Dead water phenomenon receives new explanation

[Orson]

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 What makes ships mysteriously slow down or even stop as they travel, even though their engines are working properly? This was first observed in 1893 and was described experimentally in 1904 without all the secrets of this 'dead water' being understood. 

An interdisciplinary team from the CNRS and the University of Poitiers has explained this phenomenon for the first time: the speed changes in ships trapped in dead water are due to waves that act like an undulating conveyor belt on which the boats move back and forth. This work was published in PNAS on July 6, 2020 


https://phys.org/news/2020-07-dead-w...henomenon.html

I’m sure I’m not alone in not grasping this new explanation. There’s a graphic at the link. So, please weigh in if you can help us out?

[Lake-Effect]

Originally Posted by Orson

I’m sure I’m not alone in not grasping this new explanation. There’s a graphic at the link. So, please weigh in if you can help us out?

I always thought that boats slow down in direct proportion to how badly their skippers want to go fast.

I hadn't heard about this til you posted it. From the article, it seems that waves form underwater where waters of two different densities mix, and these submerged waves can act just like big waves at the surface in impeding boat motion.

[hpeer]

I’ve read about it before. Sounds really weird.

The offered explanation do any really explain anything. Too vague.

[Lodesman]

I've never heard of, nor experienced this phenomenon - and I have spent a significant amount of time in freshets/glacial run-off. Interesting but not intuitive as to what happens. Found a neat video on Wikipedia:

https://www.youtube.com/watch?v=bzcgAshAg2o

[fatherchronica]

I believe that this phenomenon is caused by killing albatrosses.

[Lodesman]

Originally Posted by Orson

So, please weigh in if you can help us out?

Have you looked at the youtube video I posted? As near as I can figure it, when the underwater wave has built up sufficiently, so that a large trough develops right at the stern, the surface water at the boat is drawn rearward in order to fill the hole. You can see it if you look closely at 1:24; it's even clearer with the 3-layer experiment at 2:17 and 2:29.

Of course, once forward motion has reduced or stopped, wave-making ceases, the underwater wave flattens, the sudden surface current stops, and the vessel makes headway again - only to start the process all over again.

[Hoodsail]

Lodesman great video showing the effects of differing layers and associated wave formation=dead water. I looked up specific gravity of sea water and found one reference noting, that it varies, but 1.025 is a good estimate. SP varies by temp, found that at 79F, SP=1.017 that increases to 1.02 (at 85F). The model uses layers that are significantly different in density of 1.0007 (surface) and 1.0225 (lower layer), so unsure if this differential is possible in the natural environment.
Have i ever experienced "dead water"?? havent a clue...and if i did probably attributed to my inattention in trimming my sails or heading to close to the wind.
thnks

[Kelkara]

Originally Posted by Hoodsail

Lodesman great video showing the effects of differing layers and associated wave formation=dead water. I looked up specific gravity of sea water and found one reference noting, that it varies, but 1.025 is a good estimate. SP varies by temp, found that at 79F, SP=1.017 that increases to 1.02 (at 85F). The model uses layers that are significantly different in density of 1.0007 (surface) and 1.0225 (lower layer), so unsure if this differential is possible in the natural environment.

The article claims the phenomenom was first observed by Nansen in 1893 with a density change (pycnocline) caused by freshwater lying over sea water ... this matches much closer to the densities used in the model, and is not uncommon in nature.

[billgewater]

Of all the physical laws we utilize, the most powerful one is probably the law for Conservation of Momentum. It works irrespective of any energy change considerations or weird internal shenanigans. So I would like to see how this law can be consistent with a boat that slows down. To me it would require that something with equivalent momentum moves in the same direction to retain the same total momentum. I suppose it depends on what closed system is being considered.

If you consider the crew on the boat as a closed system, then if the boat slowed down then they should be reasonably aware of the event, i.e. their momentum should be conserved, or Newton's First Law of Inertia should apply. So it should be recorded in ship's logs.

[Paul Howard]

I have experienced something like this when operating heavy displacement motor yachts in the 55 to 80ft. LOA range.
One can be motoring along quite happily and then go over a shallow spot, such as a mud bank in the IntraCoastal Waterway. I was told it was due to 'bottom effect' where water moving with the hull hits a shallow spot, say five feet or so below the boat keel. The boat can suddenly veer off to a near broach, rapidly slow down and roll from side to side.

