Surfing Explained: Ep10 Surfboard Materials EPS & PU Foam

It's our TENTH episode! What started life as a 30-second animation on noseriding has turned into this series. Thanks to everyone's support and comments so far.

This weeks episode of Surfing Explained we break down the construction of a typical foam and fiberglass surfboard. Starting with the stringer, we then analysis the differences between expanded polystyrene (EPS) and Polyurethane (PU) foam, then look at the fiberglass weights and polyester resin vs epoxy resin.

Enjoy!

Transcript:
In this episode, we’re going to break down the construction of a typical foam and fiberglass surfboard to understand the general concept and resulting qualities. In a later episode we’ll talk about some of the interesting alternative construction techniques and materials that are available, but for now, let's keep it simple.

A surfboard’s core is typically made from a plastic foam “Blank”, which is shaped into what might resemble a surfboard then covered by a skin of glass fibers and resin. Running through the center of the foam is normally a single strip of wood called the stringer, which adds strength and rigidity to the board. 

Taking several materials, capitalizing on their qualities, and layering them up is known as sandwich construction (although within the surf industry we tend to reserve this term for boards with multi-layered skins). In a surfboards case, this construction style makes a structure like a steel I beam, and this contributes to the surfboards mighty strength to weight ratio. It’s the ideal system for surfboards as the inner core is thick, light, and buoyant, while the outer skin is thin, strong, and stiff.

When we look at variations in this construction method, different approaches will affect the weight, strength, and flex pattern of the finished board.

Depending on construction, the weight of a finished board is split about 50/50 between the skin and core materials. The stinger accounts for just a very small percentage of the final weight, but it’s the combination of the Skin and Stringer that accounts for most of the board's strength and flexibility.

Slide Title: Stringer

First off let’s consider the stringer as it’s the quickest to explain. Again, the stringer is the strip of wood that runs the length of a standard fiberglass board, and it's there because without it the board would probably flex beyond the tolerances of the core and skin materials. Essentially, it’s there to prevent the board from snapping.

With that in mind, it's obvious the stringer has an important part in determining how much the board will flex whilst being ridden – and although surfboards may feel very rigid, they can flex an amazing amount whilst you are onboard.

To increase the surfboard strength, especially on longer boards, the stringer width can be increased or more stringers can be added to cope with the increased stress,
and in the longboard era of the ’50s and '60s it wasn’t uncommon to have a three stringer board for maximum strength.

Stringers have been made from a variety of wood over the years, with plywood being most common for many years. Balsa and Cedar are also popular, along with regional woods such as Basswood or Redwood within the US. Balsa is a favorite nowadays on performance surfboards thanks to its lightweight, but shapers often opt for different varieties of wood-based on a particular aesthetic since the weight of any stringer is pretty low.

Using a strip of wood has its limitations; the main one being that the flex will vary depending on the grain of the wood – which is mostly hidden within the wood itself. A knot in the wood could stiffen the flex pattern in one area, potentially making the board more (or less) desirable to ride. This mightn’t be a huge problem, but if you want to make a copy of the board with the same flex pattern, all of a sudden you have something frustratingly unique.

Slide Title: Foam Inner Core

If you have ever snapped a board and investigated its contents, then you may be familiar with the inner core. Typically a plastic foam material, the core is mostly there to create the shape of the surfboard that the skin can be formed around, but it does also contribute to the overall strength of the board. By increasing the density or the stiffness of the foam we can strengthen the board without hugely affecting its flexibility.

Some pro surfers seek out lower density blanks with a lower weight for a high-performance board, or high density blanks with a higher weight for a big wave board – a tow-in board’s core might weigh as much as 5 times that of a normal board, allowing the rider to carry more momentum or be less upset by choppy conditions.

Polyurethane foam (PU) has been the standard core material since its creation in the 1950s.

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