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Today, if you’ve got somewhere to go, you might hop in a car
or even take a plane
But for centuries, a boat was your fastest way of getting around.
Except when it suddenly wasn’t
for thousands of years, sailors have been telling stories of
a mysterious phenomenon called dead water
that could stop a ship in its tracks.
We now know that dead water is just, well, water
and after all these years
scientists finally understand why it used to mess with sailors
and how it might still affect swimmers today.
Fridtjof Nansen, a Norweigian sailor and future Nobel Laureate
coined the phrasedead water back in 1893
but he wasn’t the first to get stuck in it.
Stories of ships moving under full power
suddenly slowing or even totally stopping
date back to at least 600 B.C.E.
Sailors over the years have tried all sorts of ways to escape
from pouring oil in front of the bow to shooting at the water
and even banishing monks from their ships.
But, most of the time, if you’re caught in dead water
但是 多数时候 如果到达这片死水水域
you’ve just got to wait it out
After this had happened many times
people started to notice that dead water was more
common where it was cold.
For instance, Nansen’s ship, Fram, was sailing north of Siberia
when it got stuck
This incident helped point to what was really going on
Sailors were getting stuck in places where a layer of
cold freshwater was floating on top of the salty sea.
Like, maybe meltwater from a nearby glacier.
The two types of water act almost like totally separate fluids
and the boundary between them is a lot like the one
between the water and the air.
And that can make things complicated.
In 1904, the oceanographer V. Walfrid Ekman
1904年 海洋学家V Walfrid Ekman
showed mathematically that when a boat runs
into this two-layer system, it creates massive waves.
But those waves aren’t at the surface where water meets air
they’re at the boundary between saltwater and freshwater.
Which is why, to a sailor, the surface of the water looks perfectly calm
Creating these waves takes a lot more energy than
just producing the normal wakes you’re used to seeing around boats
especially when the boat is moving slowly
close to the speed of the waves themselves.
Those underwater waves will steal the boat’s momentum
and slow it down in the water
And as long as those waves are there
the boat’s not going anywhere
at least not fast
Ships can only escape once those waves dissipate
which usually happens when they run into the boat
Fortunately, if you’re traveling much faster than the underwater waves
this effect is pretty irrelevant.
You’ll just outrun the problem in the first place.
So most modern ships don’t have to worry.
But there is another kind of slow-moving vessel
that might still need to worry about dead water: you.
Among open-water swimmers
there are surprising cases of well-trained athletes
drowning of exhaustion in perfectly calm water
and some researchers wondered
if dead water could have anything to do with it.
In a 2009 study, scientists found that
swimmers moving through water with a thin, cool upper layer
lost up to 40% of their motion.
There’s still work being done in this area
but it could help explain why even strong swimmers can
sometimes get trapped in calm water.
So it’s something to keep in mind
if you go for s swim
and feel water layers of different temperatures
We asked an expert and, if you find yourself
trapped in such a situation
your best bet is probably to stay calm
and use a slow, shallow stroke.
And, you know, maybe don’t swim by any melting glaciers.
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This episode is sponsored by NordPass.