Flex, feel, foam, fins: How 11 years of surf science is helping make better boards

“Today, this material configuration remains one of the most widely used in surfboard construction” a 2024 paper on the progress of composites in surfboard manufacturing states, referring to a sandwich of fibreglass (commonly its e-glass) and polyester resin outside a polyurethane (PU) core.

Professor Marc in het Panhuis studied chemical engineering in The Netherlands, attained a PhD in physics in Ireland, and has been both on staff at University of Wollongong and a keen surfer since 2006.

He has been researching the science behind surfing since 2014 and established the university’s Surf Flex Lab in 2015.

Among his work are two multi-year collaborative research projects supported by the Australian Composites Manufacturing CRC: Manufacturing Automation for Smart travel Surfboard and Smart composite manufacturing for marine safety applications. Both are led by industry partner Gowing Bros.

In het Panhuis was an author on a recent paper explaining the use of experimental modal analysis in measuring “feel” in a surfboard, and finding that each has a “unique mechanical fingerprint”.

ACM CRC: Tell me about how you got drawn to surfing as a field of research. 

MihP: It’s mainly because of translation time.

I used to work on hydrogel materials and living cells and trying to make gels in a way that we can implant them in the body. So a couple of years ago, we went on – there was still a show at the ABC had, The New Inventors. And we went on there with tissue mimics.

But the horizon for that to come out of the lab is usually 10 to 20 years. And I was always looking for something that could shorten that translation time. And surfing, making fins, for example, is where we started 10 years ago.

That is so fast because we can make a fin today in the lab and you can go test it tomorrow. So it's the shortening of that time between what we do in the lab versus it being used in the real world. 

ACM CRC: This is perhaps an impolite way to frame it, but you're an impatient person by nature? 

MihP: Very impatient. Yes.

ACM CRC: Well, so am I. It's not a bad thing to be, I don't think.

So why is flex such a fertile subject in your research? 

MihP: Flex is incredibly important, in particular, in the sport of surfing. Pretty much any surfer, even if they're not using the word flex, when they describe how their board goes or how their fins go, they are generally talking about the flex, either the flex of the fin or the flex of the surfboard. 

In particular, if you ski, you're a snowboard, or if you play tennis, or if you play – name another sport where you have an implement in your hand that you're using to do something with, be it hitting a ball or surfing a wave: they're going to flex, mechanically flex. So it's going to bend under you. 

We've been testing fins today, checking the flex behaviour of fins. So flex is also linked to the way we experience the surfboard, for example, or the fins. Because we feel that flex happening. It contributes to what people describe as the feel of the surfboard.

That's what that paper is trying to achieve, is to show that we can do very sophisticated mechanical measurements that allow us to capture the feel at frequencies that we actually can feel through our feet. Because we're very tactile; we're tactile through our hands, and we're tactile through our feet. 

ACM CRC: It was a long time ago, but why would you say that the move from wood to composite sandwich boards was so transformative for surfing? 

MihP: It's more lightweight, with more performance.

So it wasn't just the materials, but the new materials – I'm not a historian of surfing, but I believe all these new materials were developed as part of the Second World War. So they were lighter, they were more responsive, and probably more cool to use, I think. And that's really changed the way the sport of surfing has gone.

Most boards, most performance boards will still have a foam core. There are, of course, lots of other boards on the market. You can buy retro boards or longboards, but anything where weight is important, you'll find that there are surfboards out there that have foam cores in them, because that material is so lightweight.

That makes it also very hard to replace it, because it's so lightweight.  

ACM CRC: Would you say that feel is sort of just a function of vibration and flex? Is that safe to say? 

MihP: It's the frequency, the vibration, the flex, the feel, how something bends, how it behaves in response to forces, for example, from the waves, but also from the rider itself. So the way they would move their body around, the way it recovers. So it's not just flexing.

It's also the way it comes back to its original position. So that's called recovery. It's a very fertile field of investigation.

ACM CRC: Something that jumped out at me in the paper was the mention of the feel of a violin and the role of vibration in that.

MihP: Violins are a very good example of a wooden instrument where modal analysis has been applied. And that particular measurement technique has been used in many, many different fields. So it ranges from measuring tennis records, but they also do it, for example, for bridges.

