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


Ellen MacArthur wrote:
"Jeff" wrote
Nope. Don't work. Not Nohow.

but the boat is also going with the current. The boat speed is "through the water," not "over the bottom."

It will work. Where you go wrong is saying the boat is going with the current.

Your boat isn't affected by the current? That's quite remarkable!

It is if it's dead in the water. When it's sailing it's also going with the wind.
You never want to give the wind any credit.

The boat moves according to the wind it feels. Its not the same as
the wind it would feel if there were no current.

So the current goes two or three
knots. And the wind makes the boat go double that.

That's speed over the bottom, which you can't determine.

You can use the boat motion
caused by the wind to measure the current.

No, you'll measure the speed through the water as the same in both
directions.

You just have to go back and forth.

Won't help - you'll measure "zero" current

You don't need instruments to measure the wind. You can use Beaufort. The
wind becomes your reference point. It's better if you go back and forth at least
every forty-five degrees to really tell what's going on with the current.

You might be able to guess that there's a strong current against a
strong wind if the steepness of the chop doesn't match the wind, but
this is only a guess and certainly not accurate to a knot. Otherwise,
what you're saying is just gibberish.



The wind that the boater would call "true wind" is actually the vector sum of the wind and current (adjusting signs as
appropriate).

And if there's no boat around? There's still apparent wind if you have
a bottle drifting with the current.

Sure, but there's no way to determine the components.


nope. You're connected to the current while you're in the boat.

Only if your just drifting along. Once your sailing the wind makes your boat
move independently of the current. The current still affects it but it affects it
different in different directions.

How is that? Do you have some sort of "magic" current? Up here, the
current affects you the same no matter what direction you're pointing.


Your boat measures the wind speed.

You just contradicted yourself.

Never.


And how do you tell the boat speed over the bottom, as opposed to through the water?

The bottom's got nothing to do with this. He just wanted to know if you could know the
current with no outside landmarks and the bottom is an outside landmark.


If you know the current, and the speed through the water, then you
know the speed over ground. If you can't determine the speed over
ground, then you can't determine the current.