We get a lot of emails from listeners of the Surf Simply Podcast, asking for our thoughts on certain surfboard construction methods; particularly epoxy vs polyurethane; plus we hear in the surf media about exciting new materials being used in surfboard design, like carbon fibre and kevlar. What can we take from this mix of science, pseudo science, fact and opinion? In this two part series, starting with the stringer, we’re going to navigate the material science of surfboard construction.
Firstly, lets go through some of the facts and fiction.
Ok, Epoxy vs Polyurethane, who would win? Now the problem with asking this question is simple; epoxy is a material used to make the skin of a surfboard, where as polyurethane is used to create the core in the middle. So they’re not mutually exclusive. You can commonly buy an epoxy skinned board with a polyurethane core.
So it’s more helpful to break the surfboard down into its component parts, then look at the different methods to manufacture them.
A surfboards core is typically made from some sort of foam, which is then covered by a skin, traditionally fibreglass. Running through the centre of the foam is the stringer; typically a single strip of wood that adds strength and rigidity.
When we look at traditional construction methods, different methods affect weight, strength, and flex pattern. Changes to the core and skin will affect all three, however a change in stringer will only affect the strength and flex, as the weight is almost negligible.
The core accounts for the majority of the weight in a traditional board, approximately 2/3 – 3/4 of the finished total. However the skin and the stringer are what provide most of the strength. There are many interesting ideas out there about how to build surfboards, here we will discuss the common ones.
First off lets consider the stringer, it is the quickest to explain and a good way to introduce some of the materials we will touch on. Again, the stringer is the strip of wood that runs the length of a traditional fibreglass board, and its there because without it the board would probably flex more than the core and skin would be able to handle. Its there to prevent the board from snapping; especially on longer boards, where the stringer size is increased to cope with the increased stress.
With that in mind, its obvious the stringer has an important part in determining how much the board will flex whilst being ridden – and although the board may feel very rigid, they can flex an amazing amount whilst you’re onboard. (Note the flex in Julian Wilson’s board as he lands an air at 5:26 in Quicksilver’s 2012 film, “Cyphervision” below).
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 part, potentially making the board more desirable (or undesirable) to ride. This sounds excellent, until you want a second board with the same flex pattern, and all of a sudden you have something frustratingly unique, particularly if the original snapped.
Considering the interesting materials available nowadays, its surprising that surfboard manufacturers still insist on using wood for the stringer. There are a few companies playing around with this idea, and have released boards with fibreglass, or a carbon fibre stringer, and even a PVC plastic example – mainly to get a more measured and consistent flex pattern. A potential reason this hasn’t been pushed earlier is because only in recent times has a desired flex pattern been pursued and discussed. The professional surfer and average consumer previously considered the shape and design of the external properties, whilst ignoring the internals. Manufacturers are now moving towards where the ski and snowboard industry have been for some time. Considering the influence materials have on the user experience, and refining the internal characteristics is becoming more prevalent in surfboard design; so getting a consistent flex pattern is now a priority.
More common now even at your local surf shop, you can find surfboards with strips of carbon fibre within the skin, in an effort to engineer the flex of that particular part of the board. Another change recently, companies like FireWire and FutureFlex are taking the stringer, and placing it in the rails, rather than down the centre of the board; changing the way the board flexes, allowing more twist rather than flex along the length; similar to how a snowboard might bend. The end goal is the same however, whether wood or carbon, the rails are there to stop the board from snapping when placed under high loading or stress.
If you have ever snapped a board and investigated its contents, then you may be familar with the core. Typically a plastic foam material, that for the past 60 years has heavily been polyurethane (also Poly, or PU). Poly comes in different densities, whilst the standard for surfboards is around 3lbs per cubic foot (which works out neatly as approximately 26 litres, or a standard performance shortboard size). Some pro surfers seek out lower density blanks for a high performance board, and high density blanks for a step up or big wave board – a tow in board might weigh as much as 6-10lbs per cubed foot. Shapers might refer to blue or red foam, the colour coding system to indicate density.
Have you ever shaped Polyurethane foam? Sandpaper and a breadknife glide through it like butter. Shapers can make micrometer adjustments with little effort, due to the high density of the bubbly material. Its also cheap as its a widely used chemical, all together shapers have been happy using it for those reasons.
