[TakeFive]

I'm wondering what you guys think of this. Have any of you ocean cruisers actually experienced it?

Melting Glaciers Lead to the Freak Phenomenon Of "Dead Water"

The video is pretty convincing, but as one who makes my living in low Reynolds number hydrodynamics, I am suspicious of using a little toy boat to simulate the a phenomenon that they claim happens to yachts and ships. ("...it stops a ship in the middle of the ocean")

The lack of bow wave suggests that they are pulling the toy boat slower at Fr<1. And Fr does not appear to depend on density or interfacial tension:

...but, maybe it does depend on density. The g0 constant in the denominator is a body force, which is taken as the gravitational constant for the air-water interface. But the video shows a stern wave forming at the interface between the freshwater upper layer and the saltwater lower layer. The density difference between the two is small (much smaller than the difference at the air/water interface), and the resultant body force that would tend to resist the formation of this wave is therefore small. So at that interface, the net body force between the two layers may be very small, resulting in Fr>>1, and leading to the stern wave forming.

I am skeptical whether this stern wave would ever happen under real-world conditions in the ocean. So her statement that "...it stops a ship in the middle of the ocean" could be junk science.

What do you think?

 

[travlin-easy]

I suspect you are right in your assumptions. In four years of cruising the worlds oceans while in the US Navy, I would have thought we would have encountered this at least once, but I can assure you that we never did.

Gary

 

[RichH]

Probably a balanced three-phase system going on here, not a two phase event as described and in which the towed boat has an apparent density (Mass/unit volume displaced plus velocity) and at only which at a balanced velocity and apparent density effect of boat is very carefully balanced.

If the effect were generalized one would expect to occasionally notice such effect at the mouth of fresh cold water rivers entering salt water, etc. etc.

My preliminary hypothesis from viewing the demo would be: not junk science but a balancing act between densities (including the apparent density of the boat) and its velocity (w/r to wave formation), including the balance of downward force refraction of waves off 'a bottom' (in this case a dynamic bottom via the higher density of the underlying saline water).

Rx - A balancing act of 3 phases which includes the hydrodynamic effects of the boat including its velocity.

Pretty neat! (they've got about ~6-7 simultaneous 'dimensions' going on here) But It would be better to have all the temperature, density, thickness, velocity, etc. etc. etc. values of this demonstration to allow repeatable experimentation and validation.

Great post, thanks.

 

[TakeFive]

My "junk science" comment is not directed at the video, since that clearly needed to very carefully done to demonstrate the effect. But her extrapolation of the bathtub toy to an ocean-going ship by stating "...it stops a ship in the middle of the ocean" is junk science IMO.

You make a very good point about the importance of another length scale in the form of the depth of the water in the demonstration tub. Just as gentle ocean swells turn into breakers at shallow depth, the presence of the boundary condition at the bottom of the tub probably encourages the formation of the stern wave much more than you would see "in the middle of the ocean."

 

[Stumble]

I don't know about the demonstration, but it is a real though rare phenomina. For details take a look at papers published by Prof. Maas at NIOZ (Royal Netherlands Instatute for Sea Research).

[0810.1702] Dead Waters: Large amplitude interfacial waves generated by a boat in a stratified fluid

 

[fryewe]

The video at io9 appears to be computer generated...there is no bow wave or any other disturbance of the surface caused by the model boat.

The submerged wave shown is traveling in the same direction as the boat and faster than the boat in water of greater density than that the boat was floating in, so the combined effects should be to lift the stern of the boat and impart a force in the direction of the boat's motion.

Described effects can't be due to propellor effects because the article says the boat was towed across the tank.

The boat used (modeled) appears to be non-displacement. Coupled with the non-displacement model, no mixing of the different density fluids occurs. Not real world.

How do you conduct such a test...ie, having seen Brownian motion at work, i question how such a clean interface between the differing densities of water can be maintained for the duration of such a test.

All the articles on the subject that I read after researching the topic did a bit of handwringing about loss of glaciers due to global warming and the sources were political rather than scientific. Hmmmm, I wonder why this dead water effect, reportedly first seen in situ over a hundred years ago, is suddenly a subject of discourse.

 

[fryewe]

I don't know about the demonstration, but it is a real though rare phenomina. For details take a look at papers published by Prof. Maas at NIOZ (Royal Netherlands Instatute for Sea Research).

[0810.1702] Dead Waters: Large amplitude interfacial waves generated by a boat in a stratified fluid

The link doesn't appear to go to Prof Maas's research, but to a Cornell link to the demonstration the OP provided.

 

[Capt Len]

Perhaps not quite the same thing and not near as complicated but I know of two examples in the real world. One was a large cargo vessel ,mid south Atlantic ,encountering a large fresh water lens from the Amazon .Suddenly unsupported by the less dense water ,she dove her bow under and was in serious trouble as crew was working in the open forward hatch.(told to me by old British seaman) I would think that a vessel, having slipped unnoticed into an invisible puddle of fresh , would be surprised at the effort needed to climb back up and out. Back in the day over laden pilchard boats would sink as they came into the Fraser (2 pounds/cubic foot of displacement difference. ) That maths is good enough for the girls I go with. As an after thought, consider what will happen to Europe if the Gulf Stream dives under a big lens off Ireland and returns south,,.cooling at depth all the way

 

[Stumble]

The link doesn't appear to go to Prof Maas's research, but to a Cornell link to the demonstration the OP provided.

Try ...

