The other day I went on a trip to Husum with my sister and her family. While walking along the sea, we saw the weirdest thing: Birch trees growing in the middle of the water! From their positions it was clear that they were some kind of marker for the waterway, but it looked very strange. But google suggests that this kind of marker is very common in wadden seas, where the water is too shallow for traditional buoys.
“Pricke” – birch trees serving as markers for the water way
When coming from the sea, you’ll see the birch trees on the port side of the waterway, and on the starboard side there will be poles with branches which are tied together on the very top of the pole, branching out below. Apparently this is called “Pricke” in german. You live and learn! :-)
Today I’m going to share a long movie with you, but I’m planning to talk about ship lifts in more detail soon. But just how awesome is it that they can lift ships (SHIPS!) 40 meters up just like that? Each of the troughs carries 5,800 t water. You see the counterweights move when the troughs move, and it is totally fascinating.
I went there with my parents, but this is pretty much all they saw of me for a very long time :-) Weirdly enough I was the only person standing right at the railing. Well, maybe not so weird considering how wet I got. But you’ve gotta do what you’ve gotta do and I definitely enjoyed watching!
The movie below is sped up by a factor 3.5 because there was so much footage that I wanted to show…
Did you ever notice how when certain ferries dock, they stop, already parallel to the dock, a couple of meters away from the dock and then just move sideways towards the dock? Usually they don’t even move passenger ferries any more, just use thrusters to keep them steady while people get on and off.
Currents caused by thrusters of a harbor ferry in the port of Hamburg
But why this weird sideward motion?
One reason is the Coanda effect – the effect that jets are attracted to nearby surfaces and follow those surfaces even when they curve away. You might know it from putting something close to a stream of water and watching how the stream gets pulled towards that object, or from a fast air stream that can lift ping pong balls. So if the ship was moving while using the thrusters, the jets from the thrusters might just attach themselves to the hull of the ship and hence not act perpendicularly to the ship as intended.
But I think there is a secret second reason: Because it just looks awesome :-)
My dear ship builder and naval architect friends, if this post seems horribly oversimplified to you, you are very welcome to write a guest post and go into this topic in as much detail as you feel is needed :-)
So now my dear non-ship builder and non-naval architect friends, here is a post about ships. And be warned: it is very simplified. I have been taking pictures with a post on this topic in my mind for more than a year now, so here we go:
Have you ever noticed the bow waves that ships make?
Bow wave on a ship somewhere in Cornwall
It’s pretty easy to imagine that a lot of energy is lost generating the wave field around the ship. Energy that could be used on propulsion or on something completely else instead.
Energy wasted on creating an enormous wave field.
So what if the solution to this problem was really simple? As simple as a ball right in front of a ship’s bow, just below the water line? That would produce a wave field as seen below.
Wave field created by a submerged buoy in a current.
And indeed that is what you see when you look at big container ships like the one on the picture below.
Bulbous bow on a container ship in the port of Hamburg
So why would this work? In the picture below, I’ve sketched an over-simplified explanation. In A) you see a ship moving from left to right, and the bow wave that is created by the ship moving through the water. Then in B) you see the wave field created by a submerged ball (compare to the ball in the third figure in this post – that’s not so unrealistic!). And then in C), you see the water levels from A and B added together: They cancel each other out (pretty much). Voila!
Sketch explaining how a bulbous bow cancels out the wave field created by a conventionally shaped bow.
Of course, it is not quite that easy in reality. Having a bulbous bow is only an advantage if you are planning on driving with a set speed most of the time, since the wave field created by both the bow and the bulb depend on the ship’s speed, and both have to be tuned for a specific speed. And you will still lose energy to the wave field that you are creating as you are moving your ship through the water, but not as much as before. But still, since you see bulbous bows on most large ships these days, it seems to be working quite well, and, according to Wikipedia, yields fuel savings of the order of 10-15% for any given speed. Not too bad!
We (or I, at least) hardly ever see empty ships. For one, it doesn’t make a whole lot of sense economically to have ships driving around empty, but also the stability of ships is maximal at a certain position of the ship in the water. Therefore people will always try to drive a ship that is neither loaded too full or not enough. But don’t empty ships just look funny?
Ships in the port of Gothenburg
Especially when you see sister ships next to each other where one is full and the other is empty (below).