Re: What causes 'boat trails' in the water?

They actually are wakes.  I know exactly what you mean, because I 
have wondered about them too – they aren’t the wave trains we’re used to 
that water skiers jump over, but much longer features trailing many 
kilometers behind.

Here are some images of ship wakes taken from the Space Shuttle, which include 
examples of what you are interested in. 

It turns out that wakes are pretty complicated things. In fact, this 
particular type of feature is not yet fully understood, because most 
dynamical solutions damp out to a level that should be unobservable this 
far downstream from the ship. What you are seeing is probably a non-linear 
interaction between some of the wake features I talk about below.

Being able to see these features requires a special combination of 
circumstances. First, the state of the sea itself has to be calm enough so 
that this feature is not swamped out. Second, the lighting angle has to be 
right. Notice in all the photos in the NASA link above that nearly all the 
wake features are most visible near the point of maximum reflection of the 
sun – since the sea provides a specular surface, the reflection of the sun 
is ‘smeared out’ over a wide area, providing brilliant illumination that 
allows you to see very fine details and differences in the sea state 
(things like eddies, wakes, squalls, etc.). Third, the ship has to be 
going the right speed, and passing over the right kind of water for non-
linear effects to appear.

A ship’s wake is composed of many different phenomena:
 
1. The familiar set of spreading waves in a 19.5 degree angle V behind any 
ship is called the “Kelvin wake” after Lord Kelvin, who gave the first 
rigorous description of it. There are actually a set of waves which cross 
the V too. These waves are what eat up most of a ship’s energy. These 
wakes are long-lasting, and far reaching. There are some fantastic 
pictures of Kelvin wakes here. 
Here’s a computer simulation of a Kelvin wake by Prof. Berry of Bristol 
University in the UK. 
A lot of research has gone into how to reduce this wake and the energy it 
drains from a ship – one result was that you can reduce the transverse 
waves in the set (and the energy required to produce them) by putting a 
big bulbous part on the bow of the ship. But unfortunately, only at a 
particular speed. Here’s an 
interesting page on hull shapes and how they affect drag on ships.

2. The “turbulent wake” is all the white foam kicked up by the propeller 
wash/cavitation and the chaotic eddy shedding at the stern. This wake 
tends to wash out the Kelvin waves that cross perpendicular to the 
direction of travel. Here’s a picture of the turbulent wake caused by 
an aircraft carrier. Since it’s turbulent, it tends to damp out pretty 
quickly.

3. The “dead water” wake or “narrow-V” wake, which looks like the ship 
flattened out the waves. This wake is also always present, but is very 
hard to observe, since it’s flat! Turning ships often give a good view of 
it, and here’s a 
biggie. This is a big part of what you are seeing, but this wake alone 
cannot last as long as several kilometers.

4. All of the above are really the surface manifestations of what is 
really a 3-dimensional process – all of these wakes have a portion below 
the surface that is just as complex. One of the large parts of the sub-
surface wake is a set of twin vortices that are shed from the stern. They 
are very hard to see in water, but a parallel type of phenomenon is easily 
observed in airplane wakes, here.

5. Now we get to the weird stuff. At certain speeds, ships can set up long 
solitary waves (solitons) that precede the ship. You can think of 
them as the “draw-down” before the Kelvin wake comes crashing in. If you 
want to read some more about these, go get this PDF file.

6. The next complication is the structure of the water itself. As you 
probably know, very often water is stratified in layers of different 
temperature, salinity, turbidity, etc. This creates conditions where waves 
can diffract and reflect internally in these layers making all sorts of 
complicated effects. These can be classed as “internal wave wakes.” In 
some cases, if there is a layer of material (oil, say) on the top surface, 
this disrupts surface tension forces that cause certain wave/wake 
phenomena – these are the “oil slicks” which are generally flatter than 
surrounding waters. A detailed analysis of surfactants and radar returns 
is here.


As you might imagine, all of these things are of great interest to people 
who want to know where ships are, and how to track them. Not only the 
military and the coast guard, but also ship traffic management and 
companies that are tracking their ships’ progress (like express mail 
services that use GPS to track the whereabouts of vans, trains, etc.). 
Radar is very good at this, since it reflects really well from metal 
corners, but also very poorly from the exact features you are asking 
about. In radar images, what you see is a very bright ship followed by a 
very dark, long streak. To find out more about radar and ship tracking go 
here
 and 
here.

So..... the long streaks of calm water that reflect the sunlight better 
than the surrounding water are most probably a complicated non-linear 
interaction of the narrow-V, the vortices, and some internal wakes that 
depend on the exact structure of the water column.

I hope this helped, Lonnie!