Construction materials

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To enable you to keep up, fix up or renovate your boat (inflatable, aluminium, and iron-cement excluded), we thought it would be interesting to offer a short, yet detailed general glossary of the materials used most commonly in the building (amateur or professional) of motor and sailboats, as a hobby (light dinghies), windsurfs or kayaks. Until a relatively recent time, all boat hulls were made out of wood. But thanks to the progress in science and techniques, they are now made out of a variety of materials, each one more adapted to a given construction type.

Polyester resins, epoxy resins, epoxy, sandwich construction, monolithic mixed construction or even charged resin, these words or expressions are understandably gobbledygook to your average Joe, but soon, they will have no secrets for you. 

Polyester and epoxy resin:

A resin is defined as a viscous liquid able to harden under certain circumstances (in the case of studied resins, it is a polymerisation type reaction). This reaction is activated by mixing a component and a small quantity of catalyst (for polyesters) or a hardener with a 1:4, 1:5 or even sometimes 1:1 ratio (for epoxies).

Polyester is cheaper, tends to harden at once and its properties (lesser hardness, tendency to delaminate) are lower than those of epoxy. In any case, its use is restricted to fibreglass mixed materials.

Epoxy is more expensive, has better properties (overall water-tightness, gradual hardening) but is sensitive to UV rays.

Fibreglass/kevlar/Carbon-fibre:

A set of woven/assembled fibres under sheet form. Including them in resin allows for more resistance in the obtained surfaces. The maximal resistance lies in the length of the fibres, so the pattern and disposition are important for the final result. A unidirectional material has all of its fibres arranged in the same direction (one-direction resistance).

Fibreglass:

Has average properties but is relatively easy to procure and cheap.

Kevlar:

The surfaces obtained are very resistant, even to stamping (it is used to make bulletproof vests) but it is rather expensive and difficult to shape.

Carbon-fibre:

Light, solid in the direction of the fibres but fragile in the other directions and more expensive. This is the preferred material used for race boats.

Foam:

A material which is not full, but consists in an emulsion of a given material and air. Therefore it is filled with small air bubbles. Its resistance is generally rather low (depending on the emulsified material, the amount of air, but it is always less solid than the same material, full), but is very light.

Composite material:

As its name indicates, composite materials consist of a mix of different components, generally a lining inside a die. The advantage is to create a material which combines the qualities that the lining or the die would not have had on their own. In this case, the lining is fibre and the die is resin. The resin is used to bind the fibres and result in rigidity and a resistance to stamping, combined with a strong lengthways resistance.

Wood/epoxy composite material:

This product revolutionised the construction of wooden boats. It minimises upkeep, brings solidity, lightness and the possibility to choose from many harmonious shapes, and prolongs life. Wood being water-sensitive, the epoxy is used to make it waterproof. Every single piece of wood used in the construction of the boat would indeed be treated with epoxy. Therefore hygrometry is stabilised and the wood does not rot any more.

Monolithic composite material construction:

This construction process uses a single composite material (as opposed to “sandwich”, which piles up different types of materials)

Sandwich construction:

Multiple layers of different materials are piled up around a core wrapped in one or more layers of different kind. This also enables combining the advantages of different materials. The core minimises weight and brings rigidity whereas the composite layers protect from shocks and strengthen.

Female mould:

A female mould is a mould where the object to mould is inside (a pie dish is a female mould). With this mould, the external layer is pressed up against the mould. Once completed, the hull is taken out of the mould.

Male mould:

This is the opposite of the female mould: the hull is created around the mould and the inner layer rests upon the mould.

Charged resin (glass microspheres):

A hardened resin may be assimilated to a relatively dense piece of very hard plastic. Depending on requirements, a “charge” is mixed in to the resin before it hardens in order to alter its properties. For example, the materials in question will bring a strong resistance in certain directions. Fibres will give it a good multidirectional resistance (for filling in a hole). Glass microspheres or silica powder will make the resin lighter and thicken it, rendering it usable as glue or finishing compound (depending on the needs) without lessening inherent resistance.

Peel ply:

This fabric has little or no adherence on resin and can be inserted between the composite material and the breather so that both elements aren’t stuck to each other.

Breather:

Also a type of fabric. It drains by absorbing the extra resin out of composite layers during construction. As long as it is airtight and can absorb resin, the rest is not of great importance.

Infusion:

In this case, all of the elements necessary to the hull are laid into a female mould: outer fabric/foam/inner fabric as well as the peel ply and the breather. Then everything is put into a vacuum. All the elements end up firmly held together to one another. Using one or more pipes, resin is injected into the vacuum, which will come perfectly into place due to the vacuum and capillary action: minimal quantity, perfect gluing without a single air bubble… and without even having to get dirty! After it has hardened, the hull is ready!

Injection:

As a result of the process started with the infusion, the injection consists in fashioning a part between both moulds in order to obtain a perfect finishing on both sides at the first attempt. This technique, mastered by a few big boatyards requires large investments (you need two moulds…) and great precision in the thorough fashioning of the moulds, in order to guarantee the thickness of the air gap. As opposed to infusion, here resin in injected and it migrates from the periphery of the part to its volumetric centre. A slight dip makes migration easier. Catalysis happens in equal conditions, while the temperature nonetheless remains more stable than with an infusion.

Vacuum construction:

The point of this process is to build composite materials as light as possible, keeping a minimal amount of lining around the die, all the while guaranteeing optimal binding. The fabric is laid into the mould and soaked with (slow hardening) resin and the air bubbles are rigorously removed. The result is covered in peel ply, itself covered in absorption felt.

The whole is set into a vacuum bag (an airtight tarpaulin may do the trick) which is sealed. Using a pipe that goes into the tarpaulin, a pump sucks the air out of the bag which comes to stick firmly against the structure (therefore insuring optimal gluing) and the extra resin will take the place of the air which is pumped out of the breather. The resin is left to set in the vacuum, and the airtight bag and other materials may then be removed.

It is worth motioning:

Nowadays, in order to respect the environment, natural fibres have caught the attention of constructers, a large number of older boats go to waste and are difficult to eliminate. Plastics and composite materials frequently used to build pleasure boats pose a real environmental problem. When these boats become older, they make up waste which is difficult to eliminate, and building them creates VOCs (Organic Volatile Compounds), which damage one’s health.

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