Judging true wind direction

webcraft said:
Nail on head. Considering the amount of confusion this thread seems to have generated I don't think Raymarine and Maretron are to be commended for this, and I for one hope that their b@stardised terminology does not 'catch on'

- W
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It will catch on. Millions of sailors will read what their instruments tell them and accept it. I suppose you still use the term gay to describe a cheerful person?
 
webcraft said:
What I DO find odd is (if this is in fact what is happening) is the apparent refusal of the instrument manufacturers to move with the technology and change the way the TWS figures are calculated so it is an accurate display of true wind. Instead they seem to prefer to use or invent a little-used term, 'ground wind', to create a third display that shows the real true wind speed
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The instrument manufacturers have not invented this term, it's been in common use in racing circles for many years. They're just starting to add it to their instrument setup, so it's becoming more widely known.

while leaving available a display labelled 'true wind' that in fact shows tide induced wind.
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That's not the tidal induced wind though. Tidal induced wind is ONLY the part of the wind that is created by the tide moving the boat - not the whole value of "true wind" as displayed now.

I would like to sail with the ST70 instruments to see this in action. It may be helpful to Flaming, but it is going to confuse more people than it helps IMO, and seems rather unnecessary. The Merchant Navy use the term true wind, not ground wind, as do various oceanographic institutes.

- W
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But do we actually know what the merchant navy mean by true wind historically? I don't see how they were measuring it any differently from yachties pre GPS.
 
Woodlouse said:
It will catch on. Millions of sailors will read what their instruments tell them and accept it. I suppose you still use the term gay to describe a cheerful person?
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What a strange statement.

Millions of sailors don't use Raymarine and or Maretron instruments, and many that do won't be upgrading them. Many of those who DO use instruments that calculate a value labelled 'true wind' using heading and speed through the water are smart enough to understand that this is not true wind speed, only an approximation.

When manufacturers of geegaws are allowed to rewrite the scientific dictionary and sailors become too stupid to sail without blindly following instruments measuring dimly understood values then the battle has truly been lost.

- W
 
I have to say if I were instructing, then the tale of this whole thread would probably come up during a course to illustrate the potential confusion between true wind and ground wind. I would use it as part of teaching the whole concept
of wind direction.
Or should it be kept quiet and hope that raymarine get the hint?
 
flaming said:
The instrument manufacturers have not invented this term, it's been in common use in racing circles for many years. They're just starting to add it to their instrument setup, so it's becoming more widely known.



That's not the tidal induced wind though. Tidal induced wind is ONLY the part of the wind that is created by the tide moving the boat - not the whole value of "true wind" as displayed now.



But do we actually know what the merchant navy mean by true wind historically? I don't see how they were measuring it any differently from yachties pre GPS.
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Flaming:
Normally, I'm in agreement with you but on this one, not.

True wind is the wind strength and direction as measured from a stationary point on the surface of the earth. If you are moving due to current, tide, rolling on a skateboard down a hill, gliding, flying, sailing, riding a bike, sticking your head out of the window of a car, letting go of kedge in a river and drifting with the current .... what you feel is the apparent wind.

Correcting a race course for the tide strength is just a recognition of the effect that the current has on the true wind.

I can understand your explanation and having been around long enough to remember the original B&G hercules system that computed "True" wind (back in the early 80's) I can understand where the confusion has come from and why, with the introduction of GPS, the instrument makers had to come up with another term and why that has been introduced to the RYA performance squads ... still doesn't make it right.

The accuracy of measurement has also improved as corrections for upwash, boundary layer effects/windshear etc can also be measured and corrected for....but that's a whole other can of worms

Going back to the OP's question:
observation & judgement

ripples on the surface yes

smoke and flags on the shore: beware of the effect that crossing the coast and localised turbulence has on changing the direction of surface wind
 
The engineer said:
I have to say if I were instructing, then the tale of this whole thread would probably come up during a course to illustrate the potential confusion between true wind and ground wind. I would use it as part of teaching the whole concept
of wind direction.
Or should it be kept quiet and hope that raymarine get the hint?
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I find that just getting the concept of 'true' and 'apparent' over often takes a while. The boat I usually instruct on can display Flaming's 'true wind' (using the log) and I find it best not to muddy the waters at the start as it is a good enough approximation in most instances. I make a point however of picking an area with no or little tide to cover this point.

However . . . if someone is bright enough to ask how the 'true wind' figure is calculated I would go into the gory details.

- W
 
So the measure of wind that is the difference between the water and the wind. The measure of wind that is the actual power available to your sails, and the actual direction you are able to point your boat.

What do YOU call that?
 
Seeing as it it not something that is readily observable, I work with true wind (ground wind to you) apparant wind, and tidal stream. I trim to the apparant wind, and work out a best course normally on VMG. I don't feel the need to call it anything. I might call the change between true and the apparent induced by the tide, tide induced wind shear, but that would be about it.
 
Just seeing what B&G have to say on the matter. I can't find any explicit references to 'ground wind' but this is what they have to say about 'true wind':

"As with all true wind functions, True Wind Angle (TWA) is referenced to the water surface - therefore tidal flows affect the True Wind measurement."

All of the instrument makers seem to agree that 'true wind' is as measured when moving with any current. Not as when measured from a geostationary position.

