MadSci Network: Physics Query:

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

Date: Sun Jul 8 02:15:33 2001
Posted By: Paul Filmer, Staff, Geosciences Directorate, National Science Foundation
Area of science: Physics
ID: 993896141.Ph
Message:
```
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!

```

Current Queue | Current Queue for Physics | Physics archives

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci