All articles on the Cherub website are written by amateurs and are intended as guides only, while building boats is an enjoyable hobby some of the chemicals involved are harmful and the appropriate safety precautions should be taken.

Composite construction is based on the idea of combining materials together to give final product that shares and exceeds the properties of the individual components. Having an understanding of how the individual materials behave will enable you to make better choices in material choice for repairing, modifying and building your Cherub.

Traditionally Cherubs were made from Plywood which is in itself a composite material and is actually a very good material for boat building; however with modern materials it is possible to create a lighter, stiffer structure using Fibre Reinforced Plastic. In FRP construction the main strength comes from the properties and orientation of the fibres, these are encased in a thermosetting resin that binds them together and provides a water resistant barrier. The fibre reinforcement is the component that takes most of the load and as such correct selection is critical, however if the fibres have not bonded to the resin the structure will not work.

The choice of resins for boat building is largely between Epoxy and Polyester. Both are generally two part where a hardener if added to a resin, this starts a chemical reaction which changes the resin from a liquid to a solid, hopefully bonding all the parts of your structure together in the process.

To get the best out of your laminate you need to choose the correct material in the correct style in the correct quantities for the loads that you can expect. This is the great advantage of composite construction in that you can reduce weight by correct selection of materials; however it does make the whole thing more difficult.

UD, woven, Bi axial etc

Woven fabrics are produced by the interlacing of warp (0°) fibres and weft (90°) fibres in a regular pattern or weave style. The fabric’s integrity is maintained by the mechanical interlocking of the fibres. Drape (the ability of a fabric to conform to a complex surface), surface smoothness and stability of a fabric are controlled primarily by the weave style.

Plain Each warp fibre passes alternately under and over each weft fibre. The fabric is symmetrical, with good stability and reasonable porosity. However, it is most difficult drape, and the high level of fibre crimp imparts relatively low mechanical properties compared with the other weave styles.

Twill One or more warp fibres alternately weave over and under two or more weft fibres in a regular pattern. One result of this is the appearance of a staggered diagonal in the laminate. The twill has improved wet out and drape than plain weave although it does fall apart more easily. With reduced crimp, the fabric also has a smoother surface and slightly higher mechanical properties.

Satin Satin weaves are twill weaves modified to produce fewer intersections of warp and weft each fibre will typically cross 4, 5 before passing under one. Satin weaves are very flat, have good wet out and a high degree of drape. The low crimp gives good mechanical properties. However they do tend to fall apart easily.

Basket Basket weave is the same as plain weave except that pairs of warp fibres alternately interlace with pairs of weft fibres. It can also be done with more two pairs of fibres; this pattern has less crimp than plain weave and is often used on heavy fabrics.

Non woven fabrics are typically held together with a binder material or stitched together with a thin thread. This means that the fibres can have minimum or no crimp. This improves the mechanical properties of the laminate particularly improving the stiffness.

All the non woven fabrics are variations on a theme, with Unidirectional having fibres running in only one direction bi-axial having fibres in two directions and multi-axial having many directions.

Chopped Strand Another type of non woven cloth is chopped strand mat. This consists of short lengths of fibre randomly distributed and held together with a binding agent. Due to the short fibre lengths chopped stand has very poor mechanical properties and the binding agent is only designed to dissolve in polyester so if you use Epoxy you will get a very poor bond onto a weak structure.

The Orientation of fibres in a composite has a large impact on the mechanical properties of the laminate structure. If you were to take a length of unidirectional cloth and laminate it so that all the fibres lay in one direction (0 degrees) then the fibres would contribute fully to the stiffness only in the 0 degrees direction, at 90 degrees they would do nothing the only strength would come from the resin matrix. This means to get to get the best out of a composite the fibers

There are three main materials used as reinforcement fibres in Cherubs

• Glass
• Kevlar® (Aramid)
• Carbon

The following is a table comparing the relative properties of reinforcing fabrics.

Glass fibre is the cheapest of the reinforcing materials and is the most widely used material in general boat building

This was one of the first high strength synthetic fibres to gain acceptance in boat building, Kevlar® (Aramid) has a considerably higher tensile strength and modulus than fibreglass. One disadvantage of using aramid is that when sanded it tends to become fluffy, therefore if there is any requirement to sand after cure a layer of fibreglass should be used over the top.

Graphite fibres contain up to 95% carbon and yield the highest tensile strength currently available. These fibres woven together form carbon fabric. These fabrics offer higher strength and stiffness-to-weight ratios than any other commonly available reinforcements. The carbon fabric comes in several different quality grades this is known as the modulus.

One of the main advances in FRP construction was in the use of a low density foam core between the outer skins of FRP In a sandwich panel, the core functions like the connecting web of an I-beam, separating the face skins at a constant distance, while the skins themselves function as the I-beam flanges. The sandwich panel's bending stiffness is proportional to the core thickness, in the same way an I-beam is stiffer as the distance between the flanges increases. Doubling the core thickness yields a panel six times stronger and 12 times stiffer.

Foam core helps distribute loads and stresses on the skins, which improves impact resistance compared to a honeycomb. A core's compression strength prevents the thin skins from wrinkling failure, while its shear strength keeps the skins from sliding independent of each other when the panel flexes.

Follow up reading Links http://www.fiberglast.com http://www3.gurit.com/marine/index.php