Fiberglass boats are also called FRP or GRP, which stand for Fiber Reinforced Plastics or Glass Reinforced Plastics respectively. This is because the hull is actually made of plastic that is reinforced by fibers made of glass. Since the fibers are made of glass, FRP and GRP can be used interchangeably.
The factor that gives the plastic its strength are the fibers that run through the plastic. As with anything placed by engineers and builders, there are many ways to do the exact same thing! The three most common ways to add fibers to the plastic are:
- Chop Strand Mat
- Woven Roving
- Woven Cloth
These three methods all serve the same principle of adding fibers to the plastic. The difference is how they align the fibers that are added. Fibers offer strength only along the long axis of the fiber.
If all the fibers are arranged from left to right, the final product will only have significant strength when forces are applied laterally from the left or the right.
If all the fibers are arranged from top to bottom, the final product will only have significant strength when forces are applied vertically from the top or the bottom.
If you combine both fiber directions, and apply fibers running vertically and laterally, you will now have a final product that will have significant strength when forces are applied vertically and/or laterally! Woven cloths and woven roving offer fibers arranged in both lateral and vertical orientation, all in an easy to handle material. This may seem great, but it does still have one major pitfall, vertical and lateral oriented fibers are weak when diagonal loads are applied.
To offset this problem, woven fibers can be arranged in two different directions. The first layer will be oriented with the fibers running vertically and laterally, and the second layer will be oriented diagonally with its fibers running diagonally up and diagonally down! Now you have strength in vertical, diagonal, and lateral directions! Success!
If you can imagine, applying many layers involves more labor. More labor means more work and more cost. This is where Chop Strand Mat (CSM) comes into play. CSM is literally chopped up strands of fiberglass held together in a mat. CSM has no special orientation to the fibers, and the fibers do not run very far. They are randomly arranged and offer strength in all directions, but the fibers are all short and offer little strength to the plastic. The only reason CSM is widely used is it is cheap, quick, and easy! Woven fibers take skill and attention to detail, CSM can be applied with a gun that literally shoots it into the area.
We know that CSM is randomly arranged small fibers, but what is the difference between cloth and roving? Both cloth and roving are woven fibers arranged into a sheet that can easily be managed and worked with, but they do have their significant differences.
Woven roving is made of very large clumps of fibers that are woven together. Woven roving offers a lot of bulk and a lot of strength in a single layer of fabric. The problem with woven roving is it results in a much lumpier surface. If the outside of your hull was a lumpy mess, you would either have horrible resistance while moving through the water, or you would spend a really long time trying to fair the hull.
Woven cloth is made out of very small fibers that are woven together. Woven cloth offers very little bulk and very little strength in a single layer of fabric. While it is very weak, it does offer a very smooth finish layer to the hull. This will mean less lumps and less time spent fairing the hull!
You might be wondering why would you ever use CSM or Roving when you could simply build the hull out of woven cloth and have a very smooth finish with no lumps to contend with. The reason is the needed bulk to build the thickness of the hull. Making a hull sandwich with woven cloth for bread and woven roving or chop strand mat for the meat will result in a smooth, thick, and strong hull with plenty of strength in all directions!
In general woven cloth offers a smoothing finish layer to the meat beneath the the FRP work. Woven roving offers a lot of meat and strength to the FRP work. CSM acts mostly as a bulk filling sponge that holds plenty of plastic as it bulks up the FRP work.
Next time you are working on a fiberglass project, you can choose the correct fiber for the job and create a strong and durable result.