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I started this investigation with the hope of finding something tougher that the two layers of 3.25 oz. satin cloth used on the previous boat without gaining additional weight. All of the samples in the tests will build a structurally sound boat (for general purpose sea kayaking). But toughness varies.

The Test:

  1. An 18" by 5/16 steel rod cut square dropped from 40 inches.
  2. An 18" by 5/16 steel rod with rounded end dropped from 40 inches.
  3. A large strait slot screwdriver dropped from 36 inches.

The Fabric:

  1. 6 oz. plain weave left over from a Pygmy kit.
  2. 3.25 oz satin, 2 layers thick, from Raka.
  3. 5 oz. satin from Raka.
  4. 8.9 oz. satin from a local source.
  5. 3.75 oz. plain weave s-2 glass, 2 layers thick, from Raka.
  6. 6 oz. satin s-2 glass from John R Sweet.

Each sample had one seal coat, saturation coat, and fill coat (to better judge completeness of wet-out) of MAS slow resin/hardener applied to luaun plywood. Shop temperature was 60 deg. and resin temperature was 70 deg. The samples were hardened in a nice warm spot for several days. The samples were set on concrete for testing.

The Results:

Each sample was judged for the depth of denting and amount of fiber cutting and whitening.

Sample One was clearly the worst of the lot. Most noticeable was much more whitening than any other sample.

Sample Two was somewhat better in all three criteria than sample One.

Sample Three dented and cut deeper than sample Two, but had less whitening.

Sample Four showed minimal whitening, better resistance to denting, and partially resisted cutting. Finally, some signs of toughness.

Sample Five was very close in toughness to sample Four. It stood above only in resistance to whitening.

Sample Six was the only sample that passed the screwdriver test, and it passed very well indeed. No fibers were cut, just dented. Overall, whitening was limited to a few very small white flecks. This sample stood head and shoulders above the others.


Originally I was going to weigh each sample and test for stiffness. Now that I have seen the impact test results, I've lost interest since Sample Six, the 6 oz. s-2 glass, performed so well. Assuming a 50/50 glass to resin ratio for the tight weave cloths and a 40/60 for the plain weaves, the six samples would weigh 15, 13, 10, 17.8, 18.75, and 12 ounces per yard respectively (excluding wood). So the second lightest sample was the toughest. And also the most expensive.

I find it curious that sample Five was not tougher than sample Six. After all, it adds up to a thicker composite. I have seen discussions supporting the merits of multiple thin layers of glass, but my limited test does not support this idea. Also, it is my impression that my boat with two layers of 3.25 oz. cloth cuts much easier than it should. That's why I went a-testing.

I would like to point out that 6 oz. satin s-glass cloth is meant for vacuum bagging applications, not hand lay-up. I can verify that this cloth is very difficult to wet out completely in a large application like a boat. It tends to hang onto micro-bubbles and requires the use of the thinnest resin you can find. Even so, I have used it on both the deck and hull of my latest boat (along with System Three Clear Coat) and it looks pretty good. It would be best used where appearance is less important, like under a painted hull or inside the boat.

S-glass has a different chemistry than the more typical e-glass and possibly a different refraction index. Perhaps this is why it does not disappear as well as e-glass upon wet-out. The open weave sample clearly showed the slight ghosting that many others have observed. It's not particularly noticeable except on close inspection. The 6 oz. satin however, was much better in this regard. Perhaps it is as Sam McFadden suggested, because the strands are so closely packed that the individual strand shadows do not stand out.

Head in the clouds