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I've renamed this thread to suit the topic better.

Rodney Myrvaagnes and I have been indulging in a friendly argument about
the best strategy to adopt when making a passage across a tidal stream.
Sorry about the delay in responding, to his last message, but I've been
away in Cornwall for a few days.

I did my best to summarise the position, comparing my strategy with his
strategy, as copied below-

>>It's true that over ANY cross-current passage, when that cross-current
>>varies significantly with time, then trying to counter that cross-current
>>at every moment, by varying the heading to hold to a short and straight
>>ground-track, will not be the best way to go. It will always be more
>>efficient to stick to a constant heading, which is calculated to compensate
>>for the total net tidal displacement over the duration of the passage,
>>whatever that happens to be.

Rodney then countered (my responses are interpolated)-
>If you know what the ideal heading should be, they are indeed
>equivalent.

No, not equivalent at all. What I said in an earlier message was-

"Following a straight ground-track will turn out to be the same as the
procedure above if, and only if, the cross-current stays constant with
time. This may be the case when crossing an ocean current, or a river flow,
or making a short cross-tide passage which only takes an hour or so, a
small fraction of a tidal cycle (such as when crossing the Solent). In
those cases, following a straight ground-track will be just as good."
That's the situation in which the two strategies are equivalent.

Rodney continues-

>But a wrong constant heading in the steady cross-current
>case can be costly. For example, if the set is half the boat speed,
>heading straight across will take twice as long as with no current,
>counting the upstream slog at the end. A correctly calculated course
>will take only 11% longer than it would without current, as would the
>continuously corrected course.

Here, he has set up an Aunt Sally to shy at, which is quite irrelevant to
the argument, Neither of our strategies would head straight across in such
an absurd way. His numbers, by the way, appear correct. With such a
constant current, both strategies would be the same, requiring 11% more
distance than if there were no current.

>I don't think the 11% would be exceeded at any time if the set didn't
>exceed half boat speed, even in the periodic case.

Well, I agree, in that special case where the speed of the tide was never
more than half the boat-speed. If he always sails in such an environment,
and with such boat speed under his belt, Rodney is privileged in a way that
those of us who sail slow craft in strong tides are not. For small
sailboats in English Channel waters at spring tides, it's common to find
the tide running every bit as fast as the boat can travel. How would
Rodney's strategy of brute-force bucking of the tide, to maintain a
straight ground-track, cope then? He would find himself expending all his
boat's way in fighting the tide, making no headway at all toward his goal.
No, it's necessary then to be more subtle, to coexist with the tide rather
than oppose it.

Mariners, especially fishermen under sail, in strongly tidal waters, have
for many generations been making such allowances, in their heads, after
holding up a wet finger. Unlettered, few would understand a vector diagram.
Not having GPS, they wouldn't know, or care, if they were following a
straight ground-track.

My sailing area includes the Alderney Race, with tidal currents of up to 9
knots in places, maybe more, locally, at perigee springs. One can only
transit that race, in a 4-knot craft, by timing the passage right. One can
only cross it, to reach or leave Alderney, by even more careful timing.
Fortunately, it's only a short passage (IF you get that timing right!). By
the way, the French give this the evocative name of the "Raz Blanchard",
the washerwoman's race, which describes it well. When you pop out the other
end, it can feel like having been through the laundry. Once you're in the
tidal stream, there's definitely No Turning Back. You are in it for the
full cycle; wash, rinse, and spin. Occasionally, though, it can be quite
benign.

Apologies for that diversion about the Alderney race, which was completely
beside the point.

>For periodic tides
>one could calculate the error at the end from missing the cancellation
>by 90 degrees. It would not be insignificant for the 12-hour trip, but
>surely would be for anything over a couple of days.

>In my wanderings, the Gulf of Maine crossing is long enough so that
>tides do cancel out, about 30 to 40 hours.

I don't see the logic in that statement above. In semidiurnal tide areas, a
"tide", between reversals, takes a bit over 6 hours, so 38 hours would be
about 6 such tides and should indeed cancel out. But, by that same
reasoning, a passage time of 32 hours would be 5 whole tides, and
(depending on the phase at departure) you easily could end up displaced
sideways from your goal by one whole tide's-worth, to port or to starboard.
If the passage time is as loosely specified as "30 to 40 hours", then
cancellation can not be relied on.

>The tides are not charted in
>detail there, but the Bay of Fundy has significant effects for some
>distance. There are circulating tides, so they will have different
>effects depending on the exact track across.

Yes, the tides can be more complex than a simple linear to-and-fro motion,
and any such complexity needs to be taken into account, but that doesn't
invalidate the principle of allowing for the complete predicted tidal set
over the whole passage, from departure.

>In Long Island Sound and the area adjoining the Elizabeth Islands
>(Buzzards Bay and Vineyard Sound) the current is more bucking or aiding
>than setting sideways.

In that case there's no call for altering the heading to be anything other
than the direct A to B course, with either strategy.

>The Long Island Sound currents are altered from
>the charted values by rainfall and often are weaker or stronger than
>the predictions.

Just from looking at the topography on my atlas, it seems unlikely that
currents in the Sound from river systems are going to be comparable with
the tidal streams. But no doubt Rodney knows those waters well, and I don't
know them at all. In areas where the currents really are unpredictable,
then the strategy must be modified accordingly.

>In all these circumstances I think constant ground track is a good way
>to compensate for unknown set.

It's true that my tidal strategy requires some sort of prediction of what
the tidal streams are going to be, where Rodney's strategy needs only a
knowledge of what the tide happens to be at that moment. If the tide is
unpredictable, or if the navigator has no means of making tidal-stream
predictions, there may be no alternative to adopting Rodney's
less-efficient strategy. As a general rule, however, tides, where
important, are usually predictable; much more so than winds are.

>I'll gladly risk 11% to avoid 100%
>possible penalty.

That "100% penalty" was a fiction invented by Rodney for the occasion (see
"Aunt Sally", above). It has no relevance.

>Of course most unknown sets will be much less than 3
>kt, so both penalties will also be smaller.

>So I ask George. Can you predict just how long your crossing will take
>within an hour or so?

A sensible question. Answer: Under sail, no, probably not, even for a
12-hour passage. Within two hours, perhaps yes, most times. The tidal
displacement over a known time will usually be predictable to within a
couple of miles, but progress along the intended track depends on the
fickle wind. Sometimes, it will become more favourable, resulting in better
progress than expected. But for some reason, vice versa is more common!

>If you are not coming in where you want to, when
>do you decide to take corrective action?

A very sensible question. Answer: Throughout the passage, and indeed right
from the start, if progress is different from expected, the
direction-to-steer can be reassessed, using the same criteria as at
departure. That is; where are we now, how much longer is it likely to take
to reach our destination, how much net tidal offset to allow for over that
period? And then a new course-to-steer is set, intended to drop us right at
the harbour entrance without further changes of course. But keeping in mind
the possibility that further modiification may be called for. It's
necessary to be flexible, to adapt as circumstances change.

I will concede, however, that I hate to find myself downtide of my
destination, so as journey's end approaches I will make a reassessment that
will ensure being uptide to some extent. There's no point in making such
fine-adjustments until late in the proceedings, when few unknown factors
remain.

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

In this discussion, so far, we have considered only variation of tidal
current with time, avoiding any extra complexity resulting from the common
situation where tides run more powerfully in one area of a passage than in
another. And we have assumed that we can always head in the desired
direction, although the wind may force us to tack, and add some delicious
little complexities of its own. How do we relate what tack to take, to the
tide direction, to best advantage? Any offers?

George.


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01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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