Refraction of light in ship exhausts

Leaving the port of Gothenburg after our mini cruise, A and I obviously had to secure the prime spot for wake watching because I wanted to check out more of the weirdness I talked about in the last blog post. So while we were waiting for the ship to sail, we had the best view of the ship exhausts, too, and it looks really fascinating:

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Do you see how the hot exhausts have a very different refraction index than the cold air around it?

Stena Germanica is the first ferry that size running on methanol (super interesting, btw, how the company is exploring new technologies!) so the exhausts are mostly clear.

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Although on occasion it does still look a little more like what you’d probably expect of a ship’s engine:

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But nowhere near as bad as it used to be! Can you imagine that this is what all ships looked like not so long ago?

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Good thing there are tons of regulations in place nowadays, and companies like Stena testing out new technologies.

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Can you imagine what Kiel would look like if there was more than one ship making that kind of smoke?

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Wake of the Stena Germanica — what’s going on there?

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:

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Look at the turbulent wake directly behind the ship. Do you see how the two sides are markedly different from each other?

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One side seems to be a lot more turbulent than the other.

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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.

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Maybe that’s what is going on? But would they run two propellers at different pitches even when just steaming straight ahead?

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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.

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I love it!

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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.

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But this surely can’t be the effect of just the wind?!

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Looks great though!

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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:

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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?

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Same phenomenon again when leaving Gothenburg later that day:

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And arriving in Kiel the next morning, we could beautifully observe the different parts of the wake again:

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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 :-)

More wakes

And now you thought I was done showing you wakes? Ha, I was not nearly done! I told you, watching waves makes me happy :-)

And from Stena Germanica’s sun deck we had an excellent view over the Kiel fjord and later the Port of Gothenburg…

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