Introduction

Water sports have long drawn enthusiasts from around the globe to a playground consisting of oceans, rivers, and lakes. The essence of water sports such as surfing lies in the harmony between human and nature, and the quest for that perfect ride. While the fundamental appeal of water sports remains unchanged, the ways in which athletes engage with the water have evolved dramatically, thanks to advancements in technology. In particular, the integration of sensors has revolutionized the water sports landscape, offering athletes unprecedented insights into their performance [1].

Sensors are designed to capture and transmit data, and in the context of water sports, they can measure a multitude of variables. These variables include (but are not limited to) water temperature, wave height, wind speed, and GPS coordinates [2].

For example, in the sport of sailing, sailors have embraced sensors to optimize their strategies and manoeuvres. Sensors on sailboats can gauge wind direction, boat speed, and water depth, assisting sailors in making informed decisions during races or recreational outings. It was demonstrated how sensor-based analytics can be used to improved performance in ocean race sailing [3]. In particular, the focus was on analysing and visualizing the wind, boat and sail data generated from many on-board sensors. Regression modelling combined with visualization was used to map the predicted speed that allowed sailors to make informed performance decisions.

Other water sports such as kayaking have utilised sensors to assist athletes with quantifying their training intensity. The training loads of male and female sprint kayakers was quantified using a GPS interfaced with a heart rate monitor [4]. The study found that sensors combined with models was a valid tool for monitoring training load.

Research in the sport of surfing (generally referred to as surf engineering) has been expanding in recent years with the aim to quantify the surfer and their equipment using sensors. For example, sensors have been used to monitor their body temperature and physical exertion during paddling [5, 6]. Skin temperatures were compared between male and female surfers wearing neoprene wetsuits during surfing. It was shown that females may experience greater decreases in skin temperatures in some regions compared to males. This lead to the suggestion that neoprene distribution in wetsuits may need to be different for each sex [5].

Other approaches include the use of surface electromyography to examine the impact of paddling intensity on oxygen use, paddling cadence and surfboard motion. It was observed that oxygen use, paddling cadence and surfboard roll/yaw all increased with increasing water velocity [6]. Additional examples of research in surf engineering include assessing surfer performance on ocean waves [7, 8], computational fluid dynamics of fins and surfboards [9,10,11], and measuring flex behaviour of fins in under laboratory conditions [12].

In this paper, we describe the use of a surfboard with inbuilt measurement system fitted out with instrumented fins to measure the (mechanical) flex experienced by fins during surfing waves.

Materials and methods

Materials

The surfboard (7′4″ × 21.25″ × 2.75″ × 51 L, from Sanctum Surfboards, Australia) consisted of a poly(urethane) core, wooden stringer, and standard glassing with 3 Futures fin boxes.

The filament used for 3D printing was poly(lactic acid) (PLA, Cubic Technologies, Australia). The commercial strain gauge (Wheatstone bridges, AliXpress) used had a resistance of 350 Ohm and dimensions of 7.3 mm × 7.3 mm × 70 µm. The glue used was epoxy based (Ultraclear Araldite Adhesive, Bunnings).

All cabling and soldering were done with materials obtained from an electronics store (Jaycar, Australia). The router tool was a Makita DRT50Z (Bunnings, Australia). Epoxy resin (West System 105) and hardener (West System 207) were sourced from the Composite Warehouse (Australia). Silicone rubber (M4642) and acrylic sheet (6 mm thick, clear) were purchased from Barnes Products (Australia) and Acrylics Online (Australia), respectively.

The inbuilt measurement system was based on the following electronic components, an 8-bit Arduino microcontroller board (Mega 2560, Core Electronics, Australia) connected to µSD card reader (DFRobot, Australia), gyro/accelerometer module (MPU6050, DFRobot, Australia), GPS board (NEO-M9N, Sparkfun, Australia), a transceiver module (nRF24L01, Sparkfun, Australia) and a load cell amplifier (HX711, Sparkfun, Australia).

Instrumented fins

The set of instrumented twin fins (modelled on Futures T1 Twin) were 3D printed using poly(lactic acid) on an in-house modified printer (Creality 3D CR-10S Pro v2, with changes to nozzle, build plate, tubing, and hotend). The main fin dimensions include inside (flat foil) area (12,175 mm2), outside are (12,515 mm2), height (130 mm) and width (185 mm). The fin was designed with a Futures compatible base that was canted by 6.5 degrees as per industry standards. The printing was paused for incorporation and gluing of commercial Wheatstone bridge and cables. Full details are available in reference [12].

