In Boxfish, the keels that are present on their rigid bony carapace were hypothesized to provide the boxfish with favourable hydrodynamic properties, such as a relatively low drag coefficient and passive course stabilisation, enabling them to swim smoothly through turbulent water. However, later studies could not reproduce these results, discovering that the carapace shape is inherently unstable. An alternative hypothesis was put forward, namely that the presence of keels increases the body’s resistance to roll rotation by increasing rotational drag and added mass. This is in better agreement with their ecological needs in the spatially complex coral reefs, as they spend considerable time manoeuvring during foraging, and permanent course stabilisation for pitch and yaw rotation would make manoeuvring energetically costly while resistance against roll rotation has no disadvantage of this kind. Drag coefficients, pitch and yaw moment coefficients, rotational drag moment coefficients for roll rotation, and added mass moment coefficients were examined in five species of Ostraciidae. Boxfish models with keels of reduced height and sharpness were used to gain insight in the function of the keels by the degree to which the presence of keels determined hydrodynamic properties of the entire carapace, and whether or not the shape of the keels have a drag reducing effect for straight swimming, (de)stabilising effect for pitch and yaw rotation, and additionally if the presence of keels do increase the resistance of the body against imposed roll rotation. Computational fluid dynamics simulations were made for the adjusted boxfish surface models, whose keels were digitally removed, in a water flow. The models were placed parallel to the water flow to extract drag forces, and placed under a slight pitch or yaw angle (mimicking deviation of alignment) to extract pitch and yaw moments, respectively. Transient simulations were made of the boxfish undergoing an imposed roll rotation in still water to extract the rotational drag moment and added mass moment. Simulations were identically performed for the original boxfish surface model, as control. No relationship could be discovered between the presence of keels and passive stability or drag reduction, as these parameters were only very weakly affected by the presence of keels and differences being inconclusive. However, all species showed a strong increase in roll resistance by the presence of keels, with for some species the rotational drag moment coefficient for roll rotation being doubled. This study confirmed that the boxfish body is inherently unstable for pitch and yaw rotation, although it seems unlikely that the keels play a prominent role in shaping these characteristics. Evidence that keels play a part in drag reduction during swimming could not be found either. On the other hand, although additional research is needed to obtain statistical proof, our results strongly suggest that keels play a major role in increasing the body’s resistance against roll rotation. This observed increase in resistance against roll rotation should not be confused with roll stability which was not examined in this study. |
