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    Re: Channel Islands
    From: George Huxtable
    Date: 2010 Nov 28, 21:34 -0000

    Andres asked, about navigating the waters of the Channel Isles, and by 
    implication, the English Channel itself-
    
    "I am interested in knowing how secure was navigation in such waters with
    strong tidal currents, shallow water and shoals everywhere, without a GPS.
    Is amazing to see how the tide changes from the departure port to the
    the Channel islands, (see attached graphs).
    Did you use coastal navigation/piloting techniques to get a fix?, and the
    sextant?"
    
    ==============
    
    Just the sort of questions I enjoy answering. I will deal with them in 
    haphazard order.
    
    You need to bear in mind that I'm rather an old-fashioned navigator, 
    something of a Luddite, reluctant to adopt the latest technology until 
    after averyone else has done so. The answers will reflect the era of my 
    cruising; roughly 1970 to 2000, so this note is mainly in the past tense..
    
    Although I've always carried a (plastic) sextant, and often used it at sea, 
    and often in harbour too, I've never REALLY needed to know the answer. Of 
    course, without GPS, which was the case until the last few years of my 
    cruising, I haven't known my exact position when out of sight of landmarks 
    or seamarks, and have only been able to assess the precision of any astro 
    position lines when land mark bearings could be taken. Celestial navigators 
    today don't appreciate how lucky they are in always having GPS on hand as a 
    precise reference to tell them how well (or badly) they are doing.
    
    I remember that one pair of star-altitudes, assessed well after the event, 
    put me smack in the middle of Ireland. Never did find out why.
    
    Navigating English Channel waters, you are seldom out of sight of land for 
    more than 12 hours or so, so with careful allowance for tides, DR can be 
    nearly as accurate as astro, and good enough to make your passage. Me, I've 
    never been out of sight of landmarks for more than a couple of days at a 
    time, which was long enough for me. But then, I've always had, and 
    appreciated, Radio DF as a backup, to keep me out of trouble. Most of those 
    stations are now switched off; one of the sad side-effects of the 
    introduction of GPS.
    
    =================
    
    Andres mentions "shallow water and shoals everywhere",  but "my" Western 
    end of the Channel isn't a bit like that, though going East from the Dover 
    Strait, it certainly is.. Mostly, the shores are cliffy, so easily made out 
    from a distance, and  approachable, being steep-to (for small vessels) 
    close to the foot. There are shoals in places, of course, such as one 
    guarding the entrance to my home port of Poole, and a difficult sandbar at 
    Salcombe, but the major shoal areas are mostly at the head of big bays, 
    such as Mont St. Michel, and not often approached by cruising vessels. A 
    bigger problem is that of isolated offshore rocks, often with the tide 
    running hard through them, which particularly beset the Brittany coast, and 
    add to its interest (and danger).
    
    =================
    
    Andres draws a nice comparison between the weak tides at Bournemouth, near 
    Poole, and the immense range at Jersey, no more than a day's sail away, on 
    the same day. The Channel tides are very complex, driven by the 
    low-amplitude oceanic tides in the Atlantic, but then magnified by the 
    local constraints near the coasts.
    
    Because the English Channel is nearly blocked at Dover, it develops a large 
    standing-wave pattern with fixed nodes, one of which is near Poole bay, as 
    an "amphidromic point" with little tidal range. But of course, where 
    there's such a minimum in the tidal range, there's a corresponding maximum 
    in the tidal stream, creating nasty overfalls when it concentrates near 
    headlands and passes over rocky reefs, as at St. Albans Head or Portland 
    Bill. That stream has to pass through a constriction between Portland and 
    Cap de la Hague, and spring rates can reach nearly 5 knots.
    
    On top of that, because some tidal energy can pass through Dover Strait 
    into the North Sea, there's also a travelling-wave component going 
    Eastwards up-channel. These combine to produce a complex pattern of 
    amplitudes and phases, first studied in a cruise, made for the purpose in a 
    Naval vessel, by Edmond Halley, the astronomer.
    
    And then Coriolis forces act to deflect those tidal streams, an argument 
    used to explain why the tidal ranges are higher on the French side. In 
    addition, the shape of the Baie de Mont St Michel is such as to resonate 
    somewhere near the 12.5-hour principal period of the tide (there are many 
    other components).
    
    As a result, the tides govern every action around the Channel Isles, where 
    the flow is rotatory over a 12.5 hour period, not simply to and fro. 
    Predictions, and tidal stream charts get studied intensively, and they can 
    work to your advantage.
    
    =============
    
    Plan a passage by drawing a tentative line from the departure-point to the 
    next headland or destination, with a tentative starting time. Mark off 
    where your estimated position will be at hourly intervals. From the tidal 
    stream chart, which gives arrows for current direction, and spring and neap 
    rates, in tenths of a knot, add up all those hourly vectors for the 
    passage, taking the spring figure. I've never done that job in a formal 
    way, by plotting or calculation, though perhaps that's the way it should 
    really be done. After long practice, I just tot those vectors up in my 
    head. Then reduce the vector length appropriately, to allow for that tide 
    as a fraction of Springs. Brest coefficients are useful if you want to do 
    that formally.
    
    That gives an overall tidal offset to apply to your whole passage, so now, 
    shift your plotted end-point on the chart to correct for it. That may lead 
    to a reiteration of the whole thing.
    
    No need to go for high precision; you never know, before you start, which 
    way the wind will blow, so don't expect to keep to any schedule. Just be 
    ready to re-evaluate en-route, as conditions change. But the strong point I 
    wish to make is this; in such tidal waters, DO NOT let a GPS receiver or 
    plotter fool you into trying to stem the tide along the way, to keep to a 
    straight ground-track. Go with the flow, as it takes you, and only try to 
    counter the overall effect, over the whole passage.
    
    Following such a practice, I have nearly always got to my intended 
    destination, though not necessarily when expected. You have to be flexible 
    and be ready to replan for another harbour, and philosophical if, after 
    all, you end up down-tide instead of up-tide of your destination, and lose 
    a whole tide.
    
    ===========
    
    In our short Summer nights, at 50�N, I rather enjoy a night passage, though 
    I would hate to do one in Winter. Some sailors expect to have their knees 
    tucked under a restaurant table every evening, but we aim to be 
    self-sufficient and stay out, which allows distance to be gained much 
    faster. However, I'll admit to enjoying a few oysters followed by a big 
    bowl of moules marinieres, washed down with a bottle of Muscadet, on 
    arrival.
    
    There have been many times when my position has been somewhat uncertain, 
    but I can remember being lost only twice. By lost, I mean being sure I was 
    in one place, when actually I was somewhere else. Discovering that can be 
    pretty unnerving.
    
    ==========
    
    What we have to remember is that countless generations of mariners, traders 
    and fishermen, have sailed these waters without any of the aids we take for 
    granted. No auxiliary engine, GPS, echosounder, lifelines, radio, DF. Just 
    a compass and lead. And, importantly, an anchor. They survived, mostly. We 
    should keep hazards in proportion.
    
    Enough for now, I think. I haven't mentioned, yet, the main hazards, which 
    in order of increasing danger are-
    1. FOG
    2. BIG SHIPS
    3. FOG AND BIG SHIPS, TOGETHER.
    
    If Andres, or anyone else, has further questions, just ask. It's hard to 
    stop me.
    
    George.
    
    contact George Huxtable, at george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
    
    
    
    

       
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