'Evil water' linked to mysterious drownings

newscientist.com ^ | 17 December 2008 | Matt Kaplan

[Joiseydude]

It may sound like a superstitious excuse for a poor day's swimming, but it is not uncommon for triathletes to complain that the water is behaving badly - even that it is "evil". Now a study suggests what they are feeling is real.

Leo Maas, a fluid dynamicist at the Royal Netherlands Institute for Sea Research, and colleagues found that "dead water" - an obstructive effect encountered by ships at sea - can strike swimmers too.

As ships sail over a layer of warm water sitting over saltier, or colder, layers, waves form in the boundary between the two layers. As these waves grow, they form a gulf beneath the ship, sucking away its speed. This effect can stall boats at sea, reducing their speed by up to 80%.

[Mr. K]

freaky

[MarkL]

Fluid dynamics is always quite interesting...

I’ve experienced the “superheated water” effect in a microwave oven, where water is heated well beyond the boiling point, but it doesn’t form any bubbles, and appears to not be boiling. Break the surface tension and WHOOSH! It flashes to steam and boils violently in an instant!

Mark

[bitterohiogunclinger]

But we have the verizon network....

[JoeProBono]

[1rudeboy]

I can’t see this affecting drownings unless it affects buoyancy somehow.

[Salamander]

Horse latitudes ping.

[Locomotive Breath]

Worn out swimmers drown.

[1rudeboy]

I just don't find it very likely that this effect can "slow down" a swimmer to the point of exhaustion. (And if a swimmer exhausts himself--past the point where he or she should switch to a different stroke--then any drowning becomes attributable to "pilot error").

It's ok to compare a swimmer moving through the water to a ship doing the same. To not consider displacement in the equation is sloppy.

[TalonDJ]

I can’t see this affecting drownings unless it affects buoyancy somehow.

Serious triathletes swim multipule miles at a time. Given a big patch of this sort of water and suddenly their workout becomes 15 to 40% harder than they expect. So they wear out overly fast and suddenly get into trouble for no apparent reason.

[techcor]

bump for later

[ClearCase_guy]

When the still sea conspires an armor
And her sullen and aborted
Currents breed tiny monsters
True sailing is dead
Awkward instant
And the first animal is jettisoned
Legs furiously pumping
Their stiff green gallop
And heads bob up
Poise
Delicate
Pause
Consent
In mute nostril agony
Carefully refined
And sealed over

[TalonDJ]

To not consider displacement in the equation is sloppy.
They did not compare them with equations. They did it experimentally.

[holden]

I suppose if you're a super-triathlon geek, you pace every event to the very edge of your ability to endure. If you have to expend an extra 30% of your energy budget, on which you didn't count beyond 10%, it could bring you to a in-water collapse....
thrash, thrash, thrash, thrash...
ploomp!
HF

[1rudeboy]

If what you state is true (no reason to dispute it), then this wave effect would only affect open-water, competitive swimmers. Unless it is demonstrated (this article does not) that open-water competitive swimmers are drowning en masse (ok, I exaggerate), then hypothesizing that ships (large displacement) cause the same effect as swimmers (small displacement) seems to be a stretch.

I'm not disputing that an experienced swimmer can feel "dead water," mind you. I'm simply questioning whether it leads to drownings.

[1rudeboy]

full disclosure: I am not a triathlete (I just know a thing or two about swimming)

An accomplished swimmer (the sort that swims in triathlons) should be capable of staying afloat indefinitely . . . despite their low(er) body fat.

Someone needs to point to a case of a triathlete drowning before one can start jumping to conclusions about its cause. And again, if this wave effect is so pronounced to affect these swimmers, then more of them would be getting into trouble.

[Salamander]

Good to see somebody “Awake!” this early....;]

[TalonDJ]

Did you read about the phenomena? It has nothing to do with displacement. It is a wave induced by the bodies (ship or person)'s movement. Only the wave is induced on the layers or temperature or salinity. The surface stays pretty smooth. So a big wave builds up behind the swimmer and induces drag.

Check out their article on ships and watch the video. The relative displacement does not effect the phenomena because they swimmers are effected by a thinner boundary layer than ships are. Much like large aircraft experience 'ground effect' at a higher altitude. It is partly displacement but mainly how deep the body goes (which is not JUST displacement). Follow?

Lots of things effect the development of boundary layer. Plus the layers probably have to be smooth and still the wave to be an issue. And they have to be JUST the right depth for the wave to cause serious drag. Too deep and the wave effect does not get near enough to the moving body and too thin a layer and the swimmer of boat breaks through into the lower layer of water.

So for the perfect conditions it takes a swimmer, alone, in calm water. In a triathlon there are so many swimmers the waves would be disrupted and not cause the drag. Much the way wing-lets are used to break up the drag caused by wingtip vortexes on aircraft.

Pull up the video of their test ship and it makes a lot more sense.

[1rudeboy]

Of course I read the article. These scientists created this condition in a test lab. The headline links the condition to "mysterious" drownings.

Last time I checked, triathlons are not run in test labs, and triathletes do not regularly drown otherwise.

The "crawl" is a competitive stroke, for anyone else reading along.

[COBOL2Java]

I’ve experienced the “superheated water” effect in a microwave oven, where water is heated well beyond the boiling point, but it doesn’t form any bubbles, and appears to not be boiling. Break the surface tension and WHOOSH! It flashes to steam and boils violently in an instant!

Put a stick in the water (I use a clean chopstick) before microwaving. It gives the bubbles something to cling to. Works every time! (got the tip from Alton Brown's "Good Eats" show on the Food Network).