Carbon fins?

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

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 It was great for river touring for the same reasons before roto-moulded Kayaks superceded that construction.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.

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 .

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.