Guide to Fiberglass Cloths

What is Fiberglass Cloth and Tape?

Fiberglass cloth and tape are the most commonly used materials when performing DIY boat repair. Not only are they affordable, but they are relatively easy to manage, and provide the strength, durability and flexibility needed for a boat’s construction.

Let’s start with fiberglass cloth. This cloth consists of glass filament strands made up of various chemical compositions. The strands are woven together in different patterns to make the fiberglass cloth, which is then applied to a surface and covered with resin. The resin hardens to make a strong surface.

Fiberglass is a very light and strong material made from inorganic material which cannot be damaged by UV rays, fungus, or bacteria in the water. It can also withstand temperatures of 3000 degrees F, making it a perfect material for boat construction.

As for fiberglass tape, it is also made of fiberglass cloth that is woven together and has selvaged edges preventing it from unraveling. The tape is ideal for reinforcing areas, particularly long seams on boats, and has excellent chemical and water resistant properties.  When placed on wet surfaces it will be resistant to peeling and corrosion.  

In summary, both fiberglass cloth and tape consist of glass filament strands. However, in boat repair, cloth is used primarily for larger areas, while fiberglass tape is more for reinforcement and repairs.

Thickness of the Cloth and Its Uses

The thickness of the cloth determines where it can be optimally used. Cloth is sold according to its weight per square yard, which can be anywhere from 2 oz., to 40 oz. When choosing the cloth for your boat repair, you need a thickness that will withstand the wear and tear, while not being too heavy to wet out and mold how you need to. The weight should fall between 4 and 10 oz.

Typically, 4 oz. cloth will be used for the decks on small boats and to protect natural wood surfaces, as it allows the underlying surface to show through. When needing to cover a larger area, such as the hull or cabin top, 6 to 10 oz. cloth should be used. These thicker cloths will provide more protection against the elements.

Width of the Tape

When looking at the widths of the tape, you will want to choose the size that best suits your task at hand. Narrow tapes will be easier to wet-out when applying the resin, and are good for fixing small seams, joints and cracks. Wider tape may be used for areas that require more reinforcement and have wider cracks. Tapes range from 2” to 12”.  

Types of Weaves

There are 3 main types of weaves that are used in fiberglass.

The Plain Weave: This is a simple weave pattern, similar to a basket weave, where the warp threads and fill yarns intersect each other at right angles. They weave in and out; alternating. This is great for providing stability, but not so great for flexibility.

The Satin Weave: This type of weave involves 3-7 warp threads which have 1 fill yarn floating over them that is stitched underneath another warp thread. This weave has high flexibility and can conform to various shapes.

The Twill Weave: This weave is more pliable than a plain weave and more flexible than the satin weave design. It is created using a diagonal rib made by one warp thread lying over 2 other fill yarns.

What is Chopped Strand Mat?

A Chopmat is a sheet of weaved multi-directional fiberglass strands which are glued together. The construction makes it highly waterproof, which is ideal for use on the outermost layer of a boat. When applying chopmat, it conforms very easily to various shapes. Additionally, Chopmat is very effective to use in between layers of fiberglass cloth in boat construction, as it acts as a filler which helps to hold the cloth in place by increasing the surface contact.

Composition (compare weight, strength, cost)

E-glass (alumino-borosilicate) claims 90% of global fiberglass production due to its sufficient tensile strength of 500 ksi and low cost ($2-$4/lb). Additionally, the compressive strength measures 156 ksi. Although the “E” in E glass, comes from the glass’s excellent electrical insulation properties, it is primarily used as a plastic reinforcement fiber.

S-Glass (aluminosilicate), is a stronger type of glass with a tensile strength of 709 ksi, and compressive strength of 232 ksi. While it is stronger, it is also more expensive costing $24-40/lb.

E-glass is affordable, but has relatively weak impact resistance when compared with the more expensive S-glass. E-glass is still most commonly used in fiberglass.

Now you know the basic types of fiberglass cloth and tape used in DIY boat repairs including what fiberglass cloth and tape is, the various sizes, the main weaves, all about chopped strand mats and the most common glass compositions. Best of luck on your next step toward repairing your own boat.

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