Hull Construction – Twaron, Kevlar, Chopped Mat, Roving, Vinylester, Epoxy Resin, Airex, Balsa

Fiberglass is made of fibers composed into fabrics then layered with resins. Between layers of fiberglass, sometimes builders use a core material.

Fiberglass Fabrics
Fiberglass Fabrics

E-glass and S-glass are the common fibers. Some production builders use E-glass which is 5-10 times less expensive than S-glass. Others use S-glass which is 20-40% stronger than E-glass. Even other builders combine the S and E glasses. Kevlar is a higher quality glass used by the best manufacturers. Hylas uses a variant of Kevlar called Twaron which makes Hylases “bullet-proof” we boast. Twaron is 2 times as strong as S-glass and 60% lighter than normal fiberglass. It is 5 times as strong as steel. While Kevlar has great tensile strength, many times builders combine Kevlar with S-glass because Kevlar alone has inferior compressive strength. Carbon fiber is another fiber but rarely used in layup because of poor impact strength. Spars are composed of carbon fiber.

Chopped mat is one type of fiberglass fabric composed of short strands chopped and bound in random orientation. This randomness makes the fabric water resistant. Because of its water resistance, builders use a layer of chopped mat under the gel coat. Woven roving consists of alternating strands weaved at 90 degree angles. This roving has superior tensile strength especially between the warp and weft strands. Layers of woven roving do not bond well to each other and should have a layer of chopped mat in between. The roving quickly builds up hull thickness. A final fabric is unidirectional roving. Unidirectional roving consists of fiberglass strands all going the same direction lightly stitched together athwart. Unidirectional roving is the strong along the strands but has little strength at 90 degrees. Builders use this type for stringers, beams, and frames. Biaxial roving consists of 2 layers of alternating, 90 degree crossed unidirectional roving layers. Tri-axial roving consists of 3 layers all at different angles. These axial rovings are the strongest yet harder to handle and spread with resin.

Polyester resin is the least expensive. It should be isophtalic not orthophtalic. Builders spray the polyester resin into female molds for the gel coat. Polyester pleases cosmetically, is scratch resistant, UV protective, has impact strength, and retains color well. Vinylester resin is a higher quality resin for layup although many builders use polyester. Queen Long for Hylas uses vinylester. Vinylester shrinks less during curing for good inter laminate bonds. When owners do blister jobs say on the infamous Valiant 40 blister boats, they use vinylester resin because it prevents blisters. Finally, epoxy is the finest in quality. It is water resistant, shrinks little, but is costly, toxic, and difficult to work with. Henry Mustin, a local surveyor notes how epoxy is a bad idea for the gelcoat as epoxy is less UV resistant than polyester.

Cored fiberglass improves the stiffness, lessens the weight, and reduces the cost. This improves durability and performance. Balsa core is the easiest to work, least expensive, and has excellent stiffness and compressive strength. C&C discovered the benefits of balsa in the 1960’s. Balsa is a less efficient weight saver and more vulnerable to water saturation. Henry Mustin, a local surveyor, says balsa should not be used to core a hull underbody and “if this sounds like a condemnation of balsa below the waterline, it probably is!” Airex or rigid non-cross linked PVC foam is more expensive but more water resistant. Cross-linked PVC foam is stiffer but more brittle than Airex. Finally, honeycomb cores of aluminum or aramid paper are the lightest and strongest of all but cost much more than balsa or foam. Honeycomb coring is rare except on custom racing sleds. Hylas has used either balsa or Airex cored decks with solid fiberglass hulls.

Fiberglass consists of fibers bound into fabrics glued together in layers by resins. Core material is in between two fiberglass sheets. Example fibers include S-glass, E-glass, Twaron, and Kevlar. Example fabrics include chopped mat, woven roving, unidirectional roving, and associated bi and tri-axial rovings. Resins include polyester, vinylester, and epoxy. Cores include balsa, Airex, and honeycomb. These basic decisions by the builder and the skill in executing them are critical to a hull’s soundness. As one client noted to me, “When you are buying a boat you are really buying three things: the workmanship, the materials, and the design. And you need to have all three of these for a good boat.”