# Wake watching at Ratzeburger Segelschule

The waves that ducks (or ships, or anything else moving through water) make are called “wakes”. And wake watching is so much fun! A wake is pretty much the Mach cone in 2D — waves from the duck can’t propagate faster than the leading edge of the wake. When a wake passes by, that’s the moment you would hear the supersonic boom of an aircraft in the Mach cone analogy. That explains why the wake develops as a feathery V with the duck at its tip. Or several ducks at the tips at several Vs, as in the picture above. How awesome is that?

But wake watching is usually more complicated than just Vs with stuff at their tips. See for example below — two ducks with Vs, two row boats also with Vs, but with the additional eddies where their oars pushed through the water.

See how different the two pairs of wakes look?

Or here, we have wakes again, but they are now occurring on a lake surface that shows more different waves: The half circles that my feet tapping on the water made! And, if you look closely, you can spot algae growing in the lake in the foreground of the picture.

Isn’t it fascinating how you see the algae in the foreground, but the further you look towards the other shore of the lake, the more you see the reflection of the sky or the other shore instead?

That’s due to a phenomenon called “total internal reflection“: For light that hits the interface between two different materials (air and water in this case), there are angles at which it can cross the interface (at steep angles we can look from the air into the water and see the algae), and others where it can’t (at small angles, we cannot look into the water, light gets reflected at the lake’s surface and we see the opposite shore instead). This fascinates me time and time again!

Note how the duck in the image above doesn’t actually have a wake? That’s because it’s swimming too slowly for the wake to develop — it is just radiating ring waves in all directions.

Below, this is kind of a boarder line wake — we can see the V developing, but se also still see the ring waves in the spot where the duck first started swimming.

But of course, wakes are only straight Vs when the ship is driving in a straight line, too, otherwise it will get deformed like below.

And just because I love this picture so much: Here is a wake coming across the lake

# Some #wavewatching in Manchester

I’m in Manchester for Science in Public 2019 and I couldn’t help myself, I had to do some wave watching.

To be fair, though: These pictures really don’t do Manchester justice as a city. It is such an amazing city! Last time I was here, I spent a whole day exploring four historical libraries that were breathtakingly beautiful, and I would totally recommend you do the same if you are here, even before wave watching. And that is saying something! And I love all the architecture here, and the Science and Industry Museum! But my blog is about Adventures in Oceanography and Teaching, so I am not showing you that side of the city here, only the river Irwell and some reflections of buildings in it.

First: A storm drain run-off into the river. Do you see the waves radiating away from where the water drips into the river?

And here is a “before” picture of the river…

…so you appreciate the “after” picture with all the cute little waves made by raindrops. (No irony here — I really enjoyed watching this!)

And it does look pretty, doesn’t it? I especially like the wave rings on the boundary between the dark reflections of the buildings and trees, and the brighter reflection of the sky, blurring the line, bringing the sky and the city together…

Oh, and one of my favourite wave pattern: The V-shaped wake of a row boat and the pairs of eddies, rotating in opposite directions, where the oars pushed through the water!

# Wake comparisons: Row boat and motor boat

I just love this picture: The two boats in the front are going at the same speed (the trainer is driving right next to the person in the row boat over a long distance), yet look at how different the two ships’ wakes look!

The motor boat has this huge, breaking, turbulent wake. Even though it rides so high up in the water, it displaces a lot of water and creates a wake with a large amplitude (how large the amplitude is is visible in the picture below, where some poor people were sitting in row boats when a motor boat sped past. But also here: Look at how cool these feathery waves that constitute the wake look together!).

But then, going back to the original picture (which I am showing again below) — look in contrast at the row boat’s wake. You see the paired eddies where the oars were in the water, and you see a tiny little trail where the body of the ship went. But that’s all. Yet both boats are going at exactly the same speed! Pretty cool, isn’t it? (Also pretty scary how much energy the motor boat is spending on moving water and moving a larger hull and a heavy engine rather than just propulsion when the payload of both boats is more or less the same — one person)

# Wakes by birds, a row boat, and a SUP board

After talking a lot about turbulent wakes this week, here are two pictures of different kinds of wakes. They are of course turbulent, too, but on a very different scale.

In the picture above, see how you see pairs of eddies on either side of the row boat’s wake? That’s where the oars were in the water! But this wake stays visible only for seconds, maybe a minute. Nothing you would be able to see for a long time from afar!

Same for the waves the birds made. Can you still spot that they were swimming in the same direction as the row boat and then made a 180° turn? Maybe you can, maybe you can’t, but that’s how quickly those wakes vanish.

Now below: This is a very interesting wake. Since the SUP board is pretty much flat on the water and doesn’t displace a lot of water while moving through the water, it pretty much only creates the V-shaped wake, not a turbulent one the way a ship does when it’s displacing a large volume of water in order to get forward.

Think I’ve said everything there is to say about waves? Well, then just enjoy this one from the ColorLine ferry that sailed past a couple of minutes ago… How beautiful is this? :-)