Composite structures and manufacturing processes in the marine industry

Compared with monolithic structures, sandwich composites are characterized by their greater rigidity and lightness, but they are also more complex and sometimes more costly to install.

When it comes to manufacturing a composite sandwich structure, infusion is clearly the most suitable process for achieving a high level of quality. 

This makes for remarkable rigidity and lightness, while keeping costs under control.

In this article, you'll find an overview of the different manufacturing processes for composite materials, i.e. the choice of composite structure and the methods adopted for its assembly.

As a reminder, by "composite" we mean the marriage of two compounds of different natures whose properties may be complementary, with the aim of obtaining a new material whose capacities are different and often superior to those of the components taken individually.

In our case, this involves mixing different types of fiber with various resins to obtain a "composite" material that is both rigid and corrosion-resistant.

First: which composite structure to choose?

Monolithic or sandwich?

Monolithic construction and sandwich construction are two distinct approaches used in shipbuilding, each with its own specific advantages and applications:

Monolithic construction

• Definition: A monolithic structure consists of a single layer of composite material, with no separation or distinct layers of different materials within the structure.
• Features :
  • Simplicity: The structure is simpler to design and manufacture because it doesn't require the assembly of different layers.
  • Strength: Can offer good strength through its entire thickness, but can be heavier to achieve a rigidity equivalent to that of a sandwich structure.

Sandwich construction

• Definition: A sandwich structure consists of a light central layer (the core) sandwiched between two thin but rigid composite skins.
   
• Features :
  • Lightness and rigidity: The core provides a separation between the skins, considerably increasing the structure's moment of inertia without adding much weight, giving very good rigidity for the same weight.
  • Insulation: The core can also provide thermal and acoustic insulation properties.
  • Manufacturing complexity: More complex to build, as the core and skins must adhere perfectly to prevent delamination.

Key differences :

• Performance:
  • Sandwich construction is generally lighter for the same rigidity, or stiffer for the same weight, than monolithic construction.
     
• Manufacture and Cost :
  • Monolithics can be less expensive and simpler to manufacture in some cases, especially for small structures or prototypes.
  • Sandwiching requires more material and know-how, but offers significant structural advantages that can justify the extra cost in demanding applications.
     
• Application:
  • Monolithic construction is often used when simple robustness is required, or for parts where weight is not critical.
  • Sandwich construction is favored in performance vessels, and any application where lightness combined with high rigidity is essential.

These two methods are chosen according to the specific needs of the project in terms of performance, cost, weight, and manufacturing complexity.

The question depends on the features you want for your boat.

For our part, with Andaman Sails, we aim solely to produce high-performance boats, which is why sandwich construction was the logical choice.

Once the choice has been made, the next step is to determine how to achieve the desired structure.

Foam insert between the two composite layers, CNC-cut to puzzle shape for maximum structural strength.
GREYachting Shipyard

Second: which assembly process?

As previously mentioned, the creation of a composite structure requires the blending of two complementary components, often with distinct properties. Consequently, the mixing method is of paramount importance in ensuring a quality assembly.

Here's a brief overview of the main processes for combining fiber and resin, and we'll confine ourselves to the main manufacturing processes used in the yachting sector.

1. Contact Molding (Hand Lay-Up)

• Process: Resin is applied by hand to glass fiber fabric (or other fibers such as carbon, basalt or kevlar) placed in an open mold. A roller is used to eliminate air bubbles and ensure proper impregnation of the fibers.
• Materials: Fiberglass (or other), polyester, vinylester or epoxy resin.
• Applications: Prototypes, small series, repairs, one-offs.
• Quality: Highly dependent on the skill of the worker, which can lead to variations in quality.

2. A little more industrialized, Spray-Up

• Process : Uses a gun that sprays the glass fibers with resin directly into the mold.
• Materials: Similar to hand lay-up, but with a special machine to chop and throw the fibers.
• Applications: Faster production than hand lay-up, often for less critical parts or larger hulls.
• Quality: Less control over fiber distribution and quality, more risk of voids.

3. Vacuum infusion (VIP)

• Process : Dry fibers are placed in the mold, then covered with a plastic film. A vacuum is created, drawing the injected resin through the fibers, ensuring uniform distribution and minimizing voids.
• Materials : Various fibers, epoxy resins mainly for better adhesion and resistance.
• Applications : High quality required, medium series production, used in performance yachts.
• Quality: Very high with an excellent fiber/resin ratio.

4. Prepreg (Prepreg)

• Process : The fibers are pre-impregnated with resin at a semi-hardened stage, and stored cold to prevent polymerization. They are then placed in a mold and cured under heat and pressure in an autoclave.
• Materials: Pre-impregnated carbon or Kevlar fibers, high-performance epoxy resins.
• Applications: Competition sector, parts requiring very high strength and low weight.
• Quality: Highly precise and uniform, with exceptional mechanical properties.

These methods can also be combined to exploit the advantages of each in the construction of a sailboat, adapting the technique to each part of the boat according to performance or cost requirements.

In our approach to Andaman Sails, the quest for a high quality product at controlled costs naturally led us to the infusion method. Our production is based on a combination of sandwich structure and infusion.

The process begins with the production of the sandwich structure in a mold. This structure is then placed under vacuum, allowing simultaneous infusion of the two composite layers with the interlayer core. To prevent any risk of delamination, we incorporate fibers that run through the foam, ensuring perfect cohesion between the layers.

The adoption of this process enables us to achieve exceptionally light, rigid and resistant results, while containing production costs.

Once the structure has been chosen and the application method determined, the next step is to select the most suitable materials to achieve the best possible result. For a detailed exploration of the different types of resins, fibers and materials used in our composites, we invite you to consult our dedicated article.

Gelcoating of the mold and infusion of the three layers simultaneously.
GREYachting Shipyard