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You requested detailed replies George.  Here they are.

George:
"What arguments were advanced to back that assertion?
1. That the English Channel currents are more complex than I described.
True, and I had acknowledged simplifying the details, but so what?"
...

The "so what" bit is rather important.  It is because your theoretical method 
does not work. I have experienced the Channel crossing quite a number of 
times by sailing boat.
----------


George:-
"2. That the current can be greater in one direction than another.
Conceivable, but then you would just have to apply an offset correction that
is not exactly zero."
...

Then you are into navigating proper, to counteract the current - which is what 
I do and what 'offsets' for tidal current allowance are all about...
---------


George:
"3. That different considerations apply to sailboats, for some unspecified
reason. That could certainly be the case, if there wasn't a free wind, and
beating was called for. And the overall passage time is more unpredictable
under sail, which might well call for a mid-passage reassessment. However, I
didn't specify sailing craft or exclude them; just referred to a vessel with
known speed."
...

The whole point about the (fallacious) idea of just pointing in one direction 
and steering that way right across and letting the current one way then the 
other do the job, is because by coincidence it takes a sailing vessel approx 
12 hours to cross. It does not work with the stink and rag fellows as they 
can cross in a few hours they are so fast.
----------------

George:
"" So now I ask Douglas Denny to confirm the procedure that he uses, when
"allowing for tidal streams every half hour with the DR plot", which "gives
a much better end result". Is he really saying that at each half-hourly
point of the passage he heads to offset the cross-tide so as to keep to a
straight ground track? I want us to be certain that we are not at
cross-purposes here.""
...

Yes. DR plot every half an hour or sometimes every 15 minutes if needed in the 
English Channel. The EC is one of the most dangerous places in the world 
having: amongst the highest tidal range and currents;  dangerous lee shores; 
appalling weather which can spring up in no time at all with low cloud and 
fog; and worst of all for a slow sailboat - probably the most congested area 
in the world for commercial traffic - container ships and the two ship lanes 
you have to cross.

Procedure is probably very easy now with cheap radar available and GPS; when I 
used to do it in the 1980s, it was low frequency DF only if in fog/low cloud; 
and Mk I eyeball for lighthouses and coastal outlines otherwise - and of 
course careful compass steering (swing that compass first!) and a very good 
DR plot.
In fact the DR plot, if done regularly for tides,  gives remarkably good 
results which are reliable and comforting, and which the "straight ahead lads 
- tally HO!"  method does not give.
------------


George:
"If that really is his procedure, then I ask him whether he would apply it,
not in the complex English Channel, but in a hypothetically ideal
environment in which such complications have been swept away."
...

There is no "hypothetical ideal environment" when sailing; and yes I would 
apply it every time. It is called good coastal navigation practice.

I have said it many many times to people when this kind of discussion ensues - 
that good coastal navigation is much more difficult altogether compared to 
astronomical navigation which is very easy to do once you know the technique.
 
The curious thing is most sailors (who inevitably know nothing about astro nav 
at al these days) think the opposite - that 'astro' is somehow a 'black art' 
only to be practised by wizards. 

I suppose the above statement about good coastal navigation is now as obsolete 
as the astro navigation technique because everyone crosses the English 
Channel now with the superb Garmin or Raymarine, all singing, all dancing, 
makes a cup of tea, integrated navigation systems with radar, depth sounder, 
chart plotting moving map dispays from GPS and uncle Tom Cobbley and all 
...built-in.

I will still be doing a DR plot every half an hour regardless I expect as it 
is now hard-wired into my brain.
-----------

George:
"And if he chooses the straight ground-track, I await his explanation of what
makes him so certain that it "gives a better end result", if that is indeed
his claim".

It is my claim.  And the explanation has already been given: I have already 
experienced the variabilities that there are 'out there' in the Eng.Ch. and I 
know correct coastal navigation can only be done with a methodical approach, 
not the haphazard method you propound.

My final question to you has to be:  have you tried your method by sailing across the English Channel?

Douglas Denny.
Chichester. England.

===========================

Original Post:

Well, this becomes interesting.

I had pointed out the disadvantages of following a ground-track from
departure to destination, by continually adjusting heading to counter a
cross-tide, in circumstances when the net amount of such tidal displacement
over the voyage cancelled out. And took, as an example, a well-frequented
passage between Anvil Point and Cherbourg, across the English Channel,
adding- "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."

And indeed, it appears that we might have the good fortune to find a
proponent of that practice within our midst, who has written, in [10005]-

"Even when allowing for tidal streams every half hour with the DR plot as I
used to do,...one thing is certain - this gives a much better end result
than a 'straight ahead' strategy over the tidal cycle." I have often
wondered why this view is so widely held. Perhaps we now have the chance to
discover why.

What arguments were advanced to back that assertion?

1. That the English Channel currents are more complex than I described.
True, and I had acknowledged simplifying the details, but so what?

2. That the current can be greater in one direction than another.
Conceivable, but then you would just have to apply an offset correction that
is not exactly zero.

3. That different considerations apply to sailboats, for some unspecified
reason. That could certainly be the case, if there wasn't a free wind, and
beating was called for. And the overall passage time is more unpredictable
under sail, which might well call for a mid-passage reassessment. However, I
didn't specify sailing craft or exclude them; just referred to a vessel with
known speed.

So now I ask Douglas Denny to confirm the procedure that he uses, when
"allowing for tidal streams every half hour with the DR plot", which "gives
a much better end result". Is he really saying that at each half-hourly
point of the passage he heads to offset the cross-tide so as to keep to a
straight ground track? I want us to be certain that we are not at
cross-purposes here.

If that really is his procedure, then I ask him whether he would apply it,
not in the complex English Channel, but in a hypothetically ideal
environment in which such complications have been swept away. Take a vessel
travelling to a destination to its South across an E-W going tideway, which
in the absence of any tide would make the crossing in just 12 hours. And a
tide that will sweep 11 miles Eastward over six hours, then back 11 miles
Westward over the next six, In such a simple, predictable world, in which
the net tide over that crossing-time is known to cancel to zero, would he
then steer to counter the instantaneous tide at each point to maintain a
straight ground-track, or would he adopt a constant Southerly heading? We
can assume a sailing craft with a fair wind, which under the conditions can
maintain a speed of four knots, and can start an auxiliary if the wind
fails.

And if he chooses the straight ground-track, I await his explanation of what
makes him so certain that it "gives a better end result", if that is indeed
his claim.

George.

contact George Huxtable,



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