Recently, I was cruising on the Danube river from Bratislava to Vienna and we were encountering these small green rocket-shaped objects floating in the water roughly after each kilometer. They were not staying still but also sailing in the same direction as our boat but slower.

What are these UFOs (Unidentified Floating Objects)?

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  • @Dorothy, I would expect channel markers to be more upright, but perhaps these are for areas with a strong current. Anyway, red and green marks channels.
    – o.m.
    Commented Jul 18, 2017 at 4:52

1 Answer 1


Those are moored buoys marking the fairway in the middle of the river, where the water is deep enough for safe sailing. A green triangle marks the starboard side of the fairway (left side, as seen in the direction of water flow) and on the other side of the river, you would have seen red buoys with a square plate on top, marking the port side of the fairway.

Your impression that the buoys are moving is just an optical illusion due to the water movement.

  • 1
    That is correct. I would also expect the bottom of these to be yellow as per the Hungarian net.jogtar.hu/jr/gen/hjegy_doc.cgi?docid=a0200027.gkm regulation which I expect to the uniform in the EU. Here's a picture with dimensions: i.imgur.com/OFBLPX0.png
    – user4188
    Commented Jul 18, 2017 at 8:50
  • 2
    @chx I do not know about any regulations for the colour of the bottom of the buoy and I don't see any point in requiring a special colour on a part of the buoy usually not visible. The submerged part of a buoy will anyway quickly be covered with algae or molluscs. As you can see on this picture, it is at least not obvious that the bottom is painted in a different colour: familie-schulz.at/Dateien/Images%20(JPG)/Oesterreich/… Commented Jul 18, 2017 at 9:03
  • I just thought digging up an official source even for this is useful (even if it's not in German / English).
    – user4188
    Commented Jul 18, 2017 at 10:00

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