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In relaxed non-competitive swimming, it is possible to keep the head out of the water while gliding. This can be breaststroke or backstroke (or doing the Tarzan stroke in waterpolo). What is the physical principle for this additional lifting force that keeps the head out of the water?

I understand that generally displacement of water produces buoyancy, but typically not enough to keep the head out of the water. And I understand that treading water with arms or legs can produce upwards thrust to keep the head out of water.

But relaxed gliding without much effort in pushing downwards against water seems to also allow keeping the head out of the water. However when the glide motion stops, the same body posture does not seem to allow keeping the head out of the water.

I have 2 theories on how this works:

  • The water "hill" in front offers resistance to the moving body, the body is pushed up by this resistance like the nose of a speeboat.
  • the water below a swimmer flat in the water offers some temporary resistance against the sinking, and while moving forward the body constantly slips onto a fresh "table" of water that supports the body for a short while. That's same as when a metal plate is held under water and sinks to the bottom slower if it's horizontal than if it is held vertical.

Both would also explain why when the forward gliding motion stops, the body sinks (until the head is under water).

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  • $\begingroup$ I don't know what gliding is in the context of swimming. $\endgroup$ Aug 1, 2019 at 12:10

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It's hydrodynamic forces, similar to that of an airplane but in water. The mass of a human head is roughly 5kg. So if your body can provide 49N (5kg*9.8m/s^2) of lift (beyond what it had from buoyancy already), it can keep the head out of the water.

It is relatively easy to achieve these forces by moving forward. You will adjust your "angle of attack" in the water to create the correct lift forces. You support your head by driving water downward with the shape of your body. This is close to your "hill" analogy, only the water moves out of the way.

You mention "relaxed non-competitive swimming." You would want to avoid this in competitive swimming because it increases drag. It's hard to calculate how much drag it causes, but it will probably be less than 49N, because aerodynamic/hydrodynamic lift typically permits Lift to Drag ratios higher than 1 (a skydiver's squirrel suit can get to 2.5, a modern airliner is around 15). So the actual additional force you have to apply with your arms and legs to keep your head up is probably on the order of the force you'd feel lifting a 1-1.5kg (2-3 pound) weight in each hand. That's not a major exertion, but it would definitely qualify as waste for a competitive swimmer.

(Lift to Drag ratios are where the "hill" model breaks down a bit. I can't think of a good way to weave that into a model with a solid hill... maybe a hill made of ball bearings that move out of the way?)

You can get much more of your body out of the water too, not just your head. The price you pay is that you need to increase your angle of attack to get more lift, and that increases drag. When doing breast stroke, I can get a good chunk of my torso out of the water, for brief periods (not that one wishes to. Less drag is faster!)

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