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Tuesday, 16 August 2016

Puzzling Twin Peaks and Passing Old Harry in the Dark

I am getting to understand the tides a little better, but as you get more into tides they get more puzzling and mysterious.

The plan was to set off from Christchurch and voyage across Poole Bay to the Old Harry Rocks and then on to Swanage.

I would set off at 8.00 pm on Sunday 14th August and the tide would take me across Poole bay (starting at the bottom of the map).

The water enters Christchurch harbour via the 'run,' a narrow channel  through which vast quantities of water  must squeeze, impossible to go against in PicoMicroYacht. So I was expecting not to get out of the harbour until 8.00 pm when it was high tide. But when I arrived and set the boat up by 6.30, most of the water seemed to be in the harbour and I could set off.

Later I checked what  had happened and the tidal chart revealed three interesting but very puzzling phenomena.

Firstly, for each tide there is a mini 'stand' with twin peaks.  I have to admit I don't fully understand this mysterious double tide, but there is a description of what happens by Dr Ivan Haigh, a tidal expert from the University of Southampton, about 20 miles from Christchurch:

Apparently it is 'the combined effects of water sloshing across the Atlantic (a bit like in your bath) and spinning round because of the rotation of the Earth; this causes the water to slosh around a point which in fact is inland. When the tide ‘comes in’ water travels up along the coast but can’t navigate the corner into the North Sea and is reflected back down the coast.' He says it is not due to the proximal Isle of Wight. 

All I can recommend to understand this is sloshing water round the bath with a few objects in it; the sloshes might reveal that the height anywhere in the bath isn't a simple function of the water just moving along it and back.

Secondly, at neaps (not too much tide), the second peak is higher than the first, whilst at Springs (lots of tide), the second peak is lower than the first ... puzzling and I cannot find an explanation.

But I have just also noticed that every other twin peak high tide is larger ... even more puzzling.

So it there is anyone out there reading this who can explain any of this, please comment?

But I am not so exercised about the tides, and there are some things as a sailor you can just accept as fact; PicoMicroYacht took advantage of being able to exit a little earlier at 6.30 pm because the tide had stopped rushing in the entrance.

PicoMicroYacht was quickly out of the harbour. On a spit by the entrance I passed some of the most expensive beach huts one can imagine, a recent one selling for £260.00.00 - they have no toilets or washing facilities and shared shower block

As I traversed along the spit, a rich owner paddleboaded out to me to say hello and ask me about PicoMicroYacht.

Soon I was out into Poole Bay, looking back to see Hengistbury Head, on which are the remains of an iron age fort.

The sea was calm and I had been stemming the tide for over any hour until it turned in my favour. As the sun set and the sky darkened I looked across to Bournemouth.

It now now darkened quickly and I was across the bay approaching the Old Harry Rocks, looming out in the darkness. These chalk rock that sticks out into Poole Bay could be seen faintly in the moonlight but defied my photography. It is at this point you have look out for ships going pass Old Harry to go in and out of Poole Harbour, including high speed ferries.

As I was lining up to go past the rocks I spotted a light in the sky and a green navigation light more at sea level, just below it.

The white light was on the top of a tall mast and the green light was on the bow, indicating it was going to the right of me.  It was a large yacht motoring quite close to me as a I rowed to ensure I was out of the way, taking no risks. I could hear the crew talking to each other over the sound of their engine. I hoped they were looking ahead properly.

The yacht had distracted me and I looked round to see I was being swept through the race, a bit bumpy but not dangerous with no wind and a neap tide.

Soon I was in Swanage bay and looking for the sailing club to land, arriving just on midnight.

The next day the friendly bosun of the club chatted about boats as PicoMicroYacht  was taken from the beach.

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