Updated
I think the Bursledon Blogger was closest, in that I think this was a navigational signal used on the Huangpu River.
I was on a boat that gave a tour of the river, first heading upstream, then down, before returning to the start point. That meant it had to do two u-turns, and as can be seen by the pictures above and below the river is quite busy with a constant stream of boats in both directions. So it was useful to be able to inform the other shipping of its intentions.
It raised the ball sufficiently prior to the turn that other traffic could be aware of its intentions and then dropped it as it was turning.
However I guess Tillerman had a good point too - if you don't understand what a boat is indicating keep a good eye on it!
3 comments:
I always thought that when the ball is raised it means the vessel is anchored.
So if a vessel is raising and lowering the ball while under way I can only think that it means, "The skipper of this vessel has no idea what he is doing. Keep well clear."
That's what I thought and when I got back I checked the books and they said the same, so I wondered if I'd missed something.
Could be that local navigation regulations apply - Example the tidal Thames has a completely different set of sound signals to international ones - I think it's four short plus one long for turning right etc
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