Kevlar Comments

I got this note from someone reading a posting on Steamradio.

“Interestingly Shuttleworth recently posted on Steam radio re. Kevlar as follows:”
“We put Kevlar inside. It is partly because it is significantly better in tension than compression, but also when water penetrates the resin fibre matrix it acts like a wick and the water will migrate into the laminate – not something you want in a material that is very close to the water all the time. Small areas of damage that are not attended to can make this a problem. The third reason is that in an impact the outer skin will rupture – the foam will distort It is is either Airex R63.80 or Corecell A500 and the inside skin will stay intact even if the resin starts to fracture. The idea is that the foam and outer skin absorb a lot of the energy before it gets to the inner skin – which is ultra strong in tension. this combination has proved very successful in a number of cases of grounding or high impact on the hull.

Kevlar does need to be laid in a combination with glass, because the resin does not stick to the fibres very well. For the bigger boats we use a unidirectional fabric with alternate tows of kevlar and glass 50:50 by volume. This means that each layer is 660 gms/m2 Aramid/Glass – 237 gms/m2 Aramid, 422 Glass Unidirectional.”


John Shuttleworth
for Shuttleworth Design Ltd.

At first I thought, doesn’t he know?  So many things wrong as I see it. Those all seemed like reasons to never use Kevlar. A grounding or impact is usually bow or keel line. I cannot see tension strength helping against basically shear load.
The multihull will not sink so holing is not the worst of worlds. The disadvantage of tensile strength across in interior panel is that a small impact can make a huge delamination inside. Especially since the Kevlar is not bonding to anything else.
The Kevlar in combination with glass seemed to be a bad solution for a couple of reasons. Even if the glass is a good laminate, the poorly bonded Kevlar between glass lams means very poor shear transfer between layers. So why bother with the Kevlar at all?
Finally if the Kevlar is for impact, a woven fabric is much better than a uni or a knitted fabric.
Then I recalled my earlier comment that the only use for Kevlar is as a yuppie magnet, for boat sales.  I realized that the way he uses Kevlar is brilliant.  He puts it in the place on a hull where it can do the least harm.  Then the boat sellers can attract yuppies with it.  Like the carbon fiber cleat pads on Gunboats.  Utterly wrong material, but it confirms to yuppies that they are indeed high tech.  And that tiny bit of fabric visible is the evidence.  Brilliant.

7 thoughts on “Kevlar Comments”

  1. If kevlar lasts 5-8 years in bullet proof vests (ask a pig!) why the hell would you put it in something that should last 20+ years! Anything disposable is OK but lifetime?

  2. Kevlar and boat building is an interesting topic. What most people do not know is that kevlar used for ballistics and the kevlar you buy for boatbuilding are two totally different materials. Kevlar for boats is structural Kevlar and has a strain at failure of 2%. Ballistic Kevlar can have e>20%. Eglass has 4-5% strain at failure, carbon has 1-2%depending on the fibre type. Most of the hybrid kevlar/glass laminates that I have had mechanically tested for survey purposes have done worse than 100% glass laminates. Adding small amounts of different fibres to glass laminates eg small amounts of carbon or kevlar rarely work due to the fibers widely different properties. Kevlar is useful to hold a failed laminate togtehr after it shatters but it does not improve 1) the original laminates mechanicals or 2) its impact performance. I council my clients to stay in the glass, carbon or kevlar universes. By the way there are some very interesting high modulus glasses now available (type H, R & S2 and S3) that are 20% stiffer then E glass which give big possibilities for lighter, stronger cheaper structures. Peter

  3. Hi Kurt,

    My thinking was that the pre-preg process might overcome resin adhesion problems that you identify with Kevlar. Of course that doesn’t alter the mechanical properties of the base fabric or its suitability for combination with radically different materials.

  4. Actually the RIGHT way to use Kevlar in a laminate/marketing process is a variation of the Suttleworth method. The key is to separate the uni into individual fiber strands and space them at least 1″ apart. That way the laminate strength isn’t compromised, the marketing department has a free reign and the bean counters love the savings on high cost material. LOL

    Care to comment on the performance of kevlar or kevlar/glass/carbon pre-preg fabrics?

    1. pre-preg process and fabric choice are two different topics.
      just check the strain to failure perentages of each to see how rediculous the three together are.

  5. I added a layer of kevlar (12oz) between two layes of 18 oz biax glass on a 32’Core Cell foam hull.
    Wish I had read your Kevlar comments at that time,sighhh,several years ago,damn!You really should publish this composite build book tips……

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