Surfboard with inbuilt measurement system

The inbuilt measurement system was placed in the thickest and strongest section of the surfboard (Fig. 1A). Instrumented fins were connected with electronics via a computer data cable (Fig. 1B). The process started from drilling holes in fins boxes to accommodate wires coming from instrumented fins and creating a drawing of the paths and position of the electronic box on the surfboard (Fig. 1A). Subsequently, after determining the position, two-stage 3D printed templates were mounted on the surface. Then, paths and casing cut-outs were made with the router tool. The inbuilt measurement system was custom fitted into the surfboard (Fig. 1A), after creating space for cables and casing, wires were installed inside the box and glued with epoxy resin. Subsequently, after curing epoxy resin for 24 h, a gasket was produced with the use of a silicone rubber (Shore A37 hardness). Finally, the space was sealed with an acrylic cover.

Fig. 1
figure 1

Images of the deck A and bottom B of the instrumented surfboard. Numbers 1–3 indicate inbuilt measurement system, cabling connecting fins to measurement system, and fins instrumented with sensors, respectively. C Close-up of inbuilt measurement system. D Applied force as a function of applied stroke for typical loading–unloading cycle of an instrumented fin. Fin sensor output in response to controlled flex sequence for right (E) and left F fins. The fin flex sequences (all up to 10% flex) for right (R) and left (L) fins are indicated by roman numerals, I: L and R no flex; II: R no flex, L inward flex; III: R no flex, L outward flex; IV: R inward flex, L no flex; V: R outward flex, L no flex; VI: R inward flex, L inward flex; VII: R outward flex, L outward flex; VIII: R outward flex, L inward flex; and IX: R inward flex, L outward flex. Inward and outward flex indicate fin movement towards the centre of the board and rail of the board, respectively

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Data transmission and recording

The inbuilt measurement system was programmed through Arduino Integrated Development Environment. Data (up to 80 Hz sampling rate) from the inbuilt measurement system including instrumented fins was recorded on the µSD card reader and transferred in real-time to a transceiver module (nRF24L01 + , Sparkfun, Australia) with an antenna (2.4 GHz Duck RP-SMA, Sparkfun, Australia) connected to a standard labtop, with telemetry viewer software used for visualisation.

Mechanical, sensor testing and surfing ocean waves

Mechanical and sensor testing were performed on a custom-built setup consisting of a universal mechanical analyser (EZ-S, Shimadzu, 500N load cell), a digital multimeter (Agilent 34410A) and a sample holder for clamping a surfboard fitted with instrumented fins (see reference [12] for full details). This setup was used to measure the load (force) required to flex the tip of the 3D printed fins (with and without sensors) in the direction perpendicular to the fin at a rate of 10 mm/min using a circular tip (diameter 10 mm) over 5 loading/unloading cycles.

The sensor output from the fins was recorded (under laboratory conditions) in response to controlled flex sequences using the custom-built setup. Here, fin flex is defined as the deformation perpendicular to the plane of the fin divided by the height of the fin, see reference [12] for full details. The controlled flex involved applying an inward and/or outward deformation of 13 mm (equivalent to 10% fin flex) to either one fin (right or left) or both fins in nine different sequences. Inward and outward flex indicate fin movement towards the centre of the board and rail of the board, respectively. The fin flex sequences for right (R) and left (L) fins are as follows, I: L and R no flex; II: R no flex, L inward flex; III: R no flex, L outward flex; IV: R inward flex, L no flex; V: R outward flex, L no flex; VI: R inward flex, L inward flex; VII: R outward flex, L outward flex; VIII: R outward flex, L inward flex; and IX: R inward flex, L outward flex.

The surfboard with inbuilt measurement system and instrumented fins was used in surfing session at a surf break (Jones Beach, Kiama Down, NSW, Australia). The surfer (age 52 years, height 1.83 m, mass 83 kg, intermediate experience level) surfed for 1.5 h in ocean swell conditions (height 0.75 m, period 12 s). A summary of the data collected is shown in Table 1.

Table 1 Summary data for surfing generated using inbuilt measurement system connected to fin sensors
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Results and discussion

A custom-built electronic measurement system was integrated into a commercially obtained surfboard and connected to instrumented 3D printed fins (Fig. 1A–C). The electronic components included GPS, accelerometers, and a gyroscope inside the surfboard and flex sensors inside the fins. Data was collected from the sensors, which was simultaneously recorded on the µSD card reader (inside the surfboard) and visualised in real-time using telemetry software. The sensors provided information about the surfboard’s velocity (m/s), acceleration (m/s2), latitude, longitude, altitude (m), 3-axis rotation accelerations (rad/s), 3-axis angles (deg), 3-axis G force (g) and temperature (°C). In addition, the Wheatstone bridges provided information about fin flex [12]. It was demonstrated that sampling rates varied between 13 to 80 Hz (depending on the number of sensors employed), while the live-stream data transmission was 100 ± 2 m (data not shown).

