Seattle Yachts New-Build Manager, Phil Friedman, continues to bust marine industry myths...
Popular marine industry myth has it that adding a layer or two of Aramid (Kevlar) fibers to a laminate turn it into a super-material, able to sustain with impunity collisions at sea with timber deadheads, steel containers, and other potentially dangerous flotsam.
But, in fact, there are several reasons for not including Aramid in a glass-fiber reinforcement stack, particularly when it comes to building trawler yachts and other vessels intended for cruising. These include:
1) Aramid fabric is relatively expensive, not only as a material but also in terms of the modifications to application procedures required by its use.
2) Aramid fabric does not wet out well with polyester resin and only slightly better with vinyl ester resin. Aramid fibers generally require the use of an appropriate epoxy resin matrix — if the benefits of its mechanical properties are to be fully realized in the resulting laminate. And epoxy-based resins are many times more expensive than polyesters and vinyl esters, particularly when you consider how much more difficult they are to use when molding parts in female tooling.
3) In almost every case, just as much or more can be accomplished in terms of adding strength to a hull against potential collision by properly engineering and "beefing up" a conventional E-glass or S-glass reinforced polyester or vinyl ester laminate.
The single major advantage potentially offered by including Aramid fibers in an FRP laminate is increased strength per unit weight. However, even using the best techniques, the gain is not likely to be great enough to make a measurable difference in other than ultra-high-performance vessels — which cruising yachts are not.
The few extra pounds per unit strength required to achieve the same results in an all-glass laminate (versus a glass and Aramid hybrid) are of little concern in a cruising yacht. The bottom line is that the inclusion of Aramid in a primarily FRP laminate is usually a marketing-driven gambit, intended to create the impression of a “bullet-proof”, highly collision-resistance hull.
The myths that surround the use of Aramid fibers in yacht construction result from a misunderstanding of the mechanical properties of Aramid (Kevlar). Aramid fibers are indeed more "elastic" than either E-glass or S-glass fibers. That is, they have a higher "Young's modulus". But contrary to popular misconception, that does not mean Aramid fibers are “stretchier”.
The higher Young’s Modulus of Aramid fibers means that Aramid fibers can sustain higher loads before being pushed past their elastic limits of deformation (the range in which they will return to their original shape when the load is removed). In other words, Aramids will sustain higher loadings before being pushed into their plastic range of deformation (the range in which deformation becomes permanent). The abbreviated charts included here for comparative tensile strength, Young's modulus, and strength-to-weight ratio illustrate what I'm saying.
Moreover, all-fabric stacks of Aramid fibers, such as those used in soft ballistic armor, will absorb tremendous loads and energy by deforming and not rupturing. However, when Aramid fabric reinforcing is set into a polymer resin matrix with glass fiber reinforcing, the behavior of the composite laminate is not strictly comparable to what transpires in soft ballistic armor.
If you’re looking to improve collision-resistance in a laminated cruising yachts, rather than incur the significant added cost of incorporating Aramid fabrics — with only vague specification of the anticipated gains to be achieved — the better option is to properly engineer and beef-up the non-exotic glass-fiber laminate. Coupled with anti-collision design features such as incorporating a watertight “crush” compartment at the forward end of the waterline, just abaft the vessel’s stem and forefoot, results will be much more cost-effective and predictable. — Phil
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