If the two identical canoes weigh the same, what is the advantage of graphite over kevlar ?
cheers,
JackL
Carbon and Kevlar
Carbon is very strong in compression - it is stiff; the laminate resists oil-canning but is notch sensitive and tends to shatter when overstressed.
Kevlar is very strong in tension. It is soft; deflecting/ oil-canning easily it almost never breaks when stressed. Kev can be stiffened with foam cores.
Some of us find that combining the two materials yields the best laminate.
You wouldn’t want to build a boat
solely out of either material. Kevlar is super tough but “squidgy.” You’d have to use more than you’d expect to get a boat that is stiff enough, and then you might have a heavier boat than you wanted.
Carbon in resin is super stiff, and strong until it fails, when it tends to fail catastrophically. A pure carbon boat can be very light and very stiff, but it can’t take a hit as well as boats combining different materials.
If you know how to take care of the outside of a boat, and protect it from unnecessary dragging, then carbon outside and Kevlar inside can make for a boat that is light and tough.
In my whitewater activities, outside dragging and gouging are unavoidable. I prefer to have S-glass outside and Kevlar inside. S-glass is much heavier than carbon for the same degree of stiffening, but it is also a little tougher, and it is hard… hard enough to stand up well to dragging.
There are Kevlar/carbon weaves, but I would not want any Kevlar on the outside of the boat regardless. There is an S-glass/carbon weave available, but I don’t know why a builder would want to use that. Maybe CE knows.
Bi-weaves
I’ve always felt that bi-weaves made no sense in hull design.
Not only does it tend to zipper - carbon breaks between more pliable kev cross weave, but the engineering is wrong.
As we both mentioned, there is some benefit to a strong compression layer on the outside, carbon or S glass, and a tensile layer on the inside.
There are interesting bias weave laminates going together in larger watercraft, but the weight, costs and potential bias flaws in thin paddlecraft laminates limit/eliminate use.
Example: a four layer laminate; first layer S glass longitudinal w/ keel line, second layer carbon,pieced in at +45 degrees, third layer, carbon, pieced in at -45 degrees and fourth layer Kevlar at 0 degrees. 2nd and 3rd layers need be same fabric and we have all those overlapping seams that may collect air / peroxide bubbles and leave weakened voids. [Infusion could solve that.]
Further the hull is 4 layers thick and heavy without partials to reinforce bottom and stems and seat placements.
That Nelo? fabric, with carbon and Kevlar running in both directions looks cool, but I still do not see any benefit to having Kev on the outside of a hull, except when building a lightweight hull to a price point, using foam to stiffen that lightweight outer layer.
the black one is faster
my Nelo
Solid carbon outer, solid kevlar inner, vacc’d with no core. They’ve gotten away from the bi-weave in their WWR layup. It is a very strong construction for the weight (k1s in this layup are running 9-10.5kg). It isn’t as stiff as their carbon-core-carbon hulls but it is very impact resistant for the weight. I’d say stiffness on these hulls is comparable to a glass hull of the same dimensions at about 15kg and you to tend to just bounce off of stuff (logs, sandbars) but I wouldn’t want to challenge sharp objects since the layup is still pretty thin.
I’ve no love for all kevlar boats. I’ve seen stiff kevlar boats built using honeycomb but everytime you look at the boat wrong you smush the core. Eventually you end up with a limp noodle or a boat with lots of dimples.