A sandwich is a material (core) sandwiched between two fiber skins. Sandwiches are used in shipbuilding to reduce weight and increase rigidity.
Thickness determines rigidity
If we want to bend a sandwich plate, the two skins will be subjected to opposing forces. One will work in tension (the upper one in our diagram) and the other will be compressed. The further apart the skins are (by increasing the thickness of the core), the greater the rigidity of the plate. Without having to thicken the skins, and therefore without adding weight (the core is very light).
Not all sandwiches are created equal
In shipbuilding, skins are made of fiber, but there are also sandwiches with aluminum skins used in aviation, for example. For boats, the resins used are polyester, vinylester or epoxy. They are applied by contact (roller, brush), but today are more often vacuum-infused. Resin-impregnated fabrics include glass, carbon, kevlar, etc. The use of these different fabrics increases resistance, particularly to punching.
It all depends on the soul
For the core, we look for the lightest possible material. This can be foam (PVC, Corecell...), balsa wood for yacht building, or honeycomb (Nomex) for racing boats.
Balsa is often used for its price, but also for its ability to withstand compression. In fact, it is used as standing wood. Incomprehensible metal or hardwood cores are added to allow fittings to be bolted through, thus compressing the sandwich.
A bad image
Buildings dating back to the 70s now feature delaminated, waterlogged decks. This has given sandwich decking a bad image. Today, using foam cores or pre-treating the balsa with a layer of resin to seal it, eliminates these problems in the event of water infiltration. Well-mastered sandwiches have a long service life.
Often used
Sandwich construction is no longer reserved for racing boats. The decks of mass-produced cruising yachts are almost all made of sandwich construction. Multihulls, with their large planking areas, are often made of sandwich construction (with monolithic bottoms for greater puncture resistance).
The sandwich advantage
- Weight
- Thermal and sound insulation
- Rigidity
Sandwich disadvantage
- Weakness on punching
- Infiltration leads to risk of delamination
