Scientists from The Netherlands have investigated which hand position is more effective in the water: spread fingers and raking the water or closed fingers using the hand as a blade. Their findings suggest spread fingers has the greater efficiency.
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To investigate the best hand position, the team of fluid dynamicists built and printed a 3D hand model and tested it in a series of wind tunnel experiments. They then combined these results with computer-based fluid dynamics simulations.
The increase in efficiency from spread fingers was small, however, offering 2-5% increase in the drag coefficient related to the thrust that powers a swimmer.
“But when you're a top swimmer, this very small effect, only a few percent, can make the difference between a gold medal and no medal at all," explained Phd student Josje van Houwelingen, who was part of the research team.
The team found that spreading the fingers increases efficiency because the rake position increases the drag of the hand and therefore reduces the slip velocity between the hand and the water. This diminishes the power dissipated for propulsion and, as a result, increases your swimming efficiency.
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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° – all digits pressed together, similar to a paddle – and fingers spread progressively wider through 5° intervals to a maximum of 20° of spread.
They took measurements on various spread conditions in both the wind tunnel and through numeric modelling. Because air and water both behave as fluids, they chose a wind tunnel as an ideal setting for a 'fine-grained force analysis' of a hand swimming in water, using two force sensors fitted in tandem.
Compared to a closed-paddle hand position, even the smallest spread-finger position of 5° enhanced the drag coefficient by 2% in the numerical simulation, and by 5% in the wind tunnel experiment.
The team of researchers included scientists from Eindhoven University of Technology, Delft University of Technology and the J.M. Burgers Centre for Fluid Dynamics in the Netherlands.