Strong, stiff and light; the why and how of high-tech composite materials.
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Strong, stiff and light; the why and how of high-tech composite materials. |
Here at Shaggy Designs we are interested in making the best possible boat, and that includes not only the shape, but also the materials that go into making the boat. We make all of our boats out of high tech composite materials rather than plastic because we think that it offers lots of advantages. To learn a bit more about composite materials, read on...
Composites are materials made up of two or more components combined so that the strength of the composite is higher than the properties of the individual components. Our canoes and kayaks are made of a reinforced plastic composite. This consists of fibre reinforcements embedded in a resin matrix. The resin used is thermosetting type, which is a plastic that cures from a liquid to a solid through a chemical reaction of its two components. Once this reaction occurs, the material can not be reformed.
Even when just adding glassfibre reinforcement, the tensile strength of the resulting composite is over 4 times higher than the tensile strength of the pure resin. The composite also becomes much more resistant to impact damage. The high strength achievable for lightweight construction is why fiberglass composites are used for almost all racing canoes and kayaks. Composite parts can also be built to withstand stresses in a particular direction by aligning the fibre reinforcements along that direction, and by adding extra fibres only in high stress areas. This saves additional weight by removing unnecessary material from areas with little stress.
Fabric selection is critical when building composite vessels. Select the wrong fabrics, and your boat may be too brittle, or heavier than it needs to be. There are many different materials available, with a variety of properties. Each material is also available in different weave patterns, and fabric weights. These fabrics are often combined together, with layers of different fabric types making up the finished laminate. Most of the fabrics described are available from us in small quantities; please contact us for more details.
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Glass was the first fibre reinforcement available, and has been used successfully for many years. It is relatively lightweight when woven, has moderate tensile and compressive strength, is tolerant of both damage and cyclical loading, and is easy to handle and machine. |
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Chopped Strand Mat (CSM) is the cheapest type of reinforcement available, and so is used in many applications where light weight is not important. It consists of lots of short (~50 mm) glass fibres randomly laid down and bonded together with starch. It absorbs between 2 and 4 times as much resin as woven glass, and so the finished composite is much more brittle. The short fibre length means that CSM is not a very strong material. It has been used to make cheap kayaks in the past which is partially responsible for the bad reputation of composite boats amongst kayakers. CSM is only compatible with polyester resin. |
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Carbon fibre is a modern reinforcement that is extremely light weight, strong, and stiff. Carbon fiber is also the most expensive of the reinforcing fibers. This fact often limits its use to parts needing selective reinforcement or high stiffness with the least weight. Parts made with carbon fibre as the only reinforcement fibre can be brittle, and susceptible to impact damage. |
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KevlarTM, the most well known aramid fiber is a type of nylon. KevlarTM is the lightest of any fiber reinforcement, has high tensile strength for its weight, and is very tough. It is slightly less expensive than carbon fibre. KevlarTM is puncture and abrasion resistant, making it the best choice for hull rails and sharp seams. It also has poor compressive properties. KevlarTM is naturally yellow, but can be pigmented during manufacture to most other colours including black. |
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DiolenTM is another type of plastic reinforcement. The properties of Diolen are only slightly inferior to those of KevlarTM. The main advantage of Diolen is that it is easy to work with, and much cheaper than KevlarTM. Diolen is only widely available in Europe; nobody seems to have heard of it in North America! |
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It is possible to combine KevlarTM with other materials (usually carbon) to create a hybrid laminate. This will utilise the properties of both materials, such as the high tensile strength of carbon, and the abrasion resistance of KevlarTM, and compensate for any shortcomings such as the brittleness of carbon. |
Fabric is just one part of the finished composite; it has to be bonded together with a suitable resin. Again, the wrong type of resin can have disastrous consequences - some types absorb water! The two most common types of resin are Polyester, and Epoxy. Polyester resin is the most widely used, since it is less expensive and easier to use. However, it is much weaker than epoxy, and can be excessively brittle. Polyester resin also has a tendency to shrink slightly when curing. All of the available car bodywork repair kits contain a type of polyester resin. This resin is designed to be easily sanded, and so is too weak to be used for boat repairs.
Epoxy resins are higher performance and more expensive than polyester. They are used in weight critical, high strength, and dimensionally accurate applications. The properties of epoxy resin from various manufacturers can differ a great deal - some are more flexible than others, or take longer to set, etc. To make the best possible boat requires a cured resin that is rigid without being too brittle. Not all fabrics are compatible with both resins - some types of KevlarTM are coated so as to adhere better to eopxy resin, while polyester must be used with CSM to dissolve the starch binder.
So, once the choices of fibre and resin type have been made, these materials have to be combined and crafted into the shape of the boat. Composite boats are made in two parts; deck and hull, and then joined together. Each part is made in a female mold, also made of composite materials. The inside surface of the molds are specially treated so that the parts to be made do not bond to the mold. Precut fabric reinforcement is placed one layer at a time into the mold and saturated with resin. When the part has achieved the desired thickness and orientation, it is left to cure. When it is demolded, it will have the exact shape of the mold surface, and the mold can be re-used. The join at the seam is made with layers of KevlarTM seamtape on the inside and outside, and fittings such as cockpit rim, seat, foam pillars, etc. are installed.
Still with us? If so, then hopefully you will have learnt something about the materials that we use. We choose composite over plastic to bring you the lightest, stiffest boats that we can. Learn a bit more about how we design and test our boats on the design page.
For more information; contact us.
