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Traditional marine advice says that, if one falls into cold water, it is better to stay still rather than try to swim (unless the shore is so close that you are absolutely able to make it) to extend the survival time because heat loss is greater when swimming than staying still.

However, I have also heard that, in cold water, people need to swim FAST in order to keep warm and prevent hypothermia by generating more body heat, and slowing down means trouble.

These two are contradictory sayings. Is there any scientific research confirming which is true? Does it make a difference if one is a marathon swimmer who is trained to swim 6 hours or more in 15 - 16°C, compared to a swimmer who only swims in warm (>24°C) pools or sea?

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Because swimming takes energy that your body could be using for heat and instead uses it for movement.

The more energy you use in cold water, the more your body cools off. If you cannot climb out of the water, conserve body heat by remaining as still as possible and reducing the amount of your body exposed to the water. Protect your critical heat loss regions: the head, sides, armpits, and groin. Do not swim unless shore, a raft, or an overturned boat is nearby. Swimming accelerates heat loss. Remaining still in the water increases your survival time.

Source

Swimming may give a feeling of warmth but it accelerates muscle cooling. The body may produce more heat when swimming but it is also more quickly lost from the arm and leg muscles. Once these muscles cool, swimming becomes more difficult or impossible.

Source

Note that if you have a life jacket, then the Heat Escape Lessening Position is recommended.

  • I think I have seen a research studying the effect of stroke rate, in that searching when the subject swam at a faster rate, his core temperature actually increased, while at slower rate, the core temperature decreased. (I'm digging back out the source) – Michael Tsang Jan 25 at 2:54
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    The core temperature of someone swimming quickly will increase, but for a short time only. Think in terms of energy: if you have a fixed amount of energy you can: a) use it to shiver and stay still for long periods of time, or b) use it quickly to increase core temperature for a short amount of time, what do you think is better? If you see the shore and can quickly swim towards it then do so, but if you're unsure about how long you'll remain under water, shivering and conserving energy is a far better strategy. – QuantumBrick Jan 28 at 12:24
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    in calm water, if you can remain still a warmer layer of water forms around your body. The rate of heat loss from your body is related to the difference in temperature between your body/skin and the water it is in contact with. When that warmer layer forms, the difference in temp goes down, so the rate of heat loss is reduced. Think of it like a virtual wet-suit. Moving/swimming breaks up that warmer layer and exposes your body to the colder water all the time - which leads to maximum heat loss. – That Idiot Jan 29 at 18:25
  • Assume the water is not so ice-cold, e.g. 13°C, where channel swimmers train for long distance. If you decide to swim immediately at the fastest pace you can, you may be able to keep yourself warm without hypothermia until you run out of energy, e.g. 2 hours; in contrast, if you decide to stay still, you may be able to stay concious for longer hours, but you will not be possible to swim, or even do useful task if needed after maybe half an hour when the body enters the protective mode. – Michael Tsang Feb 14 at 5:26
  • If you see a shore about 1 - 2 km away but no civilisation there, and you are not 100% sure if there are any landing spots and/or current keeping you away, should you take the gamble or not? If you gamble your life and make it, you have escaped from immediate danger and you can plan for long-term survival, however if you don't, it means certain death. If you don't gamble your life, you are basically hoping for someone to rescue you before the hypothermia timer is up. – Michael Tsang Feb 14 at 5:30
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If you are swimming or vigorously treading water, two things will happen that make you colder:

  1. significantly increased rate of blood flow out to the moving muscles in the arms and legs, which are long and thin and have a lot of surface area to dump heat
  2. increased heat transfer from your skin to the water, because a constant flow of new cold water will replace the still water immediately next to the skin which has already been warmed by your body. This will happen anyway but happens faster if you are moving.

My understanding is that whether this is a good or bad thing depends on how cold the water is and how much heat you’re generating. An Olympic swimmer training in a pool might prefer something on the lower end of normal pool range (i.e. still not very cold).

I’m honestly not sure in an open water race whether it’s a performance issue, a comfort issue, a safety issue or just something athletes say to psych themselves to go hard. It could be that when you’re swimming hard, the endorphins numb the perception of cold. There’s a chance these effects are non-linear, and the difference in heat loss between floating/swimming is large, but moderate/fast is smaller, and if the difference in heat generated rose with speed in a non-linear way there could be some speed at which the lines of those graphs cross and speed is favorable. If I find a link I’ll post it. But there may not be a rule of thumb; it may depend on exact water temp, race length, and physiology of the individual.

But an ordinary person in a dangerous survival situation would not want this enhanced cooling.

  • However I have heard that channel swimmers have to keep their feed stops short, because once they are at feed stop, they stop swimming which produces body heat which will eventually lead to reduced swimming performance, and also reduced heat generation, causing a vicious cycle which eventually lead to hypothermia. – Michael Tsang Feb 14 at 5:15

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