening stones every time you pushed them in and pulled them out of the case. They were cheap so you could buy them in bulk but still tended to last for a lot longer, and stay a lot sharper than dressmaking type scissors or even the very expensive Kevlar 'Shears' some companies pushed. Another trick was taping both sides where you wanted to cut and cutting through the tape. Trimming the edges of the mould after lamination was very easy with a Stanley knife IF you did it at exactly the right moment in the cure, when the resin was set but still soft. Leave it too late 'till the resin was fully cured and it was a nightmare.

I have laminated Kevlar under glass in some of my polyurethane blank windsurfers in the 90's, and that worked well enough for sanding etc, but I didnt really find much advantage for the cost involved. Kevlar really needs to be used with Vinylester or Epoxy resins to take advantage of it's properties.

We made kevlar whitewater paddle blades for their abrasion resistance, but one they became worn on the tips they fluffed up badly so it went nowhere really.

I believe that many of the early Cobra Brand epoxy sandwich boards used Kevlar against the core under the PVC foam sandwich. Apart from weight saving, I cant really say what the advantages might have been.

We used kevlar patches sometimes for hull repairs on Kayaks but used the Gladwrap over the repair trick to avoid and need for sanding.

We also used a bit of carbon cloth and Carbon/Kevlar weave in Kayak decks. Those very expensive (in the 80's) materials gave us better stiffness in very lightweight racing boats, but again, there was no point unless we used epoxy, or Vinylester resins. Polyester resins would shatter and disintegrate in the laminate while the kevlar remained intact and did not adhere as well to the reinforcement . Carbon was useless for the outside layers on hulls as it had less resistance to tearing and puncturing than even glass. Some people with deep pockets got carbon, or Kevlar/Carbon weave on the inside (protected) layer of hulls for added stiffens.

I believe that modern whitewater racing kayaks are still constructed with Kevlar and Carbon/Kevlar, but perhaps flatwater racing boats use much more carbon as puncture resistance is not required and stiffness is the priority.

I cant think of any good reasons to use much Kevlar in fins, except perhaps as a lightweight core material, but I am sure there are better things for that.

But I cant think of many good reasons to use Carbon in slalom and free ride fins under 30-35cm deep either.

Awalkspoiled said..
One useful application for Innegra is impact resistance. Alex Morales (Tillo International) uses it on nose and rail areas to protect the carbon from mast impacts. Works extremely well for that purpose, with minimal cost in weight. It's touchy to lay up and I can't picture it in a fin.

Thanks for that. I still struggle using it for that application. My head tells me the properties are so vastly different to anything else in the board, you could have a smashed up pulp and still the Innegra fibres would not have broken as they have such a long elongation on breaking. That used to happen with those Carbon / Kevlar boards where the inner core and inner glass would be completely smashed to pieces, but the outer hull would still be intact. Board was still completely broken despite not looking too bad.

Having said that, I really like the concept of having an Innegra micro sandwich within the laminate. I may try that one day, when I have exhausted all my other brain farts.

Gestalt said..

Imax1 said..
I was curious about Kevlar, but I don't want to play with it now .
Where would I use it on a board build , nose guard , on the top ridges of the rails next to a sunken mast track ? The way I read it , Kevlar is good for high speed impact and abrasion resistance . Not good at stiffening such as under heels ?

my board has carbon kevlar to the rails and tail and nose. works a treat in those areas.

I presume it would be best used just under the last layer of glass ? It's odd they use carbon and Kevlar In the one weave , they are polar opposites. Unless both fibres are arranged at the correct different angles to break at the same time .???

I spoke to a guy who sells a lot of different cloths and a lot are just for looks , like glass with carbon racing stripes . If those thin carbon stripes are weaved the same as the glass , they would snap first and do nothing except add weight . If they were woven looser to match the glass stretch , it still won't do anything except cost more . Looks good though . I've used a carbon innegra cross , under heels and nose protection , but I don't know if it's more special than a good glass ??? It's thicker , so with more resin uptake , it just feels more solid ??? I just use a good quality 4 oz glass for everything . I'm tempted to make an all carbon board but that will be expensive and possibly end up a harsh tooth rattler that's not nice to ride . I don't know enough to mix and match different cloths and get a good result . Stick with what I know I suppose .

decrepit said..
where I use carbon stiffening between fins and mast box, I run it at 45deg, That way, it stiffens the sandwich but still allows the board a bit of flex.

Top or bottom ? Where in the lay up ?

Many ways to skin a cat, I use 300gsm unidirectional for stiffness and particularly buckling resistance on deck .
That gives little to no torsional rigidity increase though, so a biax layer will assist in that regard.

I know the argument about carbon and kevlar are polar opposites as one is very stiff/brittle and the other very bendy with insane tensile strength - so the interwoven cloth is nonsensical...... but I disagree. The other way to look at it is the carbon resists the movement a lot due to its stiffness so the Kevlar (or innegra etc) is not taking any real load. Then once there is a serious issue and the carbon fractures, the kevlar becomes load bearing and avoids a total failure (= board still in one piece but requiring repair)

Both sides of that fence make sense to me but all I know is the boards using full 200gsm CK or C.I decks last a LOT longer. That's why I use wood and CK on almost everything.

There are some bloody awesome flat-tow fabrics avail overseas that I'd like to try, much less resin uptake. A triaxial flat-tow carbon-aramid looks amazing too...... but double the price plus freight.