Engineer and composites expert Rick Strand is president at Impact Matrix Systems, Hampstead, New Hampshire. He has worked for Boston Whaler, ITW Plexus and TPI Composites abd consulted for Baja, Sea Ray and others.
We take it that you know the basics, such as fiber provides the strength and resin provides the stiffness in a fiberglass laminate. And that composite is just a fancy term for a structure made from a combination of materials (hey, straw and clay bricks are “composite”). But can you sift through the rest of the jabber at a boat show?
The materials used to construct boats are varied in cost, application and how they perform. This primer is intended to help you sort the truth from the marketing mush and sales hype.
Most boat hulls are made using either knitted or woven fabric created from glass fibers. In some fabrics, the fibers crisscross at 0- and 90-degree orientations (0/90). Other axial configurations include plus and minus 45 degrees and either 0 degrees or 90 degrees. The directionality of the fibers enables the engineer to orient them in the direction of the stresses applied to the hull at any location. So, the use of “0” or “+/- 45-degree” fiber doesn’t make a boat better; it simply designates the application.
Woven fabrics require simpler machinery to produce and are hence less expensive than knitted fabric reinforcements. With knits, fibers are laid flat on a surface (by machine) and are knitted through with a lightweight and tenacious fiber to hold them together. The fibers in knitted fabric lie almost absolutely flat.
Woven fabrics’ fiber bundles run over and under each other creating a bulkier material. As a result, there are several things to consider about a boat using woven fabric. First, as the hull ages in the sun, it can develop “print-through.” This condition manifests itself by visibly transmitting the fiber so that the finish is no longer smooth but appears slightly distorted in a crisscross pattern. This occurs because the resins in the hull composite shrink after sitting in the hot sun. This condition is seen in darker-color hulls (green, blue and black) more often because they can reach temperatures of more than 200 degrees F in the direct summer sun. White hulls may get to only 160 F because they reflect more of the sun’s rays. In addition, the dark, shiny colors show the condition more than a light color does because of the way they reflect light. Knitted fibers lie flat, have lower shrinkage stresses and will not produce the print-through condition.
From a structural standpoint, builders that utilize knitted fabric as their primary reinforcement will make hulls, decks and parts that are stronger and stiffer (given proper engineering) than those built with woven-fabric composites. The parts will also be lighter. For the same reason that columns supporting buildings are straight and not wavy, knitted fibers more effectively line up in the direction of applied forces while woven fibers line up like springs. They have less strength and stiffness when they are “off-axis” to the forces. In addition, woven fibers have wells of empty space between the weave that need to be filled with resin to produce the composite. Knitted fabrics have less of this space. Therefore, woven fabrics require more resin to wet out in production. The result is heavier, more structurally inefficient composites.
Finally, woven fabrics will last a shorter period of time before the composites break down and will fatigue from repeated stress (pounding and slamming) on the out-of-plane fibers (springs) formed by the weave. At the end of the day, a boat utilizing knitted fabrics and an equivalent structural design would last longer and experience less cracking. This is particularly useful in deck composites and internal components having odd or transitional shapes.