A typical loading—unloading cycle of an instrumented fin (up to 10 mm stroke) is displayed in Fig. 1D. The slopes (or “modulus”) of the loading and unloading parts of the cycles for the instrumented fins are approximately 3.6 ± 0.1 N/mm. The value observed for 3D printed fins without sensors is 3.5 ± 0.1 N/mm (data not shown). This suggests that the inclusion of sensors results in a small stiffening effect (~ 3%) compared to the mechanical characteristics of the 3D printed fins without sensors.

The output of the fin sensors in response to a controlled flex sequences is shown in Fig. 1E and F for the right and left fin, respectively. In these tests, the fins were subjected to fin flex loading/unloading of 10.0 ± 0.1% (equivalent to a perpendicular deformation of 13 mm [12]). The deformation of the fins was either towards the centre of the surfboard (referred to as inwards) or towards the rail (or edge) of the surfboard (referred to as outwards). It should be realised that both fins flex in the same direction when the right fin flexes outwards and when the left flexes inwards. Fin flex sequence VIII (see Fig. 1E and F) shows that this results in negative value for the fin flex of both fins. The opposite case (fin flex sequence IX) demonstrates that applying an inward flex to the right fin and outward flex to the left results leads to the sensors exhibiting positive values.

The instrumented surfboard with instrumented fins was used to monitor the performance of the surfboard and the response of the fins during surfing at a location in Australia. The inbuilt measurement system was used to record surfing performance data (13 waves), with a total surfing distance of 570 m (total ride time of 125 s) at speeds up to 6 m/s (see Table 1). Figure 2A shows the speed of the surfboard for a typical sequence of paddling, wave riding followed by paddling. The term “paddling” also include time spent waiting for waves, i.e. all speeds below 2 m/s. The data demonstrates that once the wave is caught (at time = 18 s, Fig. 2A) the speed increases over 3 s before decreasing gradually as the wave is ridden to completion. It is well-known that top speed is usually achieved during the initial take-off before completing the bottom-turn, a characteristic surfing manoeuvre leading to a cutback [7].

Fig. 2
figure 2

A Speed as a function of time for a typical time sequence for paddling, wave riding and paddling on an ocean wave breaking from right to left. B Fin flex for right fin as a function of time for the same paddling, wave riding and paddling sequence as in (A). C Fin flex for right fin as a function of time for the same paddling, wave riding and paddling sequence as in A). Spheres and solid lines in B and C indicate raw and processed (running average of 10 points) data, respectively. Vertical lines are to guide the reader’s eye to distinguish between paddling and wave riding sequences. Paddling sequences include waiting for waves

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Figures 2B and C indicated that both fins exhibit minimal flex during typical paddling sequences, i.e. below 2–3% fin flex. The speed during paddling (which includes waiting) is low, which suggests that forces (applied load) acting on the fins is small, leading to minimal deformation in the fins. In contrast, during distinctive surfing manoeuvres, such as bottom turns, the fins in the surfboard encountered maximum flex of up to 8–9%. The data in Fig. 2 is generated while surfing a wave breaking from right to left. Initially (time range 20–23 s) both fins exhibited flex values (up to 8–9%) indicating that both fins are flexing in the same direction. In other words, the fin closest to the wave face displayed an inward flex (bending towards the centre of the board), while the opposing fin exhibited an outward flex (bending towards the edge of the board). This is followed by both fins flexing in the other direction, i.e. up to -5% flex over time range 23–25 s. The maximum amount of flex exhibited by the fin gradually decreases with decreasing speed during the remaining part of surfing the wave (time range 25–34 s). These observations were consistent with the data generated on the other waves. In our future work, we will correlate fin flex behaviour to time-synchronised videos of surfing maneuvres on waves.

Conclusions

In conclusion, this study successfully outfitted a surfboard with an integrated (inbuilt) measurement system connected to instrumented fins containing flex sensors. This facilitated the measurement of fin flex during surfing manoeuvres on ocean waves. The inbuilt measurement system, comprising GPS, accelerometers, a gyroscope and flex sensors, enabled data collection during surfing at a sampling rate of up to 80 Hz.

The collected data revealed that both fins exhibited noticeable flex during surfing, particularly when the board was riding ocean waves at surfing speeds. In contrast, minimal flex was observed during paddling at lower speeds. Commercial Wheatstone bridge sensors recorded fin flex values of up to 10% as the fins experienced loading and unloading during surfing sessions at speeds of up to 6 m/s. It was demonstrated that fins experienced load perpendicular to plane of the fin during characteristic surfing manoeuvres such as bottom turns. In particular, the data demonstrated that when the surfboard is on wave breaking from right to left that both fins flex in the same perpendicular direction. That is, the fin closest to the face flexes inwards (towards the centre of the board), while the other fin flexes outwards (towards the rail of the board).

This paper establishes that telemetry can be used to measure flex in surfboard fins during surfing waves. It is envisaged that fin flex behavior can be used to link surfing performance to material performance.