[HankOnthewater]

I have experiences like Paul described above, mainly in motor boats, and even more so in kayaks. I too contributed that to the ground effect. Never looked into the physics of such effect. Maybe it is time.

[CATJAVA]

I can only add from what I learned in my oceanography classes at the US Naval Academy.

This phenomenon was first noted by fishermen in the fiords of Northern Europe. Even though their rpms were constant, they noticed their speed thru the water sometimes drastically decreased. It was later discovered that if the discontinuity of the layer of fresh water over the underlying salt water was at the level of the propeller, most of the power from the prop went into the formation of internal waves at the discontinuity to the rear of the boat, not affecting the layer at the front of the boat since it’s not a closed system like in a “wave tank”.

Sometimes these internal waves can have very large effects. For instance, the catastrophic crushing of the USN submarine Thresher when she was driven deeper than her “crush depth” was determined to be from a breaking internal wave caused by an underwater landslide off the East Coast of the USA. They didn’t know what sub they “heard” being crushed for at least a week or two when subsequently all the subs reported in. As a consequence all USN subs now have their “noise signature” recorded after any repairs/modifications are done. I know this because this recording was one of my jobs as an ocean engineer in the 1970’s in California off Santa Cruz Is.

I don’t really know about the experience of meeting a sudden shallow while motoring in the ICW and being thrown off course, but I would suspect the fact that when an incompressible fluid hits an immovable object it has to go somewhere, either up and/or sideways and you get a free ride to whatever direction it’s moving. Evan on JAVA

[Richard5]

The subject here is what is known as the 'Hall effect', named after the brothers who studied this phenomenon in a wave tank in Edinburgh in 1830. Some years later it was Ekman who investigated tales told by fishermen in the fjords of Norway.

Basically, all objects moving through the water expend a part of their energy in making waves (rise in pressure). In the Hall effect, the waves are generated at the interface between waters of differing densities such as fresh water overlaying heavier salt water. Vessel passage through the upper layer results in a rise in pressure at the subsurface interface which induces wave propagation.

Because the subsurface waves move through a denser medium, the subsurface wave travels at about 1/8th the speed as through air/water interface. This results in more than one wave being formed (wave train). The vessel becomes 'trapped' between waves. An increase in vessel's power results in taller waves rather than increase in velocity. This effect can be observed where melting ice will flood over salt water or at river mouths. The way out of this is to reduce vessel's power which allows the following wave to catch then pass the vessel. However, Ekman suggested a SOG >5kts to avoid being 'trapped'.

Submarines have noticed the effects too whereby their SOG increases or decreases relative to where the vessel is on the subsurface wave front.


*a search using 'internal wave formation' as key words will turn up for good reading.

*Vagn Walfrid Ekman was a Swedish Oceanographer who had compiled a great amount of research. His study of fluid dynamics yielded us the Ekman spiral and predictions of ice berg routes in aid of charting sea ice. He was a contempory of another great scientist, Fridtjof Nansen and they did voyage on the R/V Fram.

[dannc]

Originally Posted by HankOnthewater

I have experiences like Paul described above, mainly in motor boats, and even more so in kayaks. I too contributed that to the ground effect. Never looked into the physics of such effect. Maybe it is time.

I have had this happen in a kayak as well when moving from deep to shallow water or over a patch of tall grass that is a foot or two under the water surface. I could be going 5 mph and hit the shallows or grass, and it would be almost like hitting a wall when the speed of the kayak suddenly slowed down. After I figured out what this was happening it was kinda interesting when I would suddenly slow down I would know I entered shallow water.

I do not think this is the same think as deep water though but an interaction with displacement of water as the kayak passes with the bottom or grass.

Later,
Dan

[Richard5]

When the wave generated by a vessel encounters a rapidly rising sea bottom, the wave will be refracted as determined by the sea bottom contour. Exactly how the water will flow, and how turbulent, is a factor of vessel speed and bottom contour. This is not to be confused with the Hall effect.