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  • This episode is sponsored by NordPass.

  • NordPass takes the stress out of remembering your passwords and provides secure access

  • to all your online accounts.

  • Head to Nordpass.com/SciShow to learn more.

  • [ ♪INTRO ]

  • 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 sailorsand how it might still affect

  • swimmers today.

  • Fridtjof Nansen, a Norweigian sailor and future Nobel Laureate, coined the phrasedead

  • waterback 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, 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 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 boatsespecially 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 anywhereat 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 a 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.

  • Maybe you're not a swimmer, but you still keep getting stuck on simple things in life,

  • like remembering your passwords or pulling up your credit card details when you need them.

  • If that sounds like you, you might be interested in using NordPass to make your life easier.

  • NordPass is a secure app and browser extension that remembers your complex passwords and

  • credit card details and lets you access them from anywhere, so you can work and shop online with ease.

  • Like say you're interested in buying SciShow's pin of the month from DFTBA.com!

  • With NordPass, you won't need to enter your credit card information every month.

  • NordPass will also generate secure, complex passwords for you so you don't have to think about them.

  • It'll even auto-fill your online forms to save you the trouble of copy-pasting.

  • NordPass has native iOS and Android apps, and you can add the NordPass extension to your browsers.

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  • [ ♪OUTRO ]

This episode is sponsored by NordPass.

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