Mollendorf, J. C., Termin, A. C., Oppenheim, E., & Pendergast, D. R. (2004). Effect of swim suit design on passive drag. Medicine & Science in Sports &Exercise, 36, 1029-1035.

This study evaluated the passive drag resistance on male swimmers (N = 7) wearing five different swimsuits. Drag was measured during passive surface tows at speeds from 0.2 up to 2.2 m/sec and during starts and push-offs. The swimsuits varied in body coverage from shoulder-to-ankle, shoulder-to-knee, waist-to-ankle, waist-to-knee, and briefs.

Differences in total drag among the suits were small, but significant. In terms of least drag at 2.2 m/s, the swimsuits ranked: shoulder-to-knee, shoulder-to-ankle, waist-to-ankle, waist-to-knee, and briefs. The drag was decomposed into its pressure drag, skin friction drag, and wave drag components using nonlinear regression and classical formulations for each drag component. The transition-to-turbulence Reynolds number and decreasing frontal area with speed were taken into account. The transition-to-turbulence Reynolds number location was found to be very close to the swimmers' "leading edge", that is, the head. Flow was neither completely laminar, nor completely turbulent; but rather, it was transitional over most of the body. Pressure drag contributed the most to drag at low speeds (<1.0 m/s) and wave drag the least at all speeds. Skin friction drag contributed the most at higher speeds for shoulder to ankle and shoulder to knee suits, whereas pressure drag and wave drag were reduced compared with the other suits.

Implication. Increasing skin friction drag on the upper-body of a swimmer reduces pressure drag and wave drag by tripping the boundary layer and attaching the flow to the body from the shoulder to the knees. It is possible that bodysuits that cover the torso and legs may reduce drag and improve performance of swimmers.

[Editor's note: Swimmers in horizontal rigid postures were measured. Very seldom is there a time in a competitive swimming race where such a position is held. This is an instance of measuring a rigid body moving through water when in reality a dynamic body moving in two planes is what exists in competitive swimming. The difference being indicated here is that passive drag was considered in the study while in actual competitive swimming, active drag is the greater resistive force component and passive drag is very minor.]

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