This explanation is in no way meant to be a tutorial. It is a very brief explanation for boat buyers. If you have a deeper interest in composites as they apply to whitewater craft read Charlie Walbridge's "The Boat Builders Manual"
If you are searching the web for composite beta look in the links section for web resources that relate to composites and saftey. There is also the newsgroup rec.boats.building. Topics on this group tend to relate more to ocean craft though and tend to be very technical.
There are three basic types of resin.
Polyester Resin
is the workhorse resin. It is the cheapest and most readily available of the three. There are 2 types: stiff and flexi. A blend of the two is what is usually used.Advantages: cost, availability. Tends to be stiff making it an ideal resin for decks.
Disadvantages: does not bond well with Aramids (Kevlar) and synthetics (polyester). Under impact separating of the layers of the laminate is much more likely than with other resins.
Vinyl Ester Resin.
A oversimplified explanation would be to say that it is a cross between Polyester and Epoxy. This is the most common resin for tough cost effective boat building.Advantages: Very good strength for the cost. Its elongation properties exceed Epoxy making it an ideal resin for whitewater boats. High elongation values disperse an impact across a greater area reducing the damage.
EpoxyResin
. Usually considered the strongest of the resins. Due to its cost and difficulty of working with it is typically only seen in high end slalom boats.Advantages: very tough under abrasion and applications where the highest strength is needed in the least number of layers.
Disadvantages: material cost and difficulty of use make it a very expensive resin to work with. It is a stiff resin and will not disperse the energy of an impact across a greater area.
Cloths
The physical properties of composites are fiber dominant. Even the new high tech fibers as a stand alone fabric are rarely ideal just on their own. A properly built boat will combine different fabrics in different places to accentuate the positive aspects of the fiber. The cost must also be taken into consideration. A boat with several layers of cofab cloth would be extremely strong for its weight but the cost would fall outside the realm of all but the most intense aerospace application.
It is also important to know that "stiff" and "strong" are two different things. The stiffer something is, the less it will spread the energy of an impact into the surrounding area. The stronger something is, the more it can absorb the energy of an impact OR dissipate it into the surrounding area by flexing (without tearing fibers).
Glass.
Glass and E Glass are the same thing. Glass is a stiff relatively inexpensive cloth. Due to its stiffness it has poor elongation properties. S Glass is a similar material but with 15%-30% better numbers for sheer strength and other values. S glass is only used where stiffness and weight are bigger issues than cost (race boats)Polyester/ Cap
Poly/cap is a synthetic cloth. It is often called the poor man's Kevlar. It has very good elongation, abrasion, and sheer properties. Due to its high elongation properties it tends to be very flexible.You may now have begun to understand what is meant by a Glass/poly laminate. When these materials are laminated in alternating layers their strengths complement each other for a tough cost effective combination that makes it the ideal lay-up for whitewater boats.
Kevlar.
There is no shortage of hype about this stuff. How many times have we all heard "They use it in bullet proof vests". Kevlar has a very high sheer strength and high elongation properties. It has low resin absorption making it 30% lighter per layer that it is used in a boat. It is ideal as an expansion fabric only and has poor compression properties. This means that it is good on the inside of a laminate but does very little if it is on the outside other than abrasion. Kevlar has very high abrasion resistance making it a good choice for outside seams. Kevlar's disadvantages are that it is expensive, has poor compression qualities, and is not UV stable.Carbon
. Graphite fibers contain up to 95% carbon. These fibers woven together form what we call Graphite or Carbon fabric. This material has the highest strength and stiffness to weight ratio of any cloth. It also has the highest tensile strength of any cloth. Its disadvantages are that it can be brittle under sudden impact due to its stiffness. It can also be hard to find at any price. Typical availability in the paddlesport market is only from aerospace overruns and blems. Market price is typically in the neighborhood of $50 per yard.Cofab.
Cofab refers to a weave that is a combination of different fabrics. Typically this is a weave of Carbon and Kevlar cloths but can also mean any hybrid like S Glass, and Carbon weaves or combinations of the three fabrics.Dynel
Dynel has extremely good abrasion resistance. This is why it is commonly used for paddle tips. It's main disadvantage is its cloth to resin ratio. The cloth holds a lot of resin making it heavy. Its tensile strength is low making it impractical for a layer in a boat. This cloth however makes a super tough covering for outside seams and paddle tips.