So what are the problems? Well yes it flexes well, but its compression strength is terrible, so under small concentrated pressure it crushes easily; hence all those pressure dings from your feet, or in some cases your head. Those dings don’t just have an aesthetic impact, but a volume reducing one too; potentially stealing 1 or 2 significant litres from the original value. Another problem with polyurethane is that if the outer skin is compromised, through a ding or crack, the foam will take on water. This can lead to the break down of the foam, yellowing or brittleness; and lets face it, nobody wants a yellow surfboard.
PU boards are also terrible for the environment. We’ve all seen FireWire’s use of more sustainable materials, and a big push now with large manufacturers is to clean up their process. In the future, this alone could be the death of polyurethane as a core material. Through the production of the material itself, the process creates some nasty chemicals, and the finished product takes a long time to break down once discarded.
These reasons all contribute to an increase in expanded polystyrene, or EPS, as a material for the core. EPS is the foam you find packed around your TV, or lining a beer cooler. EPS densities can vary, but for surfboards its normally lighter than polyurethane; typically 1-2lbs per cubed foot. Although EPS is more brittle, its actually stiffer, and a higher stiffness directly improves the strength of the board.
Although not all the strength comes from the core, a sizeable amount does, and so since EPS is a stiffer, stronger material, you can use a lighter density to get the same strength; thus having a lighter surfboard.
EPS does have it’s problems though. It can take on a lot of water due to the gaps between the small beads. A few companies are trying to fuse the beads together in an effort to reduce this. But the big problem is that its more difficult to shape. Using sandpaper makes the foam flake and fall apart. Instead shapers use a kind of hot knife or cheese wire to carve and melt the foam – a time consuming process for hand shapers. Another method is to form the board in a mould, which is more suited to mass manufacturers like Surftech or NSP, and less to low unit shapers – this obviously reduces the customisation too. EPS blanks are a fraction more eco friendly than PU foam but only because you can recycle packing foam to make them.
The final core you might hear of is Extruded Polystyrene, shortened to XPS. Extruded Polystyrene might be considered the ‘wonder foam’, at least for now anyway. It doesn’t soak up water, is easy to shape, and has a consistent flex pattern; whilst being more environmentally friendly than PU.
The problem is XPS is hydrophobic. Shedding water sounds like an excellent quality for something you want to keep dry, and it is, until you want to glue it. When you try to glue the skin of the board to the core, it doesn’t bond well – and as a result there are a lot of de-laminations later in the boards life. Another problem is that over time Extruded Polystyrene, because of the way its made, starts to release gas underneath the sealed skin – which creates bubbles and bumps under the skin. XPS is a similar density to PU, and so a similar weight; slightly more than EPS.
Patagonia shaper Fletcher Chouinaud is a fan of this foam, and another company called XTR make most of the extruded polystyrene blanks. XTR claim to be correcting a lot of these issues but as yet you don’t see a lot of XPS blanks being used in surfboard construction.
A traditional material that’s enjoying a resurgence is wood. Wood is an strong material, it flexes well and it can also be beautiful. Wooden core surfboards can be divided into roughly two categories: chambered solid boards and hollow skin-on-frame surfboards.
Chambered solid boards are made usually made from balsa, a light weight wood to reduce weight but they are still heavier than some surfers would like.
Hollow, skin-on-frame wooden surfboards are made by laying thin skins and strips of wood (often western red cedar) over an internal skeleton like an aircraft wing. These boards tend to finish around 30% heavier than a polyurethane foam core, however wooden surfboard specialists claim that if the extra weight is properly allowed for in the design then it is possible to produce a performance shortboard. These wooden surfboards still need to be laminated (often using an epoxy bioresin and lighter fiberglass cloth) however they are more durable and therefore have a lower impact on the environment than foam-cored boards.
The final material worth discussing is …air. Quite a few companies have played around with making hollow surfboards; George Greenough and Solomon for example. This is an interesting direction because obviously if you make a hollow surfboard then it will be extremely light, but you loose all the strength that would come from the core. This means the skin has to compensate most likely by being thicker or more clever, to maintain the overall strength.
The final problem with the hollow board is that if (or when) you pierce the skin, you may sink. With no neutral buoyancy, as soon as it is compromised, the board (and you) will drop below the surface. An interesting feature of a hollow board is the plug or valve system. When under pressure from temperature change, or if you take one up into an aeroplane, then you need a pressure release valve (a feature of some of the original FireWire and Solomon boards).
Part two of the series will investigate traditional and modern surfboard skins, carbon boards and sandwich construction.