The difference between glass fibre mat and woven glass roving cloth is that in fiberglass mat, the individual glass fibres are irregularly arranged on top of each other. In woven roving, as well as in reinforcing mats and carbon fibre mats, the individual fibres are neatly interwoven. Roving is very coarse and made of strong glass fibres, which is used exclusively in the construction of large boats. Single strands of roving fabric can be used to smooth out rough edges, especially at the bow and stern, and to repair damage to the tip of the boat.
When can I use fiberglass mat?
Glass fibre matting is mainly used with polyester. This is largely because styrene, which is contained in polyester, is what breaks down the binder and makes fiberglass mat flexible.
This structure and the binder which is applied to hold the fibres together gives these chopped strand mats their strength. If you try to bend it before processing, it will break easily. However, during processing, the styrene contained in the polyester breaks down the binder, making it soft and easy to shape.
Glass fibre mat cannot be used with epoxy because epoxy compounds do not contain styrene and therefore cannot dissolve the binder. As a result, epoxy resins remain on the surface and you won't achieve the desired strength and stability.
What to know about fiberglass scrims.
So-called scrims made of fiberglass are more flexible and much easier to work with. In its raw state, fiberglass scrim cloth feels almost like textile fabric, very similar to silk. To make it, continuous filament yarn is laid parallel and fixed in an open mesh construction. This gives this composite material much greater elasticity so that it is able to withstand loads, and is ideal for reshaping and strengthening curves or other shapes.
If all fibres run in a single, parallel direction, this is called unidirectional carbon fibre fabric (or mono-axial). This type of fabric must always be laid across the area where load transfer is expected. This is because unidirectional fibres' strength is in one direction.
For a strong repair in all directions, uni fabric should be applied in multiple layers in different directions. This makes sense when making new structural components, as high strength can be achieved without adding unnecessary weight, which is especially important on the hull. Scrims can be bought ex works that are already made and sewn with several layers. However, for easier repairs, multidirectional fabrics are easier to work with.
Multidirectional strength with easier processing and less glass is achieved by interweaving the glass fibres in different directions. These glass fabrics are extremely versatile and are usually the first choice for repairs on board, which is why it is also included in most boat repair kits and should be on every vessel. It can be used for repairs on board that are strong in all directions in minutes.
Can I use carbon fibre instead of fiberglass?
Fiberglass is heavier than carbon fibre, so if weight is an issue, it's worth considering using carbon fibre fabric. The carbon fibres used in it have similar properties to fibreglass, but a considerably lower weight. Carbon fibre fabric has high strength and low density, which gives it its light weight. Carbon fibre constructions demonstrate low material fatigue, but in terms of breaking strength, slight compromises have to be made. It is more expensive and is therefore mainly used as a construction material in the regatta sector.
What is roving cloth?
Most reinforcing fabrics are woven roving cloths. A woven roving is made of a long, continuous length of interlocking fibres that are first spun into a thread or 'yarn', and it is this thicker thread which is woven to make the fabric. This makes one single layer thicker but also stronger. Many reinforcing fabrics are made using rovings.
Fiberglass mat and roving differ not only in the way the individual glass fibres are arranged, there are also differences in weight per unit area. The most common mats have a weight of 300 g/m² or 450 g/m². Glass fibre matting is often used for smaller repairs. It has a weight per unit area of 300 g/m² and is highly stable. 450 g/m² fibreglass mats are used for larger areas where multiple layers may be required. In the case of multi-layer laminate structures, a glass fibre mat with a low weight per unit area should be used first, as this is easier to shape. Glass fibre mats with a higher weight per unit area can then be used to reinforce other layers. The quality of the laminate will be much better the higher the proportion of fibres is compared to the resin.
You can find more helpful hints and tips on boat repair in our two guides "Find out polyester on board" and "All about epoxy".