Beautiful morning arriving back in Kiel… Looking downwind, the weather might seem pleasant (especially when focussing on the sunrise).
But looking upwind however, the wind rows on the water as well as the white caps on the waves indicate that it’s quite windy!
Very cool: the turbulent wake of a ship interrupts the wave field and therefore, with its different surface roughness, is clearly visible!
And below you see so many things: The sand bank running from the lighthouse towards the next headland becomes visible as waves are breaking on it. The turbulent wake of that blue ship we saw above already is still clearly visible, as is its V-shaped wake. And you see our own wake as the feathery pattern that runs all the way from the bottom edge of the picture to way behind the blue ship!
And here our own wake becomes even more prominent as we turn. Laboe in the background…
Here is another ship, waiting to enter the locks of the Kiel canal. It’s moving only very slowly (so hardly any wake visible), but you see how it’s sheltering the water from the wind so the downwind water appears completely smooth right at the ship!
And here are some more wakes and sheltered spots of water surfaces. Locks of the Kiel canal in the background!
And another look at the locks. Do you notice how the wind rows still indicate that it’s quite windy, but how it’s a lot less windy than it was further out?
And then we are in the Kiel fjord. This is the upwind shore — see how waves are only slowly forming and building up with longer and longer fetch?
And then in the sheltered port a different kind of waves: Our bow propellers mixing the inner Kiel fjord!
Wave watching from high up gives you a whole new perspective on wakes, and depending on the lighting, features in the wave field become more prominent or fade away.
See for example below the ferry: You very prominently see the turbulent wake right behind the ship, and you see the waves of the wake opening up in a V-shape.
Above, there is still a lot of ambient light from the sky. Below though, the same ferry, similar spot, 30 minutes later: The turbulence is a lot harder to see since colors fade away, but the V-shaped wake becomes really clear since one slope of the waves reflects the city’s lights while the other reflects the darkness.
Another ferry coming in, another wake… Below the surface roughness becomes clearly visible with the turbulent wake right behind the ferry and the bow waves fanning out.
That was one brilliant mini cruise! Thanks for joining me, Frauke, and for staying out on deck with me — despite the freezing temperatures — until we were far out of the port and the light was gone completely. The sacrifices we bring in order to wave watch… ;-)
Having a bulbous bow alone does not always lead to the same bow wave. Which is fairly obvious when you think about it, of course the speed of the ship or the shape of the bow influence the wave field that is created, but also how heavily the ship is loaded, i.e. how deep the bow is in the water.
What you can see very nicely on the sequence of pictures of bows and bow waves in this post are bulbous bows going from fairly far out of the water (above) to fully submerged (towards the end).
And I just love the sharp contrast of the smooth water piling up and then the turbulence and breaking waves right there. Interesting example of subcritical and supercritical speeds, btw: The ship travels faster than the bow wave (so the bow wave can’t overtake the ship, but always stays behind it, forming a two-dimensional Mach cone).
The ship in the picture below is the odd one out in this blogpost: It does not have a bulbous bow but just pushes water in front of it. This is an interesting example of a bow shape that is clearly not optimized for energy efficiency when traveling large distances, but then the purpose of that ship is obviously a different one. But isn’t it amazing how such a small ship creates waves larger than all the other much bigger ships do, just because they have better bow shapes?
But beautiful wakes nonetheless. I love those tiny ripples riding on top of the wakes!
And, of course, the checkerboard pattern of a wave field and its reflection.
Here is another example of a ship with a bulbous bow, this time it is almost submerged. Since they are designed to be fully submerged, this ship is loaded in a way that is closer to what it was made for, and you see that the generated waves are smaller than the ones in the pictures up top.
And look at its wake — really not a lot going on here, especially when compared to the much smaller ship a couple of pictures higher up in this post!
Now for a ship that is hardly creating any waves at all, the mountain of water that it’s pushing in front of its bow looks especially weird since the bulbous bow isn’t visible any more.
See? (And isn’t it cool how the chronological order of pictures in this post just coincided with ships laying deeper and deeper in the water? I love it when stuff like that happens :-D)
And then, of course, I had to include some more pictures of beautiful wakes…
Do you see, comparing the picture above and below, how the first one was taken when the wake had just reached the shore, and the second one the wake was reflected on the shoreline already?
Not many things make me as happy as wave watching :-)
P.S.: Ok, one last bonus picture (non-chronological, we saw it some time during the walk. But that’s ok, I wasn’t going to include it until the post was already done and I decided that you just HAD to see this): Someone who is clearly not using their bulbous bow to their advantage. But at least I get to show you what they look like when they are not in the water. And we got to speculate about how annoying it is to be on a ship with such a strong tilt all day :-D
Just kidding. Below you see a movie of a neat interference pattern I observed this morning. The situation is similar to yesterday in that the ferry has sailed past and the wake runs up on those bathing steps. But: today it’s quite windy and the wind waves’ crests are perpendicular to the crests of the ferry’s wake. Check it out:
That’s the kind of stuff I loooove watching! Happy New Year, everybody, may there be plenty of wave watching in 2019!
P.S.: Am I the only one who always wants to write fairy when writing about ferries? :-D
Ending 2018 in style and exactly the way I want to continue in 2019: wave watching and dipping into Kiel fjord!
2018 has been an exciting year and a lot of changes that will shape 2019 to be very different from anything I have ever done before have already been set in motion. But despite all the new adventures, some things will stay the same: Stay tuned for ever more adventures in oceanography and teaching that I look forward to bringing to you!
