"Jeff" wrote \
Sorry Ellen, its basic physics. Galileo figured it out about 400 years ago. He called it the Principle of Relativity.
(Einstein used the word "relativity" as a homage to Galileo.)

Here's a portion of his writings n the subject:
http://physics.syr.edu/courses/modul...E/galileo.html

Your just trying to confuse me with the relativity stuff. That's got nothing to
do with sailing. It's all about mass and energy and the speed of light. Physics and
the galaxy.
But, darn you, you had me thinking about it all night and I tried to think of some
way to prove it. But my bottle on a string doesn't work. Your right all it does is tell
the boat speed through the water. I was getting confused because I just kept thinking
about land and the current vs. land and it made sense. But he asked how to tell current
when no land's in sight and no instruments allowed. So I had to totally get rid of land.
I thought about that Star Trek episode where they found a planet that was just a
big ball of water all the way to the center. No land. So, if your sailing there you'd
never know if there was a current because you only have the water and the air.
There's no way to know (you can't use a sextant for a star sight) which is moving,
the water or the air. Your right on the edge between the two. You can't know.
So I was ready to say I lose and you win but I had a dream and in the dream
I saw big waves in the Gulf Stream which were really really big and breaking because
the wind was blowing like a hurricane against the current. Ah ha! That's it.
All you need is a table with wave heights and periods. If the waves are larger and
have different periods than they'd have in still water then there's a current.
So you CAN tell after all. No instruments needed just wave tables.

Cheers,
Ellen

"Jeff" wrote in message
...
jlrogers±³© wrote:
"Shaun Van Poecke" wrote in message
...
Here's one for those of you out there without onboard
instruments/knotmeter/gps....

Is there any reasonably accurate (say, within a know or so) way of
judging the current, when you are under way, and there are no fixed
objects within sight?

Yes, Shaun, there is. For your edification, I refer you to, "We, the
Navigators The Ancient Art of Landfinding in the Pacific," by David
Lewis. Specifically chapters 5, "Keeping course by Sun, Swills, and
Wind," chapter 6, "Dead Reckoning," which directly addresses your
question, and chapter 7, "Orientation Concepts in Dead Reckoning."

http://books.google.com/books?vid=IS...H54BfXwOv1l5Zs

Ah! One of my favorites books! Thanks for the mention, I think it may be
time to read it again!

However, I don't think it supports your position. For instance, in the
chapter on Dead Reckoning, it begins the section on Current Set with "This
presents a difficult and intractable problem of which the Pacific Island
navigators were only too well aware ..." It goes on to describe how they
memorized all of the currents, and then took careful back bearings on
departure to determine how the currents were deviating from the expected
norm.

The traditional navigators did make use of a variety of signs, such as the
steepness of waves (as I mentioned in another post) but I doubt the casual
Western observer could reliably use these sign to measure to within a
knot.


True, but with "enough experience" a bow wave will tell one the speed of the
boat through the water. Wind judgment with knowledge of your boat will tell
you what your speed ought to be sans current. The difference is the
current.

--
jlrogers±³©

Ellen MacArthur wrote:
"Jeff" wrote \
Sorry Ellen, its basic physics. Galileo figured it out about 400 years ago. He called it the Principle of Relativity.
(Einstein used the word "relativity" as a homage to Galileo.)

Here's a portion of his writings n the subject:
http://physics.syr.edu/courses/modul...E/galileo.html

Your just trying to confuse me with the relativity stuff. That's got nothing to
do with sailing. It's all about mass and energy and the speed of light. Physics and
the galaxy.

No, not at all. That's Einstein Theories of Special and General
Relativity. Galileo's Principle of Relativity is much more mundane.
Read that link again - its an interesting scientific observation made
almost 400 years ago.

But, darn you, you had me thinking about it all night and I tried to think of some
way to prove it. But my bottle on a string doesn't work. Your right all it does is tell
the boat speed through the water. I was getting confused because I just kept thinking
about land and the current vs. land and it made sense. But he asked how to tell current
when no land's in sight and no instruments allowed. So I had to totally get rid of land.
I thought about that Star Trek episode where they found a planet that was just a
big ball of water all the way to the center. No land. So, if your sailing there you'd
never know if there was a current because you only have the water and the air.
There's no way to know (you can't use a sextant for a star sight) which is moving,
the water or the air. Your right on the edge between the two. You can't know.
So I was ready to say I lose and you win but I had a dream and in the dream
I saw big waves in the Gulf Stream which were really really big and breaking because
the wind was blowing like a hurricane against the current. Ah ha! That's it.
All you need is a table with wave heights and periods. If the waves are larger and
have different periods than they'd have in still water then there's a current.
So you CAN tell after all. No instruments needed just wave tables.

Too late, I already mentioned this possibility twice. However, this
only works in limited situations, and is highly variable. The wind
has to be strong, and against a real current for a significant period
of time before the chop is measurable. "Wave tables" would be of
little use, it would take someone with serious local knowledge to be
able to determine the current to one knot accuracy even in the most
favorable situation.

I have a very vague memory of maybe having read that book.... is that the
one where the guy sails to the pacific in a sloop or something and does a
bunch of research on how the navigators there follow 'star paths' by having
them navigate the boat in double blind tests (he locked away all his charts
and compass)? If so it was a pretty good read. Ill have to have a look for
that one again....

If i recall, there was something about finding land by phosphoresence as
well?

Shaun

Yes, Shaun, there is. For your edification, I refer you to, "We, the
Navigators The Ancient Art of Landfinding in the Pacific," by David Lewis.
Specifically chapters 5, "Keeping course by Sun, Swills, and Wind,"
chapter 6, "Dead Reckoning," which directly addresses your question, and
chapter 7, "Orientation Concepts in Dead Reckoning."

http://books.google.com/books?vid=IS...H54BfXwOv1l5Zs

Shaun Van Poecke wrote:
I have a very vague memory of maybe having read that book.... is that the
one where the guy sails to the pacific in a sloop or something and does a
bunch of research on how the navigators there follow 'star paths' by having
them navigate the boat in double blind tests (he locked away all his charts
and compass)? If so it was a pretty good read. Ill have to have a look for
that one again....

That's the one


If i recall, there was something about finding land by phosphoresence as
well?

"Telapa"


Shaun

Yes, Shaun, there is. For your edification, I refer you to, "We, the
Navigators The Ancient Art of Landfinding in the Pacific," by David Lewis.
Specifically chapters 5, "Keeping course by Sun, Swills, and Wind,"
chapter 6, "Dead Reckoning," which directly addresses your question, and
chapter 7, "Orientation Concepts in Dead Reckoning."

http://books.google.com/books?vid=IS...H54BfXwOv1l5Zs

"Shaun Van Poecke" wrote:

I have a very vague memory of maybe having read that book.... is that the
one where the guy sails to the pacific in a sloop or something and does a
bunch of research on how the navigators there follow 'star paths' by having
them navigate the boat in double blind tests (he locked away all his charts
and compass)? If so it was a pretty good read. Ill have to have a look for
that one again....

Your vague memory may in this respect be refreshed, and also expanded,
if you lread the discussion in Wikipedia about the Polynesian
Voyagaing Society and also the material/links on the PVS's own website
-- re. which, see, e.g.

http://pvs.kcc.hawaii.edu/aboutpvs.html
http://hokulea.soest.hawaii.edu/
http://en.wikipedia.org/wiki/Polynes...yaging_Society