c/k crossweave

I saw a new Nordkapp LV with a carbon/kevlar crossweave construction. Beautiful interlacing of the black and gold strands. However, I am told by several people who ‘should’ know that there are only cosmetic reasons for the crossweave fabric, and that the two materials do not ‘work well’ in a crossweave layup due to different modulus of elasticity. Much better to use separate kevlar and carbon layers, they say. True? Why?



– Last Updated: Feb-03-08 5:43 PM EST –

Carbon and fiberglass are strongest in compression; they are stiff, resist deformation, and break when deformed past a point.

Kevlar and Twaron, both aramides, are strongest in tension; they deform easily but are very resistant to being pulled apart.

When engineering an stiff ultra lightweight laminate like a canoe or kayak, you may want to combine both characteristics. It works best with the compression layer on the outside and the tensile layers on the inside. When an object is struck, the compression layer resists deformation, but if that deformation occurs, the aramide is pulled around a greater radius because it is on the inside. It comes under tension and reinforces the compression layer sooner than if it were the outer layer.

Having repaired several sprint boats for a local dealer, I've found dual ply fabric has a tendency to "zipper" the carbon between Kevlar strands. This is not good. Crossing the weaves at 45 or 90 degrees helps somewhat, but half the carbon is where one doesn't want it - ob the inside where it will deform more and more likely break. Half the aramide is also where you don't want it - on the outside where it comes under less tension.

Carbon/Kevlar is fine looking fabric. So cool that Bell orders a 3/4 yellow Kev 49. 1/4 black Kev 29 to mimic the look.

Do you use ‘dual ply’ to mean the cross weave? What do you mean by zipper? What does that failure look like?

zipper definition
Yes, I meant dual ply to equate to C/K cross ply taffeta fabric.

A zipper would be a break of several inches of carbon strands between two strands of Kevlar.

Thanks, CE. I bought a little K/C weave
to try, but I don’t make boats. I only patch them.

inside outside
Wilson wrote: “It works best with the compression layer on the outside and the tensile layers on the inside. When an object is struck, the compression layer resists deformation, but if that deformation occurs, the aramide is pulled around a greater radius because it is on the inside. It comes under tension and reinforces the compression layer sooner than if it were the outer layer.”

I spoke with QCC and with Seda on this point. QCC puts the carbon on the inside and Seda puts it on the outside. Your logic makes sense to me but is it accepted in the industry.

Compression on inside; outside
The rationale of engineering laminates that way is pretty well accepted, excepting QCC. ??

Maybe they know something the rest of us don’t? Alternatively, maybe they don’t.

Inside/Outside = Little Difference
On solid construction boats like QCC and most other composite sea kayaks, there is not a significant difference in distance between the inside and outside layers. Under load the inside and outside layers deflect nearly the same amount. Therefeore any structural benefit of having Kevlar on the inside in negligible.

Since Kevlar has poor abrasion resistance, it makes more sense to put the carbon layer on the inside where it will better resist abrasion from paddling shoes. Most paddlers who spend the extra money for carbon also like to see it.

I personally prefer carbon/carbon or carbon/glass construction to make for a stiffer structure that is more likely to resist deflection that could cause damage to the carbon in the first place. If you are plan on hitting rocks hard on a regular basis then go with all Kevlar or HDPE.

However on cored boats, like most surfskis and K1’s, there is a significant distance between layers. This is why reputable bulders of cored structures put carbon on the outside and kevlar on the inside for their Carbon/Kevlar hybrids.

That’s on target envyabull
but I’d say that Kevlar has good abrasion resistance, even though it fuzzes and leads folk to beleive it doesn’t! Manufacturers of hard core RIB’s use it on the keels for beach landings etc. Motorcycle clothing utilizes Kevlar for because of this property.

As stated it is also very strong in tension. Boat laminates see dynamic forces, which as you know, mean the inner or outer layers will experience both compression and tension, albeit one force will dominate.

My issue with Kevlar over the years has been a tendancy for the material to break down inter-laminarly over time. It doesn’t like to stick to other layers as well as glass or carbon.

Just as Kevlar is over-sensationalized, I think carbon is mis-known as being fragile and brittle. This does not have to be the case. I also prefer glass / carbon matrixes with some core, depending on use.

Kevlar zealouts have, and will mis-interpret this as me hating Kevlar. Nothing could be more wrong. I’d use it in a kayak when it matched the use profile of said kayak, which would be along the lines of Epics ultra-race lay-up. That’s where it makes sense to me.

Kevlar Abrasion Resistance
My comments on Kevlar’s poor abrasion resistance are based upon on my experience with the material in a few applications.