The German Society for Chemistry and Physics Education (GDCP)’s annual conference started out with a 2 hour cruise on Kiel fjord, during which the participants had the opportunity to choose between enjoying the sunshine and just doing whatever they liked, and several “guided tours” on either the sights of Kiel in general, or the biggest sight in Kiel, the water :-)
Preparation is everything: charts to learn to observe waves
I had prepared laminated charts to be used in case for some reason the weather wasn’t cooperating, and they definitely came in handy even during beautiful sunshine. On those charts, I used pictures that you’ve seen on this blog before, and contrasted them with typical physics text book illustrations, either sketches or ripple tank photos.
Since I was fairly busy talking, I only snapped two pictures:
The Oslo ferry right after turning inside the Kiel fjord, right when it starts backing up towards its berth, and, more interestingly, the turbulent wake. See how messed up the wave field is? It’ll take quite a while for all that turbulence to dissipate and for the sea surface to look as if nothing ever happened there!
And then our ferry’s wake. Here we see the turbulent propeller wake and one side of the feathery wavelets of the V formed by the wake.
Beautiful day to be on a ship!
Now, if you’d like to do a guided wave watching tour, you know where to find me… ;-)
Here is the wake of the little ferry that goes across Kiel canal.
I love how you can see the different parts that a wake consists of: The V with the ship at its tip that consists of wavelets from the bow wave and that spreads outwards. And then the turbulent wake where the ship has physically displaced the water when sailing through, and that then has been thoroughly mixed by the ship’s propellers.
This second, turbulent wake actually changes the water’s surface for quite some time. Propellers put the water in rotation and it slowly entrains surrounding waters, and this turbulent motion looks substantially different from the “normal” sea surface. It can even be spotted from satellites long after the ships are gone. You’ve probably sometimes noticed streaks on the sea even though no ships were present — those might well have been the remains of wakes!
But speaking of ships that have sailed…
Wake of a ship that sailed past a while ago
Here is another example of a wake being visible quite some time after the ship has sailed past. However what we see here is the feather-y train of wavelets from the V reaching the shore.
While we were looking at it, my friend mentioned that the waves seemed to approach a lot more slowly than she had expected. That’s because the movement we first notice is the phase velocity of individual wave crests. But when you look closely, you can’t follow one individual wave crest for a very long time, it always appears and you have to start over. That’s because the signal, the V itself, only moves with the group velocity, which moves at half the velocity of the wave crests. That really looks confusing! Unfortunately I didn’t get a good movie of this. But there is always next wave watching session! :-)
And a mystery wave!
But then what would any wave watching session be without a riddle.
Any idea what caused the wave pattern below? Not the obvious, larger waves, but the concentric circle segments that radiate outwards from somewhere in the bottom right?
This is a case where it is really helpful when you recognize where the picture was taken, because there is some important information missing from the picture: The straight edge continues on for a little to the right, and then it opens up to a fairly long channel coming in.
However what you do see is the wind direction from the way the water is smooth right at the shoreline and then ripples start to form as you look further away: The wind is blowing out onto the water.
Now combine those two informations and you understand how that wave pattern was generated!
Diffraction at a “slit”
Wind-generated waves move as (more or less) straight crests out of the channel that opens into Kiel fjord just outside the right edge of the picture above. Then, suddenly, they aren’t bound at the sides any more, and what happens looks like diffraction at a slit (except the slit is fairly wide in this case): The straight crests turn and form 90 degree circle segments that radiate outwards. Voila!
So many people are surprised when I speak of wave watching as of a “real activity”. But to me it is! So I am going to talk you through a couple of minutes I spent looking out on the water where the Kiel Canal meets the Kiel fjord, right outside the locks at Kiel Holtenau.
A light breeze across the fjord
The “light breeze” part is fairly easy to observe: There are ripples on the water, but no actual waves. “Across the fjord” is also fairly obvious if you look at either side of the wave breaker: On the fjord side, there are ripples, on the shore side, there are none (or hardly any), indicating that the wave breaker is sheltering the shore-side from the wind (and dampening out the waves that come across the fjord).
And then: A ship sails into view!
We watch the ship sail past, dreaming of foreign countries and exciting adventures.
A ship leaving a wake
Behind the ship, the water looks very different from what it looks like everywhere else. The wake is turbulent and waves radiate outwards like a V, with the ship always at its tip.
Then, the ship is gone. But we can still see where it went.
There are no waves in the tubulent wake
The ship’s path is completely smooth. No waves have invaded the turbulent waters of the wave just yet, claimed them back. However, the waves the ship created in that V are about to reach the wave breaker.
Also the wind has picked up a little, as evident from the less smooth water surface shore-ward of the wave breakers.
Diffraction at a slit
Right after the waves from the V reach the wave breaker, they reach the opening at the end between the pylons. And what happens now is that the waves get diffracted at a “slit”: they propagate outwards as semi circles, even though the wave fronts were straight when they reached the slit.
How awesome is that? And all of this happening in a matter of minutes!
The weather changes
I said earlier that there was hardly any wind. Later that afternoon, it still wasn’t very windy, but the wind direction had changed: now the smooth and sheltered part has moved to the other side of the wave breakers. There are a lot more waves on the shore side of the wave breaker now, the ones with crests parallel to the wave breaker due to it moving, and the ones with crests perpendicular to it generated by wind. And you see gusts of wind on the sea side of the wave breakers in the different surface roughness.
So if you were wondering, too: That’s the kind of stuff I look at when I am wave watching. And I still find it super fascinating and relaxing at the same time! :-)
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 :-)