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    Steering a ground-track. was: leeway
    From: George Huxtable
    Date: 2006 Jul 8, 05:43 -0500

    I've renamed this thread, intending to expand the discussion somewhat.
    
    First, thanks to Bill for explaining set and drift, as corresponding
    to direction and speed of tidal current. In British English, those are
    the terms we would usually use, tidal direction and speed, but I see
    that set and drift are accepted American usage, to be found in both
    Bowditch and Mixter.
    
    I don't think there's a lot of difference, between Bill and me, about
    how we understand tidal offsets and allow for them in practice.
    
    ========================
    
    Doug wrote-
    
    "....Yes, there are procedures to check or calculate leeway.
    
    Here's how it works on a commercial vessel. The OOD and helmsman are
    given the intended course line at the start of a watch. With today's
    computer navigational graphics and GPS it is easy and almost immediate
    to see the effects of set and drift and leeway.
    The master usually denotes the amount of distance off the intended
    track
    line is acceptable in routine situations. As an example he may denote
    1/8 n. mile either way of the intended track line is acceptable but to
    correct anything outside of that limit to get the vessel back on
    track.
    But, in reality, the helmsman is constantly adjusting the wheel to
    keep
    the vessel on the track line. There is a monitor with the electronic
    chart of the area with the vessel's pos, the intended track line, the
    track actually followed and the vessel's position on the current
    heading
    at some set time in the future on the immediate heading.
    It's pretty easy now to compensate it all out as one is moving along."
    
    It seems to me that such offsetting of the heading results from the
    combined effects of leeway and ocean current and tidal current, and
    there's no way to distinguish between them to deduce what was the
    leeway component. Not that it really matters. Doug lumps them together
    and calls the result leeway.
    
    In principle, though there may be no call for it in practice, it may
    be possible to introduce an instrument to measure actual leeway. Down
    near the keel, a small crosswise pipe could be introduced, joining
    exactly symmetrical points on the starboard and port sides of the
    hull, like a tiny version of a bow-thruster tunnel. Then the flow rate
    through that tube should indicate the leeway. If it mattered.
    
    The practice of allowing for leeway (and currents) in the way Doug
    describes can only have arisen in the last 20 years or so, since exact
    ground positioning has become available. Before those days, commercial
    vessels must have had to estimate their leeway and current offsets as
    best they could. What did they do then?
    
    ========================
    
    Here's where I wish to expand this discussion. It's a matter that has
    arisen before, but perhaps that was in the days before we had Doug's
    exprience to call on.
    
    Steering a straight ground-track from A to B is not necessarily the
    optimum way to travel. It's exemplified, particularly strongly, for
    small-craft sailors travelling between my home-port, Poole, on the
    English Channel, and the most popular destination, Cherbourg, to the
    South of Poole. About 48 miles of that passage is exposed to the full
    strength of the Channel tide.
    
    Simplify things by assuming that Poole-Cherbourg is exactly
    North-South, the Channel flow is exactly East-West' the relevant
    distance is exactly 48 miles, the vessel's speed is just 4 knots, and
    the tide displaces the water by exactly 12 miles in a six-hour period,
    then reverses.
    
    If, from Poole, a vessel sets off at the start of the West-going
    stream, heading due South, ignoring tide, then in 12 hours she will
    arrive at Cherbourg. The tide will have taken her 12 miles Westward,
    at mid-passage, from the ground course, but then will bring her 12
    miles Eastwards, right to the port entrance.
    
    On the other hand, another vessel trying to follow a ground track, as
    navigation electronics will usually direct, has to head off quite a
    lot to the eastward,  to counteract that westerly tide. During the
    first six hours, until the tide turns, she has on average to head 30
    degrees of-track, at a course of 150 degrees rather than 180. Because
    of that, the Southward component of her speed is not 4 knots, but only
    3.46. As a result, at that time, she has made only 20.8 miles of
    southing rather than 24. Similarly, during the next six hours, she has
    to head about 210 degrees, to counter that eastgoing current, which
    results again in only 20.8 miles of southing. As a result, when the
    first vessel is entering harbour, after 12 hours, she still has more
    than 6 miles to go.
    
    Most navigators making that passage have learned not to follow such a
    ground track; but not all. Some blindly follow their electronics.
    
    Of course, ours are slower vessels, much more affected by tide,
    relative to their speed, than ocean carriers such as Doug's. However,
    I wonder whether his navigational practice allows him to deviate from
    the defined ground-track, and under what circumstances.
    
    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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