Glass and carbon fibres are the reinforcements that give the skin laminates their strength.

They come in wide range or weights, weaves and finishes. For any given component in the boat your plans will specify the weight and the fibre orientation of the reinforcements required. 

Composites engineers and designers tend to work with reinforcements and resin systems from particular manufacturers and suppliers they have developed a relationship with through the course of their work. Because there is such a wide range of fabrics available your composites supplier may not stock the specific reinforcements or resin system specified on your plans but may be able to source the material or supply something very similar.


Unfortunately there is no universal standard for labelling reinforcement types so be prepared to see different codes for a similar or effectively identical product from various suppliers.


There are four basic criterion that determine the reinforcements that are specified throughout the boat.

•Panel stiffness

• Impact resistance, 

• Properties strength in torsion, shear and bending for highly loaded items such as beams, spars, rudder stocks and chainplates.

• Criteria determined by a certification standard such as ISO and possibly incorporating any or all of the above three points..




Panel stiffness is one of the properties covered by the rules of the various classification societies, however at this time (2019) the rules for multihull design are not comprehensive and so in some cases the laminates are specified by resorting to engineering from first principles.


Impact resistance is fairly subjective. The question is how hard and how sharp is the object you are protecting against, and what is the impact velocity? The general consensus is that 600gsm of reinforcement is the minimum you need to protect hulls, decks and other surfaces from local impact loads, regardless whether the reinforcement is glass or carbon. 

On a typical cruising catamaran the requirements of panel stiffness will call for a laminate heavier than 600gsm for most external surfaces and this will override the impact resistance requirement. 

On a lightweight racing boat the weight might be reduced below 600gsm and the likelihood of some damage being suffered from time to time is an acceptable penalty to pay for the weight saving.


If your boat has been professionally engineered the plans and specifications will include a list of the properties the builder is expected to achieve in the cured ply laminate. These properties are likely to include the laminate thickness, tensile and compressive modulus, in plane shear modulus, longitudinal and transverse tensile strain and in plane shear strain. 

These properties will have been calculated on the the predicted properties of the laminate depending whether the laminate is applied by wet hand layup, vacuum pressure, transfusion or other method such as an autoclave cured prepreg. 




The warp refers to the longitudinal threads on the roll. The weft threads run transverse to the roll.

Plans that specify reinforcements will include a compass to show the required orientation of the fibres. Zero degrees indicates the fibres that are aligned with the warp. 



For unidirectional reinforcement the fibres are mostly or entirely in the zero axis. The exception is a transverse uni where the fibres are aligned across the roll. The unidirectional fibres are usually held in place with some lighter fibres or stitching on the 90˚ axis. 



How easy the fibres are to wet out and how easily they drape over contoured surfaces depends on the fabric weight, the yarn size, the type of weave and the finish or sizing of the fabric.

Satin, twill weaves and crowfoots generally drape more easily than plain weave reinforcements and are easier to wet out than the heavier triaxial and quadraxial stitched fabrics. We recommend to purchase a small sample and try it out on the job before ordering a large quantity.






With stitched reinforcements the fibres are more closely placed and so the resin content is lower and the fibres are straighter which gives them more strength than a woven reinforcement.



Biaxial means the fibres run in two axes at 90˚ to each other. They can be at ±90˚ on the roll or at  ±45˚ on the roll. the ±45˚ reinforcement is sometimes referred to as a double bias fabric.



There are two types of Triaxial; weft and warp. A weft triaxial has the weft fibres at 0˚ and a second set of fibres at +45˚ and a third set at -45˚. A warp triaxial has the warp fibres at ±90˚ a second set of fibres at +45˚ and a third set at -45˚



The quadraxial reinforcement has fibres at ±0˚, ±90˚, ±45˚


Diagrams showing fibre orientation on the roll

Above: Diagrams indicating fibre orientation on the roll.


Note that the orientation of warp and weft is not necessarily the same as the fibre orientation on the job. Each drawing in the plans with a laminate specification should have a fibre  direction orientation diagram like the one shown here to demonstrate the orientation of the fibres relative to the part shown on the drawing in question. So for example if a biaxial that is oriented at ±45˚ on the roll is laid with the roll orientation at 45˚ on the job, then the fibre orientation on the job becomes ±90˚.



If you were to do a very detailed analysis of the load distribution throughout the boat you could optimise the fibre orientation at different areas of the hull and decks, and for every component throughout the entire boat. To some extent we do this, even on cruising boats and a typical cruising cat might use four or five different reinforcements over the entire structure. A high end race boat might use a lot more, or it might simply use a narrow range of very light unidirectional reinforcements at different orientations, and reinforce highly loaded areas by increasing the number of plies.


Ultimately the decision on how much laminate optimisation to do comes down to practicalities. Increasing the range of laminates means more joins, more waste, more potential for mistakes on the shop floor and the added cost of purchasing in smaller quantities. These disadvantages have to be weighed against the actual weight saving to be achieved optimisation, and it may not be significant on a cruising boat.


There may be cases where you have to do some basic calculations to determine the number of layers to use, especially where a highly loaded component such as a beam requires multiple layers of reinforcement and your supplier doesn't stock the reinforcement weight specified in the plans.

So if the plans call for 8 layers of 300gsm (a total of 2400gsm) unidirectional and your stock is 400gsm, you can reduce the number of layers from 8 to 6.

The fibre orientation should not be changed.



Fibre orientation compass
Fibre orientation on the job

A sample fibre orientation diagram that might be included on a plan with laminate specifications. Note that the 0˚ orientation is not alway left to right, it might also be in the vertical plane depending how the part is orientated on the drawing.

Examples of reinforcement styles


Top Row Left to Right: Satin Weave Glass, Uni Glass, ±45˚ Biaxial Carbon, ±0˚/90˚ Carbon


Bottom Row Left to Right; Uni Carbon, Plain Weave Glass, ±45˚ Biaxial Glass, Crows foot Glass