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This was triggered by Jeremy's reference to the effect of current, but it's
about a rather different situation, of tidal currents.

It's arisen here before, a long time ago. I can illustrate it best by a
little problem that faces many small-boat navigators crossing the English
Channel, between Anvil Point, a headland South of Poole, and Cherbourg, to
its South by about 50 miles. I will simplify the details, but for many small
craft that passage will take about 12 hours, or just two tides' worth. Say
the tide flows at about 2.5 knots, a total displacement of about 11 miles
East, over the first 6-hour period; then 11 miles West, over the next 6
hours..

Many navigators will set the destination of Cherbourg as an intended
waypoint, then religiously adjust their heading to keep their ground-track
along that intended line, angling against the current to keep it so. They
are, of course, wasting time and energy. Because, over 12 hours, the net
tidal displacement, East then West, will sum up to be close to zero, there
will be no overall effect of tidal current. In which case, the correct and
easy procedure is to head due South, allowing your craft to be swept
up-channel first, then back again later. By ignoring the instantaneous tide,
and the resulting cross-track error, the vessel is heading exactly
Southwards all the time; the best that can be done.

Indeed, set out like that, it seems pretty obvious, but it's hard to
convince many navigators that they can, and should, ignore those warnings of
cross-track error.

George.

contact George Huxtable, at  george@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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