The water's lightness of touch and the physics of swimming

It was a most peculiar sensation. On getting out of the water, exiting the pool using the gently graduated steps, to re occupy my body and meet gravity. The transition was, just for a few moments, unmistakable and impossible to ignore. One moment, my body had felt weightless. The next, I met myself, and my (healthy) weight once more...

Physics was never my strong point. But I've spent enough time in the water, to have gleaned a few nuggets. Whilst the electromagnetic spectrum and I never gotten to be chums, buoyancy is something of a dear friend, and one I try to connect with as much as possible...

What neither myself nor my swimming student could quite believe was the degree of weightlessness we had just been enjoying, without even realising it. 

I recognise now a little better why it is that sometimes when I swim, I feel as though I'm flying:  Buoyancy, or upthrust, is described as the force on a body when displaced in a volume of fluid and is in the opposite direction of gravity; this buoyant force is what gives us the feeling of weightlessness when we’re in the water.

When swimming, we can enjoy and make the most of the upward force exerted by the water that opposes and supports our weight.  But, in order to enjoy this to the max, we have to first acknowledge and understand our own, individual, relationship with buoyancy.  Regardless of swimming technique, some of us have a natural advantage when it comes to floatation.

Buoyancy differs from person to person as various factors affect how a person floats...

The distribution of body tissue causes each individual to float differently.  When adipose tissue is concentrated more in one part of the body (e.g., around the thighs and hips of "pear-shaped" women) the centre of buoyancy moves closer to the centre of gravity, reducing the rotational sinking effect in the lower part of the swimmer.  Such individuals would need to devote less effort to force production to streamline the total shape.  Conversely, swimmers with very little fat below the centre of gravity, but with some above, will sink markedly.  They will need to work harder with kicking to maintain streamline while swimming.

The proportion of fat in an individual's overall physique will govern the tendency to float.  Long distance open water swimmers are benefited by being 'larger', not only because the extra adipose tissue acts as an insulation factor, but because it assists flotation.  Less energy is used for internal body heating, and for maintaining streamline.

The volume of air in the lungs has a pronounced effect. After inhalation, a greater amount of water is displaced without any increase in weight.  Thus, floating is easier when the lungs and chest are expanded but the angle of float will be increased.

As the relative proportions of the major body tissues change with age, so does a person's specific gravity.  Usually, specific gravity is lower in children and aged persons.  Therefore, it is flawed theory to expect a young girl to float with the same proportion and parts of her body above the water as a champion woman swimmer.

Females tend to have a lower specific gravity than males, because they are predisposed to having a higher percentage of body fat.

The degree of tendency to float causes inter-individual variations in floating positions. Synchronized swimmers tend to have low bone density, will often be larger than competitive swimmers, and have good lung volumes. These features facilitate the repertoire of skills and stunts they need to perform. Competitive swimmers are benefited by the same physical tendencies.

Racial differences. Asian Indians tend to be leaner and more prone to sinking, because they have little fat and a high percentage of bone and muscle in their physical make-up.  On the other hand, the Inuit are fatter and rounder, adaptations that minimise heat loss. That combination also makes them float well.  

The density of water also determines how a person floats.  Salt water is denser than fresh water.  A swimmer would float slightly higher in salt water than fresh water. Water can reach a saturation level of saltiness.  The Dead Sea is perhaps the most famous body of water that consists of salt-saturated water.  In it, people have no problem floating, usually horizontally.  

Having recently qualified as a swimming teacher, I realise now how much science is involved in refining technique to ensure proper alignment and maximise efficiency and streamline to support someone to develop effortless, easy, sustainable swimming strokes.

© Lucy Snowe Photography

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