Forces on the Tail Surface of Swimming Fish: Thrust, Drag and Acceleration in Bluefish (Pomatomus Saltatrix)

Abstract

1. Pressures on the right and left sides of the tails of swimming bluefish were measured and found to have a range of +5.9 to - 5.9 cm H2O. The pressures were resolved into their forward and lateral vectorial components of force to allow calculation of forward and lateral force and power at speeds ranging from 0.26 to 0.87 m/s. 2. The peak to peak changes in force of acceleration of the body, measured with a forward accelerometer averaged 209 g or 2.05 N at 0.48 m/s, and were compared with the maximum to minimum excursions of forward tail force averaging 201 g or 1.97 N at the same speed. The mean difference was 8 g, s.d. of the mean difference ± 29, s.e. of mean difference ± 10 g. 3. Mean tail thrust was calculated as the time average of tail force in the forward direction. It averaged 65 g, or 0.64 N, at 0.48 m/s. The mean forward power was 0.34 N m/s at 0.48 m/s. The drag of the gauges and wires accounted for 10% of this figure. 4. The mean lateral power of the tail was 1.28 N m/s at a mean speed of 0.48 m/s. 5. The propulsive efficiency of the tail, calculated as the ratio of forward power to forward plus lateral power, was found to be 0.20 s.d. ±0.04, s.e. ±0.01 and was not related to speed. This suggests that 80% of the mechanical power of the tail was wasted. Turbulence in the water may have contributed to this large drag and low tail efficiency.