"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.