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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.
----- Original Message -----
From: 
To: 
Sent: Thursday, October 01, 2009 12:54 AM
Subject: [NavList 10005] Re: Allowing for current. was: [NavList 9997] Re:
Noon sun fix example



Now I don't want you to get paranoid George and think I am 'getting at you'
as I assure you when I disagree with you it is not a personal attack! (which
I am beginning to feel you might think);  however, I have heard this
chestnut a number of times and it simply does not work with a sailing
vessel. (i.e. a 'stick and flag' type of vessel).
The 'stink and rag' boys get over there so fast it doesn't matter anyway.

The differences actually experienced with the tidal streams in the English
Channel as you cross for a variety of reasons are not symetrical and you get
shoved one way more than the other. I know I have done it.
Probably such things as the topology of the Channel sea-bed; swirls of
current around bays; the Alderney race flushing out across the Channel;
faster stream near the centre but more one side than the other ..who knows?
I guess a thousand and one reasons for it.  Another fine theory in
principle, that does not work out practically.

Even when allowing for tidal streams every half hour with the DR plot as I
used to do,  there can still be differences appearing which make the passage
lop-sided as you cross, but one thing is certain - this gives a much better
end result than a 'straight ahead' strategy over the tidal cycle.  There are
too many  variables for such a simplistic approach.

I have to admit I have not done it for some years so would be interested to
know of anyone who has done it recently with GPS to guide them. I only had
LF beacons such as St Catherine's Point on the IOW, and Nab Tower... or best
of all:  Mk I eyeball when there wasn't fog or rain or cloud.

Douglas Denny.
Chichester. England.
==========================

Original Post:

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,




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