judging current; rules of thumb?
 

"Jeff" wrote in message
. ..
Nope. Don't work. Not Nohow.

Ellen MacArthur wrote:
"Jeff" wrote
OK, please explain. Maybe this will bring Jax out of the woodwork to
tell us that Einstein proved that dead reckoning is impossible.

I can explain it too. I've been thinking about a way off and on
since you said there's no
way. Remember way back when we were talking about me getting in irons
because of wind
going the same speed as the current? You said unless there's land there's
no way to tell.
Well there is and I'm going to tell how.
1-- tie a long string on an empty wine bottle.
2-- throw the wine bottle off the boat when your sailing.
3-- get a stop watch time on how long it takes for the string to go
tight and pull the bottle along.
4-- pull the bottle back aboard.
5-- turn around and go the opposite direction
6-- repeat the first three steps.
Since the bottle goes with the current it lets you measure the
current.


You're right Jeff... but Neal is an idiot, so what do you expect.

--
"j" ganz @@
www.sailnow.com

judging current; rules of thumb?
 

"JaxAsby" wrote in message
...
Jeff wrote:
jlrogers±³© wrote:
"Jeff" wrote in message
. ..
Shaun Van Poecke wrote:
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?

Shaun
No, there aren't.

--
jeff


Yes, there is.

OK, please explain. Maybe this will bring Jax out of the woodwork to
tell us that Einstein proved that dead reckoning is impossible.


Use a venturi/pitot boat knotometer and rotate it 360 degrees noting the
speed accurately for increments of angle.

If it reads 0 knots for all angles your GPS speed is the current speed.

If it reads 5 knots forward, 0 knots aft and the GPS speed is 4 knots the
current is 1 knot from aft.

The same procedure is done for sideways currents but one must use
trigonometry, a feat easily accomplished on my Mensa circular mind meld
slide rule. I can do all the calculations in my head while wearing just a
set of Speedo briefs.

I've devised a set of Lorentz contraction gauges to measure all absolute
motion precisely in 3 dimensions. Aliens are after me to keep this gift to
mankind from falling in human hands so I must travel in disguise.

Jax


Stupid sockpuppet can't even read:

"Here's one for those of you out there without onboard
instruments/knotmeter/gps"

--
"j" ganz @@
www.sailnow.com

judging current; rules of thumb?
 

"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.
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. So the current goes two or three
knots. And the wind makes the boat go double that. You can use the boat motion
caused by the wind to measure the current. You just have to go back and forth.
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.

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.

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.

Your boat measures the wind speed.

You just contradicted yourself.

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.

Cheers,
Ellen

judging current; rules of thumb?
 

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.

judging current; rules of thumb?
 

Shaun Van Poecke wrote:
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?

Shaun



The ocean current at the surface is mostly wind driven. The surface
current diminishes rapidly with depth. Researchers have used with
limited success a long thin float dropped into the water. About one
meter sticks above the surface and about 3-6 meters below. The angle
that the stick tilts indicates the surface current speed relative to the
deeper layer current and the direction of tilt shows the current
direction. A bamboo pole on a weighted clorox bottle works ok. If the
float is anchored it is a better indicator.

Popeye

judging current; rules of thumb?
 

jlrogers±³© wrote:
"Jeff" wrote in message
. ..
Shaun Van Poecke wrote:
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?

Shaun
No, there aren't.

--
jeff


Yes, there is.

No, there aren't.

judging current; rules of thumb?
 

"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?

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


--
jlrogers±³©

judging current; rules of thumb?
 

"jlrogers±³©" wrote in message
et...
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

Would they be the '6 o'clock swills' they had in Australia before they
changed the drinking hours? It was a helluva job keeping any proper course
after those... :-)

judging current; rules of thumb?
 

"Edgar" wrote in message
...

"jlrogers±³©" wrote in message
et...
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

Would they be the '6 o'clock swills' they had in Australia before they
changed the drinking hours? It was a helluva job keeping any proper course
after those... :-)



Swills, smells, swells. Every data bit plots the navigator home.

--
jlrogers±³©

judging current; rules of thumb?
 

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.