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.