In our cored construction methods (plywood or foam core), we use directional glass or woven glass. In a few rare cases, we also use mat (matt in Europe) to increase stiffness. Here are some excerpts from a unfinished book about cored boat building. In the text below, the terms foam sandwich or plywood sandwich are interchangeable. Pictures are not included.
First time boat builders mostly know mat which is common for auto body repairs but inadequate for foam sandwich.
Mat ( or matt) is a felt type fiberglass with minimal strength, fibers are short and oriented at random. It is often used in polyester production boats between layers of woven glass.
The bottom of most production single skin fiberglass boats are made from a combination of mat and roving.
Roving is the name of a rough type of woven fiberglass made from thick glass strands. Mat and roving are cheap but absorbs much resin and the end product is heavy.
Those types of glass are used in polyester boats. The mat's function is to improve interlaminar bonding which is a problem with polyester. Mat and roving also improve stiffness by increasing thickness. In foam or plywood sandwich, we get the stiffness from the core thickness and the bonding with epoxy is far superior to polyester. There is no need for mat or roving and the associated excessive weight and resin cost.
Better types of glass are woven and stitched fabrics.
Woven fiberglass is made from woven thin strands of glass and comes in different weights.
For example, 6 oz. fiberglass means a fiberglass fabric that weighs 6 ounces per square yard. (For our metric builders, each 3 oz. per square yard equals about 100 gr per square meter. A 12 oz. glass is about 400 gr/m2.)
Woven glass is far superior in strength to mat but there is better: directional glass also called stitched glass.
Directional glass is made from fiber strands stitched together, not woven. This makes it, at equal weight, 20% stronger than woven.
The fibers are oriented in one or several directions. For example, a "9 oz. Uni 0" means that the glass weighs 9 oz. per square yard and that all fibers are uni-directional oriented at 0 degrees, along the length of the fabric roll.
Uni 90 means all fibers are at 90 degrees.
Biaxial means that the fibers are oriented in two directions (bi), For example, biaxial 12 oz. 45/45 means that the fabric weighs 12 oz. per square yard and that half of the fibers are oriented at 45 degrees of the edge of the glass and the other half at the "other" 45 degrees, at 90 degrees of the first layer.
Biaxial 0-90 is common as well. There is also triaxial, quadriaxial and directional glass combined with mat. For example, 1708 is a layer of 17 oz. Biaxial stitched to a layer of 8 oz. mat. That type of glass is very common in polyester production boats where the layer of mat is recommended to improve the bonding between layers and overcome secondary bonding problems resulting form the use of polyester. We rarely specify those combined fabrics in small boats but use them in larger units to increase thickness at low cost. They are often sold under the name Stitchmat.
For most of our foam sandwich panels, we specify directional glass. The direction of the fibers is important. In many cases, we aim for a isotropic combination of fibers (one with equal strength in all directions). In some cases, we put the accent on strength in a particular direction. The builder must respect the fiber orientation that we specify.
Fiberglass comes in wide fabric rolls 50" wide or tape, 4 to 6" wide. Biaxial tape 12 oz. 45/45 is used a lot in our designs. Many seams between panels are made from that tape. Used as tape to join panels, it is at least two times stronger than woven tape and much easier to apply.
In a seam made from woven tape, half of the fibers runs along the seam and do nothing to hold the panels together but with biaxial tape, all fibers are put to work.
With woven tape, the fibers at 90 degrees of the seam must bend along a tight radius and this creates air bubbles. The larger radius that results from the 45 degree angle of the biaxial is much easier to shape and comes out cleaner and stronger. Use biaxial tape as specified, not woven tape.
One last word about fiberglass selection: the binder.
In mat, woven or directional glass, the fibers are held together with a binder, a chemical that acts like a starch.
That binder is formulated to dissolve in the resin when the glass is wetted. Some binders will only dissolve in the styrene from polyester. There is no styrene in epoxy and in that case, the binder will keep the resin from wetting the glass. This will produces a weak laminate. When selecting glass, be certain that it is compatible with epoxies. Almost all directional fiberglass types are compatible with epoxy.
Further in the book, I mention substitutions:
When substituting fiberglass fabrics, consider the total weight of a skin, the direction of the fibers and the type of glass.
Two layers of 6 oz. biaxial can be used instead of one 12 oz. with the same fiber orientation but not 2 layers of 6 oz. Woven. Woven fabrics are about 20% weaker than stitched ones.
Keep in mind the stiffness of the dry material. It may be tempting to use one layer of 24 oz. instead of two layers of 12 oz. but a problem will arise when laminating around corners. The 12 oz. will turn around a 1/2" radius, the 24 oz. will refuse to bend and air bubbles will appear. Thicker material is also more difficult yo wet out.
One last point:
An email question motivated this post. The builder asked about using CSM. CSM is chopped strand mat. We never use CSM: it is weak and waste a lot of resin.
If a fiberglass store clerk propose to substitute mat for biaxial, run! A good composite materials seller will know that CSM can not be used to replace biaxial.
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