Structures
By Filip Sochaj, Kraken Yachts Head of Design
A brief summary of the Kraken approach to structural design
Build them strong
I am sure most of our readers are now very familiar with our philosophy of what makes a good blue water cruiser. Solent rig, central cockpit, integral keel, skeg rudder, strong hull etc. Over the next two articles, I’d like to focus on how we achieve the requirements for our structural design of the hull and deck.
I will focus on our design brief, material and technique choices, some of our key designs like the zero keel and what new developments we are working on in preparation for the K58.
As always, for those following our progress on the design spiral, we are now at hull and deck scantlings. We have gone over the midpoint in the first round of design.
Complex Composite Construction
Strong. The yacht has to be strong to survive the riggers of the seven seas and everything that’s floating within them. Given that Kraken yachts are heavily marketed as “ocean-going blue water cruisers”, it is only natural that we get asked about our construction and especially; “why fibreglass?”
This is a fair question. When you think of a tough material, metals come to mind, not plastics.
I think the best short answer would be ‘engineered strength’. What I mean by that is by using something like a fibreglass based composite construction, we can precisely design the structure to be as strong as we need, in the direction we need it to be. Materials can be categorised into two groups based on their characteristics: Isotropic and Anisotropic.
Isotropic
uniform structure
Isotropic materials have the same properties in all directions, like steel, aluminium or glass.
Anisotropic
layered structure
Anisotropic materials have different properties in different directions, wood, composite materials are in this group.
This means our structural engineers can design each layer of a composite hull layup to act exactly how they require it to. Also, by varying the type and grammature of the individual layers in the stack, it is possible to further tailor the laminate to the requirements. We can use aramid fibres to add abrasion resistance into crash-prone areas. We can use carbon fibres where the highest degree of stiffness is required. We can alternate between e-glass and s-glass or even basalt fibres to adjust any one parameter we need. At Kraken, we base our laminates on e-glass cloths. Here is a selection of what we use:
Materials
E-glass unidirectional
As the name suggests the fibres run only in one direction. This is very useful in areas where the loads are clearly only applied in a single axis or to strengthen an area that will experience increased loads in a given direction. Examples of that include wide UD strips that span the hull in the mast area, effectively tying the V1 chainplates to the mast base as well as along the rope channels.
E-glass double bias
Here you have two layers of fabric with fibreglass laid down parallel in the – 45° and + 45° axis. Very widely used cloth in all areas of the boat, the ∓45° fibre orientation works well on tight radiuses and corners. It also distributes loads well from various directions
E-glass biaxial
A simple 0/90° cloth with two layers of fibres stitched together at right angles. Again a good universal fabric that is used throughout the boat based on what load orientation is expected. We often use one big, low curvature panel in the hull and deck.
Chopped stand mat
The most basic form of fibreglass. Very useful in hand layup as a bedding layer. In infusion, we almost only use it in the skin coat right after the Gelcoat, to prevent print-through. Because the fibre strands are very short (usually around 2-3 cm) it does not have very good strength characteristics.
This means our structural engineers can design each layer of a composite hull layup to act exactly how they require it to. Also, by varying the type and grammature of the individual layers in the stack, it is possible to further tailor the laminate to the requirements. We can use aramid fibres to add abrasion resistance into crash-prone areas. We can use carbon fibres where the highest degree of stiffness is required. We can alternate between e-glass and s-glass or even basalt fibres to adjust any one parameter we need. At Kraken, we base our laminates on e-glass cloths. Here is a selection of what we use:
Resins
Principally there are three types:
Polyester is the basic resin for the most simple applications. When used properly it will give suitable results but it will always be outclassed by vinylester and epoxy. In 2022, it is essentially ‘old tech’ and is being phased out from the top end of the yacht building industry.
Vinylester is essentially a hybrid between polyester and epoxy. Both polyester and vinylester use styrene that gives off the “fibreglass smell” as it evaporates during the curing process. It is a better product in all aspects when compared to polyester due to its mechanical and chemical properties, especially when it comes to water absorption and potential osmosis.
Epoxy is the best performer across the board when only top performance matters: Volvo ocean race boats, F1 cars or rockets. That being said as technology advances and becomes more readily available to the market, epoxy resins are seeing use in the production sector. Some builders are playing with epoxy/vinylester combinations. Unfortunately with mixed success at the moment, but who knows what the future holds.
At Kraken, we build our boats in vinylester. It is the best choice when we look at our requirements; strength, elasticity, minimum thickness and long term stability of the laminate. Especially now when we are switching production technology to infusion, vinylester makes the most sense.
So, let’s work on an example. Consider a chainplate mounted to the side of the boat. In a steel or aluminium yacht, the designer will need to distribute the load either by adding frames or stringers or increasing overall panel thickness – greatly increasing the weight and volume. In composite construction, we can introduce localised unidirectional tapes that fan out over the hull side from the chainplate to spread the load evenly over a greater surface area. This way we also remove stress concentrations which are usually where failure occurs. This way we can keep the dimensions and weight to a reasonable minimum while achieving an arguably better solution.
Now with a crash course in composite materials behind us, we will look at how we implement that in next month’s edition of this series.
Join us next month for Part 2 where we cover the design and placement of the structural elements throughout the yacht.
