Wakes of ducks

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 :-)

Awesome wake of high speed catamaran

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.

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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 all directions, then how come we see linear wakes?

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!)
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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…).
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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! :-)

Reading the water

Just because it’s fun! :-)

I’ve mentioned before that I tend to stare at water when nobody else seems to find anything interesting to look at. So just because I’m weird, let’s look at some more water.

For example here. What could have caused waves like those below?

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What could have caused this pattern?

Yes. These guys went past and what we see are both the circular waves caused by the oars and the stern wave of the boat.

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Rowing boat. Seriously, why would anyone want to go backward all the time???

Ok. So on to the next riddle: What could cause what we see below?

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Bubbles on water. What could have caused them?

Right, that was him:

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Alsterdampfer!

And this?

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More waves.

Yes! Him again!

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Alsterdampfer.

Does anyone see where we are going with this?

Correct. Here.

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Hamburg town hall.

And a last glimpse on the way back:

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Lombardsbrücke.

Isn’t this the most beautiful city in the whole wide world? :-)