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

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 H20. 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. ±o-oi 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.