And if you want to argue about what definitions are used by the hydrographic office, merchant navy etc, etc, who do you think makes their instruments?
 
Woodlouse said:
. . . if you want to argue about what definitions are used by the hydrographic office, merchant navy etc, etc, who do you think makes their instruments?
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They specifically mention the need to know COG and SOG, so I am not sure I get your drift there. One thing is certain, they don't read it off a Raymarine ST70 'true wind' screen.

- W
 
webcraft said:
Eh?

I call it 'apparent wind' - what do you call it?

- W
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Love the half quote you did there.

I'll try again.

Your boat garners power only from the difference between the velocity of the water, and the velocity of the air. If the air is doing the same speed and direction of the wind you have no ability to move your boat relative to the water. Regardless of the speed over the ground that you might be doing.

The difference in velocity between the water and the air is calculated by modern wind instruments and called "true wind". You seem to think that True wind is something else.

So what is this measure of the available power? What do you call it?
 
Woodlouse said:
Just seeing what B&G have to say on the matter. I can't find any explicit references to 'ground wind' but this is what they have to say about 'true wind':

"As with all true wind functions, True Wind Angle (TWA) is referenced to the water surface - therefore tidal flows affect the True Wind measurement."

All of the instrument makers seem to agree that 'true wind' is as measured when moving with any current. Not as when measured from a geostationary position.

And if you want to argue about what definitions are used by the hydrographic office, merchant navy etc, etc, who do you think makes their instruments?
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So does this mean that the Met office changes its forecast of true wind based on the current/tide ...... I think not.

PS: While you're at it, you'd better update US sailing too
http://www.sailingusa.info/true_wind_calculator.htm

Ockham Instruments actually provides a good explanation with better terminology than has been used earlier in this thread:

from http://www.ockam.com/truewind.html

Relativity
There are two frames of reference for wind, each with its own advantages.


Ground relative.
This is the 'normal' way of thinking when on land. "Wind's from the nor'east at 60". If you strapped an instrument on a car and drove around, the wind direction would always report as 45° (ignoring magnetic variation of course).

All meterology is earth-based and true north oriented, so earth-true is the number you have to use when route planning.
Using earth-based boatspeed & heading prevents the calculation of current.
When converting to water-true, say for next-leg apparent, the local current and magnetic variation must be considered.

Water relative.
On the water, the true wind that the boat feels is a combination of the earth-true and current. If the current is significant, water-true wind will be different from earth-true.

The boat responds to water-true because the keel is immersed in the water. Polar plots always use water-relative reference.
True wind angles are the same on both tacks (except for wind shear). If wind direction were calculated using COG/SOG, the true wind angles would be different on the opposite tacks (more on the difference here).
When being used for performance, because of this symmetry, water-true is the number to use.
Current is calculated by comparing water-based boatspeed & heading+leeway with COG/SOG.\

so: toys back in cots & handbags back to their non-combat postions
 
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Gay Faggots Fannying Around

At the end of the day the terminology is irrelevent if one understands the vectors. They could be called red, blue and green if tacticians wished to avoid the competition overhearing their tactical discussions in the bar.

If one races, where it matters, understanding the wind instruments definitions is vital to make use of them.

Personally it has probably never mattered that much to me as all my racing was Firth of Clyde or inshore lochs. However, the examples that Flaming has given, illuminates why the matter is important for racers.

Many things in life have different names which do not limit understanding and their use: hoovering my home, gathering faggots, putting something into a fanny bag.

In life there can be misunderstandings due to ignorance of the basic concepts and I suspect many weekend racers have a gay time fannying around with their instruments.
 
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flaming said:
The difference in velocity between the water and the air is calculated by modern wind instruments and called "true wind". You seem to think that True wind is something else.

?
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It is and if I take yoru meaning correctly then I am afraid you are just plain wrong. True wind is not related to tidal flow. True wind is the wind that would be experienced if your vessel was stationary with respect to the earth, NOT if it is stationary wrt the water. The latter is just a convention adopted in the past by instrument manufacturers because they were only able to take the boat's movement vector from the log. I thought you had grasped that, but obviously not, even after 140+ posts.

Apparent wind is what moves the boat and determines how high it can point, nothing else. Apparent wind is the result of all vectors, which are summed up by SOG and COG. SOG and COG can only be calculated by GPS.

I am sure you do not actually mean what you appear to be saying :eek:

- W
 
webcraft said:
It is and if I take yoru meaning correctly then I am afraid you are just plain wrong. True wind is not related to tidal flow. True wind is the wind that would be experienced if your vessel was stationary with respect to the earth, NOT if it is stationary wrt the water. The latter is just a convention adopted in the past by instrument manufacturers because they were only able to take the boat's movement vector from the log. I thought you had grasped that, but obviously not, even after 140+ posts.

Apparent wind is what moves the boat and determines how high it can point, nothing else. Apparent wind is the result of all vectors, which are summed up by SOG and COG. SOG and COG can only be calculated by GPS.

I am sure you do not actually mean what you appear to be saying :eek:

- W
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Try reading what I actually asked.

The power that drives your boat through the water is the difference in velocity between the water and the air.

Agree or disagree?

This is effectively what is measured by the "True Wind" display on your raymarine instruments. I also call this True wind.

What do you call it?
 
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