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Heat loss through the head with cites. Ending that discussion.
USCG's Federal Requirements and Safety Tips for Recreational Boats
Immersion in water speeds the loss of body heat and can lead to hypothermia. Hypothermia is the abnormal lowering of internal body temperature. If your boat capsizes it will likely float on or just below the surface. Outboard powered vessels built after 1978 are designed to support you even if full of water or capsized. To reduce the effects of hypothermia get in or on the boat. Try to get as much of your body out of the water as possible. If you can't get in the boat a PFD will enable you to keep your head out of the water. This is very important because about 50% of body heat loss is from the head.
The Wilderness Medicine Newsletter - Frozen Mythbusters, Nov/Dec 2004
Now, what about hypothermia and heat loss through the head?
If the hypothermia victim is not shivering, they are at rest, and the heat loss through the head remains about 7%. But, this is important, if they are shivering, the percent of heat loss via the scalp can increase to upwards of 55%, so protecting the head well is a very important part of treating the hypothermia patient. And as you can imagine, the primary defense against the cold and hypothermia is vasoconstriction of the peripheral circulation, this shunts blood to the core, reduces circulation to the skin, and increases the percent of heat loss through the scalp.
#1 The difference is that the shivering hypothermia patient is indeed exercising, but they do not vasodilate the peripheral circulation; the shivering muscles increase metabolic demand and cardiac demand so the patients do increase their cardiac output; therefore, they do increase cerebral circulation; therefore, they do increase the percent of blood loss through their head.
#2 How does being in water change the equation?
Life-preserver, personal flotation device (PFD), research has shown that when in the water, if your head and neck are wet, you cool faster. This is why modern PFD’s hold the person in the water with their head and neck out of the water; even if unconscious, to decrease the rate of heat loss into the water.
#3 What difference does hair on your head or facial hair make?
None. In order for hair or fur to provide a protective thermal barrier, it has to be much denser than what we humans grow and it has to be in layers of different types of fur to provide a thermal barrier. Beards are great, but they do not keep you any warmer. And bald is beautiful.
Mariners, I've enjoyed reading your thoughts about how to keep warm. It gives food for thought. However, you do lose 50% of your heat through your head as you can see and Wilderness answered the why.
Mamu, the sailor.