Contrary to popular marine industry myth, adding a layer or two of Aramid fibers (e.g. Kevlar) to a laminate does not turn it into a super-material, able to sustain with impunity collisions at sea with steel containers and other potentially dangerous flotsam.
I am not personally a big fan of including Aramid in the laminates of purpose-built cruising vessels because:
1) Aramid is relatively expensive,
2) It does not wet out well with polyester resin and only slightly better with vinylester resin and, indeed, generally requires the use of an appropriate epoxy resin matrix if it is to develop its full mechanical properties in the laminate, and
3) More can be done in terms of adding strength to a hull against the potential collision with flotsam at sea by properly engineering and “beefing up” a conventional S-glass or even E-glass reinforced polyester or vinylester laminate.
The only advantage that Aramid (Kevlar) potentially offers is increased strength per unit weight. However, the difference is not great enough to be significant in anything but ultra-high-performance vessels, which cruising yachts are not. And the few extra pounds per unit strength required to achieve the same results in all-glass vs a glass and Aramid hybrid are of little concern in a cruising yacht. The bottom line is that the inclusion of Aramid in the laminate is a purely marketing gambit, generally intended to give the impression of a serious addition to collision-resistance.
Moreover, most people misunderstand the mechanical properties of Aramid (Kevlar). It is more “elastic” than either E-glass or S-glass fiber (namely, it has a higher “Young’s modulus”). But contrary to popular misconception, that does not mean it is stretchier. What it means is that it can sustain higher loads before it is pushed out of its elastic range of deformation (the range in which it will return to its original shape when the load is removed) into its plastic range of deformation (the range in which deformation is permanent).
SEE ALSO
Mythbuster No. 1: Engines With Higher HP Ratings Burn More Fuel.
Mythbuster No. 2: Infused Hulls Are Stronger.
Mythbuster No. 3: Oversized Generators Don’t Run Better or Last Longer.
This means that in all-fabric layups such as soft ballistic armor, it will absorb tremendous loads and energy by deforming and not rupturing. However, when set into a laminate composed of a polymer resin matrix and fabric reinforcing, the behavior of the composite laminate is not strictly comparable. A few minutes perusing the charts below for comparative tensile strength, Young’s modulus, and strength-to-weight ratio clearly illustrate what I’m saying.
I’m happy to include Kevlar in a laminate if a customer absolutely wants it, but I cannot in good professional conscience say I believe in it. And I personally much prefer to properly beef-up the glass laminate and employ design features such as a watertight collision compartment at the forward end of the waterline (extending back a few feet from the stem) than do the “Kevlar marketing jig,” as I am much more confident that the results will be predictable and ultimately more to my client’s or customer’s best interests.
