## Materials and Methods  
### Surfing Hydrodynamics  
For a wave height hwave of 1.5 m and a corresponding wave velocity uwave of 4 m/s, a typical surfer's velocity usur is approximately 6 m/s [31], as shown in Figure 1. Increasing the wave height increases the wave velocity and, consequently, the surfing velocity--up to a certain limit [32]. The following basic equations describe the velocity uwave of a shallow wave Equation (1) [33] and the velocity of a shallow wave with the wave height multiplied with an empirical factor of 1.28, see Equation (2) [34]. The velocity of the surfer is described with two different equations Equation (3) [35] and (4) [34]. Whereas the velocity in Equation (3) is only determined by the speed and height of the wave without considering the prevailing drag of the surfer-surfboard system, Equation (4) is based on the kinetic and potential energy of a surfer and the wave. All velocities are absolute values. Figure 1: The velocity of the surfer (dashed lines) and the wave (solid lines) versus the wave height at the breaking point of the wave. The green dots show the simulated inflow velocities of 5, 7.5, 10, and 18 m/s, which were used in this study, and the corresponding wave heights of 0.9, 2.8, 6.0, and 25.6 m. The red dot shows the Guinness World Record of [PERSON] in 2017 with the confirmed wave height of 24.38 m (80 ft.). The small subfigures show corresponding waves of Hossegor (France), Pipeline (Hawaii), Mavericks (California), and Nazare (Portugal) in order of the maximum reachable wave height. \[\text{u}_{\text{wave, [PERSON]}}=\sqrt{\text{g}\cdot\text{d}}\text{ with d }=1.28\cdot\text{h}_{\text{wave}} \tag{2}\]  
\[\text{u}_{\text{surfer, [PERSON]}}=\frac{1}{3}\big{(}\text{u}_{\text{wave}}+ \sqrt{\text{u}_{\text{wave}}{}^{2}+256\cdot\text{h}_{\text{wave}}}\big{)}, \tag{3}\]  
\[\text{u}_{\text{surfer, [PERSON]}}=\sqrt{\frac{3.28\cdot\text{g}\cdot\text{h}_{ \text{wave}}+1.13\cdot\text{t}\text{s}\cdot\sqrt{\text{g}^{3}\cdot\text{h}_{ \text{wave}}}}{1+\text{c}_{\text{D}}\cdot\text{A}_{\text{Surfer}}\cdot\frac{ \rho}{2}}} \tag{4}\]  
g is the gravitational acceleration, h\({}_{\text{wave}}\) is the wave height at the breaking point, s is the gradient of the steepest surfable part of the wave's pocket; see Figure 2, (1.0 for 45\({}^{\circ}\) of the slope of the wave), t is the time in the wave's pocket (we chose t = 2 s), c\({}_{\text{D}}\) is the drag coefficient in the air of the surfer (1.0 for a standing human), A\({}_{\text{surfer}}\) is the reference area of the surfer (1.5 m\({}^{2}\) for a standing human), u\({}_{\text{surfer}}\) is the surfer's velocity, and \(\rho\) is the fluid density. Nevertheless, an increasing board speed is also increasing the overall drag of the surfboard that is limiting the surfing velocity. The Guinness World Records confirmed in 2017 that [PERSON] surfed a 24.38 m (80 ft.) wave [36]. Putting this wave height into the basic kinetic and potential energy equation (see Equation (4)), the velocity of the wave results in 17.5 m/s, leading to the assumption that, currently, u\({}_{\text{surf}}\) has a maximum of around 18 m/s [37]. When the wave peels to the right from a viewing perspective of the surfer looking to the shore, the wave is called a right-hand wave, and when the wave peels to the left the wave is called a left-hand wave [38]; see Figure 2.  
After dropping into the wave, the surfer gets up and surfs down the pocket of the wave with the surfing velocity u\({}_{\text{surf}}\): see Figure 2. To stay in the wave's pocket (energy zone), the surfer performs a bottom turn, which is a common maneuver that brings him/her back to the pocket of the wave. In everyday surfing, the surfer surfs up and down the wave in alternating order traveling with the wave. The declaration of the fins within the investigated 4-fin configuration and for the basic surf maneuver of a bottom turn analyzed in this study are as follows, see also Figure 3a,b:  
Figure 2: [PERSON]. [PERSON] surfs a bottom turn in a left-hand wave at Uluwatu (Bali).