Hydrodynamic Force Measurement on Surfboards During Surfing Maneuvers

Abstract

Surfing is a complex sport that encounters the interaction between the surfer and the highly unsteady water hydrodynamic of ocean or river waves. With the availability of off-shore river waves or wave pools, it becomes more and more popular being included in the Olympic Games 2021 in Tokyo/Japan. To analyze the biomechanical characteristics of surfers, all forces that act on the surfer have to be measured which has not yet been done before.

Four pressure sensors were integrated water-tightly in a 3D-printed surfboard fin, each two sensors on opposite side. The fin was mounted at the rear end of a surf board on the left side of the centerline within a three-fin-configuration. The sensors were connected to a micro controller with micro SD-card within the board that performed and stored the measurement data with an acquisition rate of 27 Hz. For the measurements, a male surfer performed a set of turning maneuvers on an artificial river wave as shown in figure 2. Thereby, the surfer’s motion was recorded by a camera using frame-rate of 30 fps. The synchronization of the pressure and the video signals was achieved in a post-processing step.

The pressure signals showed a negative pressure difference between the inside sensors and the outside sensors at all surf maneuvers, see figure 3. It generated an outwardly directed force that becomes largest during the turns. The smallest pressure difference was detected on the surf-path from the left to right side of the wave or the other way around.

Taking both lateral fins into account, the pressure forces are contrarily directed apart from the surfboard. This arrangement generates the stabilizing effect of the board’s rear end which helps the surfer to keep the board on the desired track. Especially during the turns, the forces became largest giving the surfer a stable center of rotation at the rear end of the board. This stabilizing effect of surfboard fins was reported and observed in computational models [1,2,3]. The measurement of hydrodynamic forces enables to develop a biomechanical model of a surfer that show the characteristic biomechanic stress level peak and location within the surfer’s body.

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