My sister took this amazing picture — and from a train no less! And I got super excited. Can you see the feathery wavelets* of the bow wave of that large ship? And then the wakes of both ships, spreading out at the same angle? I should definitely start spending time on high bridges going across canals, there is so much unused potential for wave photography!
*I was super convinced at first that they were called “winged wavelets”. But then doubt started to kick in, so I asked google. Turns out they are called “feathery”. However, apparently “winged wavelets” is a very poetical expression, all google hits are in some poem or other! And one that I really liked by Mary Bamburg, where it goes like this (as part of a longer poem):
“… waves wring sand from the shore,
strew shells, strech after them
white wash and wild winged wavelets
glass green, blaze blue, slick silver …”
Does it create the same beautiful image before your inner eye as it does for me?
After having talked about wakes quite a bit recently, I’m going to show you a couple more pics today (bit not as many as I did two days ago, sorry! ;-))
And today we are focussing on the wake of one specific ship, the Stena Germanica that we sailed on for a mini cruise to Gothenburg (highly recommended!!). And even though that cruise sadly ended a month ago, I am still puzzled by what we saw:
Look at the turbulent wake directly behind the ship. Do you see how the two sides are markedly different from each other?
One side seems to be a lot more turbulent than the other.
And I didn’t find an explanation for that. According to the internet, the ship has two propellers and on each of those propellers the pitch of the blades can be changed so that the propeller can always run at a constant speed and the ship’s velocity is changed by changing the propeller blades’ angles.
Maybe that’s what is going on? But would they run two propellers at different pitches even when just steaming straight ahead?
Btw, look at that beautiful wake pattern! You can see both the feathery V going out from the ship and those half circles filling in the V. I don’t think I have ever seen that this clearly on a ship this size before. But then maybe I just wasn’t observing well enough.
I love it!
Now. The next morning: It is quite windy now, and the difference between the two sides of the wake is quite pronounced. Also very interesting: The foam on both sides is behaving very differently! One side (the upwind side) has a much sharper edge than the other.
But this surely can’t be the effect of just the wind?!
Looks great though!
Also very interesting: When arriving in Gothenburg, we met another Stena Ferry, and the same thing could be seen. Here she is after she passed by:
And her wake stays visible for quite a while with this marked foam stripe right in the middle of the wake, as if she had counter-rotating propellers that set up a convergence zone there?
Same phenomenon again when leaving Gothenburg later that day:
And arriving in Kiel the next morning, we could beautifully observe the different parts of the wake again:
What do you think is going on here? Why is the wake of the Stena Germanica not symmetric? I’m having sleepless nights over this :-)
When I said that wake watching made me happy last week, did you really think those were all the wakes I was going to show you? Ha! No, I have plenty more! :-)
Today, I want to show you a couple that have one thing in common: the way that they show up against the sun’s reflection and thus become a lot more visible than they would be if they were just reflecting a uniformly blue or grey sky.
I really like observing the wakes of ducks. Much easier to watch than those of ships because the scale is much more person-standing-on-the-sea-wall friendly. Also much less turbulence messing up the pattern. And you can get closer than you usually could.
All three pictures from the same morning, taken within a minute of each other!
Another awesome way to make my point about how waves look really differently depending on the angle one looks at them. Below for example my “other” way to observe waves: See how the wake leaves a shadow on the sea floor?
And some more ducks from a different day, because I like them :-)
This post has been in the making for a very very long time. I have now decided to stop overthinking and just share the movie with you, because who wouldn’t want to watch the wake of a high speed catamaran? This is from my not-so-recent-anymore trip to Heligoland.
Since I am too lazy to annotate, you will have to figure out by yourself what is happening when. But I give you this: It’s speeding, then slowing down, and then speeding up again. And in any case, it’s mesmerising to watch!
If waves spread equally in each direction along the water’s surface, then how come ships (or ducks) have wakes that are just those long lines of waves and not circular at all?
So. Kids are typically familiar with what it looks like when you throw stuff in the water (for proof see below: my godchild on a “Tour de Ruhr” where I learned tons of stuff about mining in Germany. I had no idea that stuff was so interesting! Anyway, I digress. Obviously you had to throw stuff in the water when the reflections are this awesome!)
But then wakes are seemingly behaving in a very different way. For a nice example of a wake, see the movie below. In that movie, you are looking backward from a boat at its starboard wake. The boat has been sailing straight ahead for a bit after turning to the starboard side (and you will see the resulting curve in the wave in the movie).
Even though slightly curved due to the ship’s change in heading, that wave clearly doesn’t look like a ring around the boat (from where I found the video on my phone I think it must have been a touristy boat in Bergen that I went on with my friend Leela).
So. Good question, isn’t it? Why does the wave look straight? Now don’t tell me it has something to do with interference and stuff, because I need to explain it to a young kid.
I have attempted an explanation, but I am really not sure if it works. What do you think? Check it out and let me know!
The image below shows a sketch of what it looks like if you throw a pebble into the water (or the pattern a raindrop would make). Ideally, we would only see one ring, but since a secondary drop is typically thrown into the air (and sometimes a tertiary) let’s work with three concentrical rings of waves so that the pattern looks as much as possible like what the kid would be likely to observe. The fading colors indicate that the second and third ring have a smaller amplitude than the first one (whose amplitude should be decreasing as time goes on, but let’s not get too technical here…).
So now how to go from the pebble to the wake? Continuous pebble drops!
From this we have the bow wave and the choppy water inside those two rays of waves. Of course, there we would also have turbulence due to the ship’s propeller or the duck’s feet etc, but maybe this is enough for now?
Except to add that those kind of waves are shock waves (the source of the waves traveling faster than wave speed) — in 3D and in air, the same physics would lead to sonic boom! :-)