A theoretical and experimental analysis of diver technique in underwater fin swimming

This research was theoretical and experimental, with an objective of a better understanding of the physics of fin swimming. The theoretical work followed Lighthill’s slender body theory (1960). Video measurements were made on underwater fin swimmers swimming in an annular pool (58.6 m in circumference). This study considers only SCUBA divers. Five male swimmers swam at five speeds between 0.4 m/s-0.8 m/s. Skilled divers consumed less oxygen and had lower kick frequencies at each speed. The skilled divers adhered to the requirements of the theory, but not the unskilled. Specifically, the fin’s trailing edge (TE) lateral velocity was greater than the relative velocity of the water at the TE (requirement 1); which is the most forgiving. The second requirement is more stringent, and requires the TE lateral velocity and the relative velocity of the water at the TE to be of the same sign. Violating the first requirement sometimes causes a change in the instantaneous thrust’s direction. Violating the second requirement almost always produced instantaneous thrust in the opposite direction than the diver’s desired swimming direction. This study found that the Lighthill model can be used to describe diver fin swimming, which was optimised when the diver met the two requirements required for efficient thrust.