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Composite construction background

Generally I would advise to use either a full carbon sandwich construction for a custom yacht or glass sandwich with carbon reinforcements. I would not recommend polyester resin because epoxy is much easier to work with and is by far the more superior material. Overall the material costs are only a relatively small fraction of the overall cost of the boat. It does not make sense to cut corners in this area especially on a one off custom designed yacht.

Why sandwich?

A sandwich core adds thickness to the laminate without adding much weight. Increasing thickness has a large effect on the bending stiffness of the structure. Doubling the thickness increases bending stiffness by a factor of 4.

A large laminate bending stiffness allows increased distances between stringers and frames. This makes construction easier and cheaper since less stringers and frames have to be build. It also gives greater flexibility in the interior design because there is less of a chance that required structural elements interfere with furniture.

Since sandwich core materials are light weight they are also excellent thermal insulators. This keeps the inside of the boat cooler in summer and warmer in winter.

In the past sandwich construction was only used above the waterline. The reason for this is that water ingress into the core would damage the hull and require extremely expensive repair.

With modern materials this is not an issue any more. Epoxy resin is a much less permeable to water than polyester resin. A properly build epoxy hull will not let water into the core. The water issue only exists with open celled cores like balsa wood which soak up moisture like a sponge. Most foams and honeycomb cores have a closed celled structure. Even if water leaks into the core it can’t spread throughout the hull. If epoxy resins and closed celled cores are used there are no reasons why sandwich construction can’t be used below the water line.

Epoxy or Polyester?

Epoxy resin slowly finds its way into boat construction. Epoxy has two big advantages over polyester. One is that it is a much better glue and the other is that it is less brittle.

The job of the resin is to bond neighbouring reinforcement fibres to each other to create a rigid part. Good bonding strength is therefore essential. If the loads exceed the bonding strength microscopic cracks appear in the laminate where the resin debonds from the fibre. The overall structure still holds together but it is irreversibly damaged. Over time the accumulation of micro cracks compromises strength and stiffness. This is the reason why older boats that have been used heavily are generally softer than new boats.

Polyester structures typically micro crack when they are stretched by 0.x% whereas a good laminating epoxy micro cracks at 0.x%. Under normal loading one would aim to never exceed the micro cracking strain. If all other parameters are equal a polyester boat would need X% more fibre in the laminate simply because the resin does not stick as well to the fibre as epoxy. An epoxy boat can be deformed much more before damage occurs. An epoxy hull is therefore much less sensitive to occasional high loads under extreme conditions or during collisions with objects. A well designed and build epoxy hull will have a very long life and degenerate much less over time.

Epoxy also can’t create osmosis because the chemical reaction between the resin and water simply can’t happen. Epoxy is sometimes used to protect polyester resin from water.


Glass fibres are by far the most commonly used reinforcements in yachts. They have good strength properties and are very cost effective. The main disadvantage of glass fibres is their relatively low stiffness and the density is more than twice that of carbon fibre. Compared to a carbon yacht glass fibre boats are therefore generally more flexible and heavier. To increase stiffness a lot of fibreglass is required which also increases resin and labour costs. In some areas the cost advantage of fibreglass over carbon therefore diminishes due to increased labour and resin costs. For these reasons the most economical way to build boats can be the use of glass fibre for the majority of the components and unidirectional carbon fibre for reinforcements on frames, stringers and bulkheads. This approach requires very careful engineering since the much higher stiffness of the carbon attracts most of the loads. The surrounding glass structure does not contribute significantly to the strength. If not enough carbon is added the carbon fibres will break first followed by the glass fibres which will lead to a complete collapse of the structure.

Fibres should be as long as possible and therefore woven, unidirectional or stitched fabrics are much preferred over chopped strand mat. With short fibres the loads have to be constantly transferred from one fibre to the other via the resin which reduces strength and stiffness. There is no reason to use chopped strand matt if epoxy resin is used. Polyester boats require layers of chopped strand mat between roving reinforcements to ensure adequate bonding between fibre layers. Since epoxy is a much better glue these layers are not required when using epoxy.


An ideal core is light weight and has great strength and flexibility. Closed cells are also important to limit resin take up and prevent water ingress.

There are quite a few suitable foam cores with different weights and strengths available. In the slamming area high ductility is important. Brittle cores are not suitable for this application because they would break under repeated impacts.

Honey comb cores have very high strength and very low weight. Unfortunately they are also very expensive and difficult to work with. They require an expert boat builder who has experience with this sort of material if it is used on curved surfaces.

A relatively cheap option is to use aluminium honey comb instead of the regular nomex paper honeycomb. Aluminium honeycomb has very high strength but in combination with water corrosion issues can arise. If the aluminium comes into contact with carbon electrolytic corrosion is also an issue. For this reason aluminium honeycombs are mainly used for high performance racing yachts which only have to last a few seasons. It would be possible to use aluminium honeycomb for interior components where good protection from the environment can be assured.

We would not recommend balsa cores for a custom boat. The main advantage of balsa is that it is cheap. Compared to foams balsa is quite heavy and brittle. Additionally properties can vary wildly because it is a natural product. The open cell structure of balsa leads to a lot of resin uptake during construction and moisture can spread through the core. Excessive moisture will lead to rot in the core and require very costly core replacement.