My water aerobics instructor says to press my fingers together to increase resistance. The total surface area is the same with fingers open or closed, so why does it make a difference?
Stephen Johnson
Eugene, Oregon, US
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The question of whether or not fingers should be pressed together to increase resistance, or drag, in water sports has been debated for decades. In a lifetime of competitive swimming, I first encountered this discussion in the early 1970s, although at that time there wasn鈥檛 any real science on the subject.
Since 1979, there have been numerous experiments and simulations studying this topic. The current consensus is that drag is increased with the fingers held slightly apart compared with when the fingers are held together.
This is because water still isn鈥檛 able to easily flow between the fingers, and the hand now acts as a slightly larger paddle. In addition, the hand is more relaxed in this position, making it easier for an athlete to maintain.
David Muir
Edinburgh, UK
Fluid dynamics can be counter-intuitive due to the effects of boundary layers.
A boundary layer is a region of fluid moving relative to a nearby surface. It is slowed by the fluid鈥檚 viscosity and its adhesion to the surface.
If a swimmer or water aerobics participant wants to maximise the force generated by their hand moving through water, they should spread their fingers by a few millimetres 鈥� 10 degrees between each. Water won鈥檛 flow smoothly between the fingers due to their boundary layers, which act like invisible webbing, thus increasing the effective size of the hand. Watch underwater video of elite swimmers to see their innate feel for the water with their slightly spread fingers.
Good golfers of the past knew that scratched and roughened golf balls flew further than perfect spheres long before they knew anything about fluid dynamics. The dimples on modern golf balls affect the boundary layer between ball and air, decreasing resistance and allowing for longer golf shots.
Nick Canning
Coleraine, County Londonderry, UK
Different shapes, even those presenting the same cross-sectional area perpendicular to their motion, will experience different resistance.
A hand with fingers pressed together in a flat plate experiences greater resistance than the five separate cylinders of spread fingers, due to the different patterns of turbulent flow they cause in the water, which dissipate energy differently. Greater energy loss means you have to work harder to maintain the same speed of your stroke.
Think of the old, boxy shape of an early car compared to the round shape of a modern one. The old one experiences greater air resistance than the new one for the same area and speed, due to the different fluid flow patterns.
Steve Agar
Ninebanks, Northumberland, UK
Imagine trying to scoop up a handful of sand. You will end up holding more with your fingers closed than with them open.
Similarly, when you move your hand through water, you have to move some water out of its way, and the resistance you experience arises from the amount of water that obstructs your hand.
With fingers open, this amount of water is less, so the resistance experienced by the hand is less.
Pat French
Longdon-Upon-Tern, Shropshire, UK
When the fingers of your hand are spaced apart, they present a series of five curved fronts to the water as it flows between them. Even though the surface area may be similar, your individual fingers are more 鈥渁qua-dynamic鈥� than the flat paddle that is formed by fingers held tightly together.
The looser formation means that water escapes, rather than providing the resistance against your arm鈥檚 movement that drives you forward.
Looking at my own hand while pressing my fingers close together, I see that it tends to curve or dish as I squeeze. This might further impede the water flowing out of my palm and provide even more thrust. A water wheel fitted with cupped blades 鈥� a so-called 鈥� is, similarly, more efficient at creating drag than a flat-bladed water wheel.
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