Paddle or rake to improve your swimming stroke?

That's the conclusion of new research from the Netherlands in which a team of fluid dynamicists built and printed a 3-D hand model using the public domain software Make Human, tested it in wind tunnel experiments and then combined those results with computer-based fluid dynamics simulations. While previous studies suggested spread fingers do, in fact, boost swimming efficiency, this is the most comprehensive evidence to support the concept. The team presents their findings at the American Physical Society annual meeting, Division of Fluid Dynamics, held Nov. 20-22 in Portland, Oregon.

The increase in efficiency from spread fingers is small, offering 2 to 5 percent increase in the drag coefficient related to the thrust that powers a swimmer. "However, when you are a top swimmer, this very small effect, only a few percent, can make the difference between a gold medal and no medal at all," said doctoral student Josje van Houwelingen, a swimmer herself, who does research at Eindhoven University of Technology as part of a team that also includes researchers from Delft University of Technology and the J.M. Burgers Centre for Fluid Dynamics in the Netherlands.

Spreading the fingers gives a small efficiency advantage by obstructing flow with the spaces created between spread fingers. This increase in drag also increases thrust. The higher the drag coefficient, the more efficient the pull.

Researchers measured force and torque under five different conditions of finger spread in which the thumb remained in a fixed position. Measurement began with the closed position of 0 degrees all digits pressed together, similar to a paddle and fingers spread progressively wider through 5 degrees intervals to a maximum of 20 degrees of spread.

They took measurements on various spread conditions in both the wind tunnel and through numeric modeling. Because air and water both behave as fluids, a wind tunnel seemed an ideal setting for a fine-grained force analysis of a hand swimming in water, using two force sensors fitted in tandem.

Results favored a spread finger position. Compared to a closed paddle hand position, even the smallest spread-finger hand position of 5 degrees enhanced the drag coefficient by 2 percent in the numerical simulation, and by 5 percent in the wind tunnel experiment. Investigators also found the optimal finger spreading of 10◦ was the same in experimental and numerical simulations.

While the day-to-day practical effects may be small, given the few swimmers who swim well enough to benefit, the team sees a wider philosophical and aspirational benefit to this finding. "It may inspire other swimmers who are not Olympic-caliber to think about fluid dynamics, and contemplate the fluctuating forces which their hand and fingers experience while doing those boring laps."

The findings also set the stage for the next phase of research in which drag is analyzed under conditions of acceleration. Explains the team: "We will next put our hands in a big water tank and make realistic swimming movements using a robot, and again, combine this with numerical simulations."