"Jeff Morris" wrote in message
...
"Donal" wrote in message
...
Joe, I was pointing out that sailors have been sailing with very little
external input for a very long time.
It is possible to sail with a fair degree of accuracy without much in
the
way of inputs. Little over a generation ago, many people would cross
the
channel (60 - 70 miles) with no instruments at all, apart from a
rudimentary
compass.

I've been wait for you to describe what you meant by "no external inputs."
Clearly, if you had no inputs, (and no inertial guidance system) it would
be
difficulty indeed to deduce where you were. And if all you meant was no
GPS,
then the discussion is almost as meaningless.

But you claim in your previous post 1 mile accuracy for a 70 mile trip,
crossing
a channel the has (I think) a current of several knots. Is this really
feasible? This implies a distance accuracy of about 1.5 %, which thus
implies a
speed accuracy of about 0.1 knots. I doubt there's anyone here that would
claim
to be able to "eyeball" a boat's speed with anywhere near this accuracy.
I'm
not sure I could reliably calibrate my log this well, and I certainly
wouldn't
trust it a day later.

And the course accuracy? That's better than 1 degree. Again, I know I
can't
hold a course that well over time, but I would also argue that most
compasses
aren't that accurate to start with. In fact, there are some locations
where the
variation isn't that stable either.

And can anyone give the leeway figures for their boat that accurately? If
the
wind is variable can you integrate the net affects over time? And how
accurate
does one know the current? Even if you adjust for the state of the tide
and the
phase of the moon, can you predict the current to 0.1 knot?

Can one argue that many errors are possible but they tend to cancel out?
Nope,
it doesn't work that way; although there will likely be some canceling.

No, I can't buy your claim of 1 mile accuracy in a 70 mile trip, with only
a
compass; I doubt you ever did it, and you certainly couldn't do it
reliably.

Its certainly true that there were many, many coastal passages 100 years
ago,
and I'm not going to claim that the accident rate was infinitely higher
back
then. (It was, but that's not important.) But they did have other tools
that
we tend to forget about: Lighthouses, fog horn, lead lines etc. They
weren't
quite as blind as you're making them out to be.

BTW, I have done the experiment of sailing "blind" a number of times. I
served
as a guide for a blind sailor a number of times, so I tried it out to see
what
it was like. I refined it into a "parlor trick" for my students, where I
would
lie in the bottom of the boat while they sailed about a mile around the
river
basin. I could tell them where we were to within a 100 yards or so.
The was
that because I had sailed this one square mile almost exclusively for 15
years,
I could locate myself any time by a number of cues, such as the traffic
noise
from shore, or the wind patterns from buildings, or the different sounds
of the
boats from different clubs. Like I said, a parlor trick. But someone
who
crossed the Channel daily all their life would have the same ability.

Good questions, and I admit that I cannot give precise technical
explanations for the accuracy.


I will try to give as honest an answer as I can.

When I did the Yachtmaster Shorebased course(a classroom course), the
instructor told us that there "should be only one navigator" on any trip.

On the first Channel trip that I did I was surprised at the accuracy.

My second trip was in a charter boat. One of us was a RYA (coastal skipper)
instructor. He was a bit of a "know it all". On our return trip, a 23 hour
crossing from Guernsy to Salcombe, he woke up, looked at the Decca, and
ordered a 15 degree course change.

I asked if he wanted to take over the navigation ("there should be only one
navigator"). He declined.

He made me nervous enough to advise a 5 degree alteration to our course.
After all he *was* an instructor.

We hit land 5 miles **downtide** of our destination, and had 3 dreadful
hours of sailing into wind and tide. That night, I decided that I would
trust my own abilities in future. I'm not a great sailor, but I *am* good
at maths.


Where I cross the channel, the distance is about 60-70 miles. In fact, the
trip to Cherebourg is 74 miles, but I have to go to Bembridge Ledge buoy
first. From there, it it is 64 miles.


The trip time is about 12-14 hours. This fact has an impact on the
navigation. I tend to have 6 hours of ebb, and 6 hours of flood. Thus,
the tide will tend to push me as far one way, as it will the other.

Many people will check their position on the GPS when they are half way
across. If they are off course, then they assume that they were steering
the wrong course. I don't do this because it is more likely that the tidal
stream was not as predicted. If it came in faster than predicted, --- then
it will go out faster than predicted. People (like Joe), who react tend ti
over-correct.


Regards


Donal
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