Economy and efficiency of swimming at the surface with fins of different size and stiffness

The aim of this study was to investigate how fins with varying physical characteristics affect the energy cost and the efficiency of aquatic locomotion. Experiments were performed on ten college swimmers who were asked to swim the dolphin kick while using a monofin (MF) and to swim the front crawl kick with a small-flexible fin (SF), a large-stiff fin (LS) and without fins (BF, barefoot). The energy expended to cover one unit distance (C) was highest for BF (C=10.6±1.8 kJ m−1 kg−1 at 0.8 m s−1) and decreased by about 50% with LS, 55% with SF and 60% with MF, allowing for an increase in speed (for a given metabolic power) of about 0.4 m s−1 for MF and of about 0.2 m s−1 for SF and LS (compared with BF). At any given speed, the fins for which C was lower were those with the lowest kick frequency (KF): KF=1.6±0.22 Hz at 0.8 m s−1 (for BF) and decreased by about 40% for SF, 50% for LS and 60% for MF. The decrease in KF from BF to SF–LS and MF was essentially due to the increasing surface area of the fin which, in turn, was associated with a higher Froude efficiency (ηF). ηF was calculated by computing the speed of the bending waves moving along the body in a caudal direction (as proposed for the undulating movements of slender fish): it increased from 0.62±0.01 in BF to 0.66±0.03 in SF and 0.67±0.04 in LS reaching the highest values (0.76±0.05) with MF. No single fin characteristic can predict a swimmer’s performance, rather the better fin (i.e. MF) is the one that is able to reduce most KF at any given speed and hence to produce the greatest distance per kick (d=v/KF). The latter indeed increased from 0.50±0.01 m in BF to about 0.90±0.05 m in SF and LS and reached values of 1.22±0.01 m in MF.