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    Re: Pilot charts
    From: George Huxtable
    Date: 2008 Apr 16, 11:01 +0100

    | glapook@pacbell.net wrote:
    |
    |
    | I have attached a portion of the January Pilot Chart of the South
    | Pacific Ocean published by the Defense Mapping Agency. Until I looked at
    | this chart I had no idea that currents in the South Pacific could be so
    | strong. The green arrows show the current  direction and speed in knots.
    | For example, the current shown passing the southern end of New Zealand
    | is 10 to 20 knots! And in the Gilberts it shows up to 30 knots! How do
    | ships manage to maneuver against such strong currents?
    ================================
    
    Response from George-
    
    Relax, Gary. No currents anywhere are nearly that strong, or navigation
    would never have been possible.
    
    I presume that there's a discrepany between the numbers on the chart itself
    and the information panel. The numbers alongside the ocean current arrows
    appear to be in tenths of a knot, but the panel clearly states that they are
    in knots. The chart authority may or may not appreciate having it drawn to
    their attention.
    
    Tidal stream charts of European waters, produced by the UK Hydrographic
    Office, adopt the same ruse of printing in tenths, which avoids the need for
    all those "damned dots", but at least they say so clearly.
    
    That current, South of New Zealand, shows one good reason why sailing
    vessels out of Melbourne would make their passage back to Europe via Cape
    Horn, though they had usually arrived via Cape of Good Hope, in the days
    before the Suez Canal.
    
    The strongest current I've experienced is the tidal one through the Alderney
    Race, between the Channel Isles and the French mainland. At springs, patches
    of it run at up to 9 knots, though around 6 knots over most. It doesn't get
    too rough, as long as you can avoid wind-against-tide or any strong wind
    conditions, because it's sheltered to some extent from wave action by the
    land it passes between. There are overfalls, where the current flows over a
    uneven rocky bottom that remains quite deep, but these can be avoided. The
    French call it "Raz Blanchard", or washerwoman's race, and under adverse
    conditions it can feel like a being in a washing machine.
    
    It can be great fun transiting the Race under good conditions with a spring
    tide behind, which I have done a few times. You just have to hang on, and in
    less than an hour, it's done;
    you just whistle through. And indeed, the worst is yet to come, because when
    that jet of water from the race joins up with the main current along the
    English Channel, things can then get quite turbulent.
    
    In my early sailing days, 40 years ago or so, I was sailing, not in my boat,
    under another skipper, from Cherbourg aiming for Braye, the harbour on
    the far side of Alderney. In spite of my protests that he wasn't making
    enough offing to get round the North of the island, we got sucked into the
    race, facing backwards. So we had to pass South of the island instead, and
    wait a few hours until the tide turned in the Swinge. We found a superb
    low-tide anchorage amongst half-tide rocks, and spent a gorgeous few hours
    on an empty little islet. Sometimes such happy consquences ensue from an
    error; but not often.
    
    Don't know why I'm telling this tale; just thought you might like to know.
    It's not very relevant to Gary's question.
    
    George.
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
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