Drag Forces

In general, the drag increased with increasing velocity (Figure 3.6). The variation in data also increased with increasing velocity. No transient drag peaks were observed at the higher recording frequency of 1000 Hz, and so the lower recording frequency of 10 Hz was sufficient for capturing all relevant velocity-dependent changes in drag forces. The highest recorded forces during the flume tests were almost 300 N for the two largest individuals (i.e., individuals II and V in Table 3.1). The increase, however, often deviated from the second power of the velocity as predicted by the standard equation for drag (Equation 3.1) and was nearly linear.

Correlation tests of drag and the four measured morphometrical parameters for flume specimens yielded only low correlation coefficients (Figure 3.7). The best correlation with drag was found with length (R2length = 0.63). Planform area and mass both showed a slightly lower correlation with drag (R2area = 0.58 and R2mass = 0.58), whereas only a poor correlation was found between drag and volume (R2volume = 0.36). The correlations, however, improved considerably by taking the wave-dependent morphology as an additional independent variable into account so that the combined information on length and wave-dependent morphology of individuals (exposed or intermediate/sheltered) gave the best correlation with the measured drag forces (R2length + wave exposure = °.71).

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