My first experience with Kevlar was in racing sails on many of the sailboats I raced on in the late 80’s through the 90s. An older Kevlar sail would get fuzzy and abrade through. However, its competiveness as a sail (one season or less) was long dead before abrasion was an issue. Nothing would kill Kevlar faster than letting it flap in the breeze. The sail would shake itself to death.

I had a bag made from Kevlar that was all the rage in the sailing community. This bag was short lived as holes were wore through the material. To be fair this bag was often wrinkled and kevlar fibers break down pretty quickly if creased.

My first sea kayak was an all-Kevlar model with the exception of a light layer of glass used inside of the gel coat for abrasion resistance and to prevent print-through. In about 5 seasons my heels had worm completely through the Kevlar (2 - 3- layers) and the light glass layer was the only thing between me and the sea.

I think other products use Kevlar in their porducts for the marketing wow factor. It is very light for its strength so it does not hurt to add it to a product. Probably the biggest benefit it adds is to the marketing.

Absolutely agree.
I can see the sail situation for sure. It makes sense for a race sail as it’s light and strong in tension, but I would not expect long term durability compared to other materials. I can see it in a “cored” race kayak application where toughness to weight (kevlar) combined with core (stiffness) means a stiff, reasonably tough lightweight boat…NOT a long term durable boat!

It will resist abrasion better than other materials, but, in your case I believe the resin never saturated the material as well as it did the glass, and that was the difference.

Dupont spent a lot of $ pushing Kevlar, and it worked. Marketing has taught the consumer that it’s da stuff… It’s a superb material in the right application. Materials are only optimized when applied according to their properties.

We don’t see kevlar in paddles much anymore, masts, bike frames, fishing rods, yacht hulls, aircraft skins, etc. There’s a reason. Doesn’t mean it can’t add value to a kayak, or that kevlar kayaks are bad, just means it has to be used wisely, and with a purpose.

Kevlar and Sails
Aramid fibers, (Kevlar if bought from DuPont) is still used in sails, however quite differently than 20 years ago. Originally Kevlar was finely and uniformly woven just like dacron sail cloth or canvas before that. Today aramid fibers are applied as long larger diameter fibers glued in between sheets of Mylar. There can be relatively large gaps in between the fibers. The Kevlar fiber’s orientation is specifically set to be parallel with the load paths. UV resistance has always been an issue with aramid fibers and todays’s specialty fibers are coated with materials to provide better protection.

The latest and most expensive sails are made with graphite fibers (carbon). So despite aramid fiber’s excellent properties in tension, it seems carbon has become preferrable even in the tension heavy application of sails, when price is not an objection.

By the way, we all continue to use the word “Kevlar” to describe aramid fibers. Kevlar is a trademark name for aramid fibers like Band Aid is to adhesive bandages. Trademark protection lasts forever as long as the product continues to be marketed. DuPont’s patent on aramid fibers expired years ago. There are many aramid fiber alternatives to Kevlar that are less expensive and may have preferrable properties for the application. However, many companies continue to use “Kevlar” in their products because of its marketing factor. If it were just mechanical properties they were after, they could use any generic aramid fiber and save the consumer money.

Sailmakers have moved beyond the need to market or sell “Kevlar.” They use generic forms of aramid fibers in most of their sails. Some sailers will still refer to it as “Kevlar” but its not. Unlike sea kayakers, sailers will not pay a premium for “Kevlar.”

You don’t see many kayak manufacturer’s offering their kayaks in “aramid fiber” construction. They sell “Kevlar” construction because they can charge a premium by attaching DuPonts trademark to their product. If they used generic aramid fibers they might have to justify the use of this non-stiff material in a product that benefits form high stiffness to weight ratio.

i’d be quite content with a kayak made primarily out of s-glass and assorted materials but it looks like glass,no perceived premium.

I agree once again
You sound like my yachting pals, and my aerospace composite friends. The world of composites is very broad and complex. Kevlar does sell in this market, and you and I both know that most of the Kevlar kayaks are more glass than Kevlar, but folk like the yellow look, and are sold.

You also know that once in a matrix with cured resin, Kevlar 49 can shear and tear. Believe me, I’ve split surf boats, and torn ends completely off Kevlar kayaks. My experience has been far better with carbon, glass, cored laminates. Current surf kayak is a Vectran / Carbon co-weave with glass, and a thick foam cored bottom. It’s remained as stiff as day one and endured some abuse.

Again, I’ll say that some damn fine kayaks are made with some Kevlar in them, but it’s over sold, and I’d prefer the same boats in carbon / s glass.

The yacht, aerospace market is more sophisticated in regard to composite technology.