They do it for skyscrapers to find resonant frequencies. And it is a way of capturing how something feels. And the violin was just one of the nice cool references that we could find in that.

The other ones are hockey sticks. Tennis rackets are a really good example. Usually people that use this technique, I think they do it in their own passion areas.

I know there was a person before that did hockey sticks, possibly because they were obsessed, just like me with surfing. But they're obsessed with hockey sticks or with violins or with tennis rackets. 

ACM CRC: Well, you don't want to pick something to investigate that you're going to be bored by. So I guess that's fair. 

MihP: Yeah.

ACM CRC: Could you speak a little bit about the influence of different composite systems on feel? Or is it mainly concerned with just the different core, what you found? 

MihP: There's a couple of aspects there. So a surfboard is a sandwich composite structure that has an inner foam core, and then it has an outer fibreglass shell.

And sometimes, in a lot of modern surfboards, they also put in there what are called flex control systems. They help to modulate the flex in the board. The best known example of that is a wooden stringer which you'll find in a lot of traditional boards.

One of the types of boards that we tested in this paper had a different type of flex control system in it, which is something that lies on the surface on the deck of the surfboard. And that helps to modulate the different behaviour in terms of the foam cores. 

For cores, there are two main foam types of material that are used. One is polyurethane. That's a very traditional material. And then there's expanded polystyrene.

And these foam cores, they have different flex characteristics by themselves. So by adding in these flex control systems, be it stringers or being these deck inserts, that can influence the way these boards feel. And surfers have many, many, many, many discussions about the differences between a PU board, a board with a polyurethane core, or an EPS board, a board with an expanded polystyrene core.

ACM CRC: Is it mainly around weight and stiffness? 

MihP: It's mainly about vibration behaviour, but also weight. So a board with an EPS core is relatively lighter than a polyurethane board. 

And they can only work with certain resins in the shell?

It depends. You can actually use the same resins and the same fibreglass material on both boards. There is one particular resin that you can only use for a polyurethane board, yes.

ACM CRC: So how is this research being used on the two CRC-funded projects?

MihP: So for the travel board project, we are building prototypes where you take a whole surfboard and come up with a way to make it into two parts or three parts that then can be clipped together. So the key characteristic there is no matter how many parts your surfboard consists of, when you clip it together, it should have the same mechanical feel as if it was in one piece.

So the modal analysis that we use allows us to identify specific vibration behaviour. And we can also see the change in that vibration behaviour when we measure our travel board. 

So we have done that already. We haven't published those results yet. That's why we're still working on the analysis. But that's one of our main drivers and one of our main quality control systems to demonstrate and to provide a scientific backup to show that if you do make a board out of several pieces, it can still behave as an individual piece.

For the shark deterrent project, because we're putting electronics into fins, we're now taking the same approach, but for fins. So we're looking at the modal behaviour of fins. 

We were just measuring some flex behaviour as well. Again, the key is if we change something to the fin, we want to be able to demonstrate and provide scientific data to show that it hasn't changed the actual mechanical performance behaviour of the fin. 

ACM CRC: You've spent a few years in this quirky field, and I don't say quirky in a disrespectful way, but quirky as in not everyone's studying it. What are some remaining unanswered questions? 

MihP: When I started this work, I took in a couple of skis. I took in a couple of surfboards. And I thought, ‘I'll have a look at these surfboards. And then probably at the end of this year, I will move on to fins. I will move on to skis.’ So this is 11 years ago.

The two pairs of skis are still sitting in that room. They haven't been touched yet. 

So there are many other aspects that I would like to apply this particular technique to, because it's a very versatile and very interesting technique. The application of modal analysis is actually, it's a very, very broad field that people are investigating. 

The niche for us is that we are applying it to surfboards and fins. 

Pictures: supplied

The paper in Advanced Engineering Materials, “Experimental and Numerical Modal Analysis of Composite Sandwich Structures Using Surfboards as Model Systems”, can be accessed here. Its authors are Brett Connellan, Luca Lomazzi, Mathis Mercier, Kara Stark, and Marc in het Panhuis. The study was supported by University of Wollongong's Global Challenges Program.