ASME DC | Journal of Offshore Mechanics and Arctic Engineering | Dead-Water Effects of a Ship Moving in Stratified Seas

Dead-Water Effects of a Ship Moving in Stratified Seas - ResearchGate

https://vincentduchene.wordpress.com/2014/06/25/96/#refDuc11

 

[MarkofSeaLife]

Its all true. Its happened to me a few time... But worse, I have been sucked under! My whole boat with me at the helm sucked 100 feet underwater. But fortunately each time a UFO has appeared and saved me

 

[jzk]

"Climate change is bad. Melting glaciers are also bad."

So when recreating in the arctic, there may or may not be an occasional time where we might lose boat speed for a few seconds?

That is the final straw. We need a global treaty on climate change, and we need it now.

 

[mad_machine]

not the first time I have heard of this phenomenon. I remember reading of a sailor in the gulf of mexico coming across a "lens" of fresh from the Mississippi. It stopped his boat dead from both the differing densities and directions and speed the two different types of water were moving at.

Very weird, I always wanted to experience it

 

[Minnewaska]

I'll take my chances on this phenom, if it were even remotely true.

The author starts the article with, "Climate change is bad".

It's made at least two or three full heating and cooling cycles over the life of the planet already.

It may suck, but it's neither good nor bad. It's just a fact. It's like saying gravity is bad, because you may fall.

 

[mbianka]

Doesn't the same thing happen when just a regular surface wave catches up to ones boat and passes underneath it? Or even a wake from big power boat? Does not matter the densities of the water as I recall.

 

[travlin-easy]

First and foremost, there is a significant amount of mixing of the freshwater and saltwater as those rivers, fjords and estuaries enter the oceans. That has been well documented many times. There would be no sudden blockade of freshwater on the ocean's surface that would impact a ships speed. And, because of the difference in densities between fresh and saltwater, the freshwater sinks to the bottom and the saltwater becomes a layer above it - just like the depths of Chesapeake Bay, Delaware Bay, etc...

I guess governments need to piss away more taxpayer dollars to keep burgeoning scientific communities gainfully employed, but for the life of me, I never understand why such ridiculous studies are employed by anyone with a single grain of common sense. i guess that my father's statements of many years ago still hold true. "Common sense isn't very damned common these days, and the average person is well below average."

Cheers,

Gary

 

[TakeFive]

...And, because of the difference in densities between fresh and saltwater, the freshwater sinks to the bottom and the saltwater becomes a layer above it - just like the depths of Chesapeake Bay, Delaware Bay, etc...

Wrong. You have it 100% exactly backwards. All other things being equal (temperature, etc.) higher salinity water is more dense.

 

[Stumble]

First and foremost, there is a significant amount of mixing of the freshwater and saltwater as those rivers, fjords and estuaries enter the oceans. That has been well documented many times. There would be no sudden blockade of freshwater on the ocean's surface that would impact a ships speed. And, because of the difference in densities between fresh and saltwater, the freshwater sinks to the bottom and the saltwater becomes a layer above it - just like the depths of Chesapeake Bay, Delaware Bay, etc...

I guess governments need to piss away more taxpayer dollars to keep burgeoning scientific communities gainfully employed, but for the life of me, I never understand why such ridiculous studies are employed by anyone with a single grain of common sense. i guess that my father's statements of many years ago still hold true. "Common sense isn't very damned common these days, and the average person is well below average."

Cheers,

Gary

Just because you don't understand why it's imporant doesn't mean it isn't. Most research like this is reviewed before being funded for usefulness. In addition there is something wonderful about expanding our knowledge about the natural world in the abstract, long before we understand how a particular piece of knowledge can be monetized. Much like going to the moon had no commercial purpose, but the knowledge gained has driven the worlds econmy for the last fifty years.

But to answer your questions the paper that did this research has had effects in the following:

1) iceberg effects on micro environment
2) gravity wave detection
3) electromagnetic research in striated fluids
4) basic hydrodynamics. Particularly in striated fluids
5) shallow water cargo vessel resistance
6) electromagnetic detection of ships wakes
7) wave generation in finite depth areas

Your right common sense isn't that common, and it is also rarely correct.

 

[aa3jy]

Wrong. You have it 100% exactly backwards. All other things being equal (temperature, etc.) higher salinity water is more dense.

Yep..that's why you'll find jelly fish in lower depths of the Bay as they ride the denser salinity levels up the Bay in the summer.

Also having dived the deeper Mexico's Cenotes where I've encountered haloclines, fresh water over salt layer, where ones dive lights are earily deflected off the denser salt water layer.

 

[Faster]

We sailed for over 20 years in an inlet fed by a sizable glacier field.. in spring the surface would be 'glacier green', but any larger vessel would 'stir up' salt water from below the fresh layer and leave a 'blue' wake.. our depth sounders would often read the interface at anywhere between 6-10 feet, esp where it was deep enough that a true bottom return was unlikely.

With daily twenty knot winds typical, I can assure you that surface waves were always present.. pushing hull speed they became significant, and I can't say we noticed much in the way of inexplicable losses in speed.

Of course, we wouldn't see the sublayer waves and they may well have been there, but I never saw a lack of surface wash if the boat was moving at any decent speed.

Perhaps the depth of the layer is important? or the relative depth of the hull? or?....

This satellite shot shows the fresh water extent quite well...

 

[travlin-easy]

Sorry I got the density thing ass backwards, guys. Sometimes my mind isn't functioning as well as I would like. I know, however, there is a great deal of mixing at the mouth of Chesapeake Bay and witnessed some experiments by National Marina Fisheries Service and ASMFC when they were trying to determine exactly where the crab spawn took place and what the ideal conditions would be for a high yield spawn. Having entered the mouth of Chesapeake and Delaware bays thousands of times in my lifetime, I would have thought I would have encountered this phenomena at least once. Never happened, though. So, common sense and practical experience do play a major role in sciences, just some of those folks don't believe it's the case.

All the best,

Gary