## 4 Discussion
### Comparison of Two 3D Roving Models (BIFC vs. LIAC)
#### 4.1.2 Auxiliary Aspects of Rowing Movement  
During actual rowing outings, rowers usually may need to stop the boat. Thus, rowers immerse the squared blade into the water to apply a braking motion. In the simulator, expert rowers were also asked to simulate a similar scenario, in which they would immerse the squared blade into water to decelerate the boat (Question 2). In this aspect, the perceived horizontal braking forces were not perceived differently for BIFC and LIAC. Some rowers described that braking forces were higher than they expected, which also decelerated the simulated boat more compared to an actual boat. This observation may be explained by the fact that effects of the Froude number and the turbulence were omitted in both models. In real blade-water interaction, the immersed area of the oar blade in the water may reduce the drag and lift force coefficients due to a change of the Froude number [46]. In addition, the turbulence was also shown to reduce the drag and lift coefficients in a computational study [47]. Thus, both effects should be implemented to yield more realistic interaction forces. Another simplification was the geometrical modeling of the oar blade. In both rowing models BIFC and LIAC, the oar blade was initially modeled as a flat rectangular plate, i.e., the longitudinal and lateral curvature of an original rowing blade [34] design was omitted. Based on the feedback from pilot study rowers regarding the vertical forces in the catch and release, \(\varphi_{bias}\) was implemented in LIAC to account for a simplified form of a lateral curvature of the blade. However, due to the simplified blade geometry, the contact area between the blade and water surface increased. Thus, rendered horizontal and vertical forces above the water surface were generally perceived as higher for the simulator than a real rowing scenario. In real rowing, due to rolling of the boat to one side, rowers may sometimes unintentionally touch and slide the blade on the water surface when moving the oar from release to catch with a feathered blade. In the simulator, participants were asked to make a deliberate attempt to slide the feathered blades to simulate this scenario (Question 1). Rowers reported that the BIFC model rendered horizontal forces that decelerated the boat when the blade was slid parallel to the water surface (featured ear). According to the mathematical modeling of BIFC, the ideal forces along the \(B_{y}\)-axis should be zero. However, the lack of curved blade design in addition to the binary modeling of the oar immersion, i.e., either fully in or fully out of the water, might have facilitated the blade immersion during the feathered position, which may have induced the reported braking forces. Although LIAC was rated more realistic than BIFC for the horizontal forces resulting from feathered blade sliding, the mean rating was only above average (\(\mu_{LIAC}^{Q1}=5.95\)). Braking forces were also reported for LIAC; however, the magnitude was perceived as lower in comparison to BIFC. The reason for the reported forces for LIAC was the introduction of \(\varphi_{bias}\), whose initial value was set to \(2^{\circ}\) to simulate a rotated blade instead of a (laterally) flat one. Thus, especially a rapid movement towards the catch while sliding the blade on the water surface resulted small resistive forces. For both rowing models, the lack of curved blade design also resulted in excessive vertical contact forces. Naturally, the more parallel a blade is oriented with respect to surface, the greater the vertical forces become acting on the blade due to the water contact. In Question 3 and 4, experts were asked to rate the realism of the vertical forces resulting from water surface contact for a range of oar blade rotation (\(0^{\circ}\leq\varphi\leq 90^{\circ}\)). For BIFC, experts judged that immersion or tapping with all \(\varphi\) orientations rendered higher vertical forces than real rowing.