## 4 Discussion  
### Lift and Drag of the Investigated Commercial 4-Fin (T:80/A:150) Configuration  
In principle, the lift forces cause a torque around the center of mass of the surfboard that forces the board to align with the main direction of the wave propagation directed to the shore. Considering the interaction between the surfer and the board, the lift constitutes the force that the surfer has to overcome to deviate the board from its actual direction to perform a turn maneuver. Thus, for high stability during the maneuver, the absolute value of the lift force and its gradient with regard to the AoA are the most relevant parameters. Comparing the lift coefficient and lift force between the 4-fin and the 3-fin configuration reported in [11], the 4-fin configuration produces a lower lift coefficient in the AoA range up to the stall point. However, the overall absolute lift forces produced by the 4-fin configuration are higher than those of the 3-fin configuration; see Figure 17a. Figure 16: (a) Lift-to-drag ratio versus the angle of attack for six entire fin configurations. The rear fins are fixed in one axial position and shifted in six positions of the transversal direction of the surfboard. (b) the lift-to-drag ratio versus the angle of attack for five entire fin configurations. The rear fins are fixed in one transversal position and shifted for five positions of the axial direction of the surfboard. Furthermore, the increase in force with rising AoA is also much more significant, meaning that the surfer gets a much larger and clearer lift feedback. Both effects indicate a better stability and controllability of the board by the surfer. However, the surfer has to apply much more work to perform a turning maneuver, which may impede the performance of fast maneuvers, as described in [1]. Analogically to the lift coefficient and the lift force, the drag coefficient of the 4-fin configuration is lower, but the overall drag force is higher than that of the 3-fin configuration; see Figure 17b. Moreover, reaching higher AoAs after stall, the drag force and the force fluctuations become dominant, as reported for the 3-fin configuration [11]. These fluctuations of the lift and drag forces originate from the vortex shedding, which potentially leads to the reduction of stability; see Supplementary Materials Videos SV1-SV6. We suppose that in the 4-fin configuration, the drag force and the force fluctuations are more significant than the ones in the 3-fin configuration, showing the worse controllability and stability of the 4-fin configuration in high AoAs. This again indicates the better applicability of 3-fin configurations for the performance of fast and advanced maneuvers. These findings support our Hypothesis 1. The trends of the lift coefficients of the OF profoundly influence the lift coefficient of the entire configuration. The drag coefficient of the rear OF significantly decreases after the stall point and therefore positively affects the drag coefficient of the entire configuration, causing it to stay low and the velocity to stay high during a turn. The higher the AoA during a turn maneuver, the more velocity a surfer loses. The trends of the lift and drag coefficients of the four single fins and the entire configuration follow those of [11; 12; 13] and show the trend of the basic airfoil theory [16; 47]. Comparison of 3- and 4-Fin Configurations Using FCS Accelerator Fins from This Study and [PERSON] et al. 2019, and FCS k2.1 Fins from [PERSON] et al. 2009  
This section of the discussion compares the lift and drag coefficients and the lift-to-drag ratio of the entire 3- and 4-fin configurations using (1) the _FCS Accelerator_ fins from this study and [PERSON] et al. [11], and (2) the _FCS k2.1_ fins that were used in [PERSON] et al. [13]. The 3- and 4-fin configurations from [11] and this study are the commercial 3- and 4-fin positions of _THE DREAMCATCHER_ surfboard.