Nick......I'm thinking of building a kayak using the stitch and glue method. But instead of using marine ply I would like to use DIY fiberglass sheet. The idea is to use 3/4 ply covered with Formica then waxed. The ply would be first cut to the pattern. The deck would be strip planked.
What are the advantages/disadvantages of the different cloths use use (Glass/Carbon/Kevlar/Carbon-Kevlar)? Why would you use one over the other in any given location? Why not use the Carbon-Kevlar for the whole inside for instance, instead of switching back to the glass for the deck?
Stiff is subjective considering weave patterns and the weight of the clothes. All things being equal stiffness wise Carbon > S Glass> Kevlar. The ability to flex MORE makes kevlar and glass more suitable for repeated impact (such as boats hitting wake/waves). Bar none all of these clothes come to life when epoxy is added to them. Use a high shore hardness epoxy and they'll be stiff. Use a low shore hardness epoxy and they'll flex more. Kevlar will handle more damage than any of them....its ability to take ripping style damage makes it great for thin layups such as kayaks/canoes.
I know carbon is brittle and less abrasion resistant than kevlar, but I wouldn't think that would matter on the inside of the kayak. Amy I wrong? Why use kevlar on the inside?
Hey slingshot, now see I heard just the opposite somewhere.that Kevlar was supposed to be real stiff. Not saying your wrong but I am saying that there are more than one opinion about when and where to use Kevlar. Frankly I don't know why that is. Take a piece of kevlar carbon and glass, Epoxy all the same and check for modulus of elasticity (stiffness) whats the big deal?
Carbon is much, much stronger for a given cross section. So, you can either reduce the weight of your cloth for the same strength of glass, or keep it the same and have a much stronger hull. Remember, most of the weight of a laminate is the resin, so if you can use a thinner cloth for the same strength, you make a lighter structure.
+Nick Schade Thank you for the reply. Does the over lap of mat also create a structural element as well, somewhat like a partial bulkhead or is the result negligible?
+Nick Schade "newbie" That sounds logical. Thank you for responding and sharing your knowledge. You've been very helpful. BTW: You have a great website.
AKWintermute Carbon is lightest, so you want to use it where you can, but it is not as abrasion resistant as kevlar, and degrades when exposed to UV. So kevlar or carbon+kevlar is used on hard wear surfaces. Areas that need extra abrasion resistance like the keel and gunwales are often reinforced with straight kevlar. Fibreglass is lighter than carbon+kevlar and much cheaper, but itsn't as strong, and so can be used for low stress surfaces as an alternative to expensive carbon.
The polyester or nylon fabric used on Skin on Frame boats works differently from fiberglass and carbon/Kevlar. It is not appropriate for this application.
Kevlar is pretty pointless on the inside of a hull. It's strength is in tension, the inside of a hull is in compression most of the time. Plus, you could've used Kevlar's abrasion resistance to better effect on the outside of a hull.
While "most of the time" the kevlar may be in compression, when you hit a rock it is in tension, that is when you care. The boat will be finished bright, so you can see the wood. Kevlar is not transparent. For this application, it would not work on the exterior.
I'm glad we have your expertise to set us youtubers straight. The data and cogent arguments you brought to the subject of kayak failure modes are most enlightening. After 30 years of using and building kayaks, I am consistently amazed at how much I can learn from one pithy line from an expert such as yourself.
Nick Schade I'm guessing you've used expert in the traditional sense for this case remembering the origin of the word. "Ex" as in has been and "spurt" as in drip of water under pressure. Appropriate for this "expert". A has-been drip. ;-)
Using three different materials is a pointless if not stupid waste of time. Never mix materials with a different Tensile strength and fail point as the weakest ie the most rigid will fail under load first the pas the load to the next and that fails overloaded the third which will catastrophically fail as the item is destroyed. One material only and throughout. Get proper advice on the various options efficient of those materials, do not use just because carbon fibre and Kevlar look and sound flash and cost a lot. Kevlar is abrasion resistant Carbon is rigid, E glass is strong and flexible what a waste of money mixing them only for it to fail under load, maybe injuring or killing somebody. Don t believe me look it up.
Thank you for your comment. In the process of building these kayaks there are a lot of different decisions the need to be made. While strength is obviously a concern, there are tradeoffs with weight and appearance that must also be considered. As a practical matter, these kayaks are built with a huge safety margin. In order for the weakest link to fail, the kayak has to be exposed to loads and forces way beyond what most paddlers would ever risk. The situations where these kayaks are at risk of catastrophic failure are dangerous to the paddler regardless of the strength of the kayak. I.e. large waves crashing into hard rocks. In this layup we are not using just fiberglass, Kevlar and Carbon, it is all in a stack with wood in the middle, the fiberglass at the opposite end of the stack from the Carbon/Kevlar. The failure mode for the layup is one sided: a hard/sharp object impacting from outside the kayak. In this scenario, the fact that glass and CF have different tensile strengths is not that relevant, as only the CF is in tension, the glass is hard against the rock, in compression. In this layup, the glass adds waterproofing, abrasion protections, surface hardening and tensile strength while being transparent. The wood adds thickness for stiffness as well as its own tensile and compression strength while being light weight and beautiful. Of course the wood has different properties depending on grain orientation. The carbon fiber adds cross grain stiffness. The cloth being used here has the CF running across the width of the boat where it provides the most reinforcement to the wood grain. While the wood needs little reinforcement along the grain, the Kevlar serves to add toughness. If everything else should fail, the Kevlar can hold the bits together while you paddle to safety. Given the strength, weight and aesthetic tradeoffs involved I think this layup works efficiently. My practical experience through 30 years of using these boats hard, bares out that conclusion. While there is certainly room for improvement, there are a lot of tradeoffs involved, and it is not as simple as "don't mix materials with different tensile strengths". If you are interested in more information about the strength of sheathed wood strip construction, please see my pages where I put various layups in an Instron machine: www.guillemot-kayaks.com/Building/Testing/index.html
If it is such a big problem to mix different fibres then why is that the most expensive motorcycle helmets is fabricated with a combination of the three fibres? I need to replace my boats transom and was thinking of using 4 pieces of 9mm marine ply and insert a cloth of kevlar, carbon and fibreglass between them and bond it as a unit to get the best properties of all three in one assembly. But i`m not sure about my plan hence my research on RU-vid before i start using the material. should i just use the fibreglass and go for two 21mm bonded marine ply instead (already got all the materials)
Composite design is not as simple as Mr. Dawes would suggest. There are valid reasons to mix various materials together because each material contributes something different to the final product. You want to be fairly knowledgeable about both the loading and the potential failure modes of what you are building. Since you would be going with a sheathed wood core, the interior and exterior will react in different ways to various loading scenarios. You want to design your layup to account for how your boat will be used.
I would encourage Graham to look at how recurve bow limbs are made. Materials with different properties deliberately combined to create a laminate that is intended to be repeatedly subjected to extreme loads and shocks. It's a whole industry built around mixing materials in a composite lol.
