Judging true wind direction

[The engineer]

Now that would make an interesting poll

 

[NealB]

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?

I answered this one earlier (at least I think I did!).

I'd call that "apparent wind whilst drifting". This will change:

- as I start to sail or motor through the water

- as the strength or direction of tide changes

- when the direction or strength of the, errrr, true wind (to me) or 'ground wind' (to you) changes.

 

[snowleopard]

Quick question for those who think true wind is irrelevant - how do you estimate what the wind is going to do on the next leg of a passage?

 

[The engineer]

or whether an anchorage will be suitably sheltered.... but then that is ground wind
But seriously I have seen it so many times with people going into a "sheltered" bay to then realise that it is not sheltered from the true/ground wind.

 

[bbg]

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.

I guess I'll add another instrument manufacturer that disagrees with you: NKE

Every instrument I've seen that calculates True Wind (speed, direction, angle), does it with reference to the log ie speed through the water. Including my NKE system.

Although I knew this would be affected by tidal flow, I had not known, until this thread, that there was another name for sailors to use for reference against the ground (ie a stationary object).

 

[webcraft]

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?

I don't have a name for it personally as I have never seen a reason to give it a name. I suppose I would call it true wind without the tidal vector. Tide-influenced wind seems a reasonable name if you have to give it one. Water-referenced true wind wojld be another. When I am teaching at a fairly basic level I DO call it 'true wind' because that is all we have and it is a reasonable approximation, but I will on occasion explain to the comprehending and curious that it is not genuinely true wind as usually defined because it does not take tidal stream into account.

The statement that the 'power' that is available to move the boat is the difference between air speed and water speed seems to be a little simplistic, but I can see where you are coming from. Again though I do not find this helpful, either as a thought exercise or practically.

What I do not agree on is that this is the definition of true wind, which according to all the sources I have ever consulted is referenced to the earth, not the water and which I would therefore use COG and SOG to caclulate rather than speed through the water.

I teach what works and helps people to sail better, and do not get into this level of sophistry. I am sure it helps with your sailing, but you seem to have confused a few people here.

I do understand what you are saying. (I think . . . ) - it is your definition of what is meant by 'true wind' I disagree with, not your vector analysis.

Here's some web definitions of true wind - they all seem to imply it is land-referenced, not water-referenced:


Direction and velocity of wind as measured on land, distinct from apparent wind.
www.boats.com/glossary/T.jsp


The wind you feel when the boat is not moving
www.maritimetraining.co.nz/index.php/downloads/sailing-glossa…


The wind as felt by something that is not moving.
www.kitesurfingschool.org/glossary.htm


The speed and direction of the wind. The motion of a boat will cause the wind to appear to be coming at a different direction and speed, which is known as apparent wind
www.terrax.org/sailing/glossary/gt.aspx


Moving air that you feel while stationary.
www.windsurfingbible.com/glossary.htm


the direction and speed of the wind felt when stationary, at anchor or on land.
www.comminfo.rutgers.edu/~elfox/terms.html

the wind relative to a fixed point the observation of which is not affected by the motion of the observer
http://www.merriam-webster.com/dictionary/true wind

FWIW, this is not the first discussion - at great length - that has taken place on this subject - there is a lengthy and sometimes acrimonious thread over on Cruisers Forum as well.

- W

 

[webcraft]

I guess I'll add another instrument manufacturer that disagrees with you: NKE

Every instrument I've seen that calculates True Wind (speed, direction, angle), does it with reference to the log ie speed through the water. Including my NKE system.

Although I knew this would be affected by tidal flow, I had not known, until this thread, that there was another name for sailors to use for reference against the ground (ie a stationary object).

No-one disagrees that this is what yachting instrument manufacturers label 'true wind'. That stems IMO from the historical fact that boat speed was all that was available to add to the computation in the past. True wind ('true' true wind) has now apparently been renamed 'ground wind' by them. That does not however make it right.



- W

 

[webcraft]

Without wishing to appear rude it is becoming easy to understand why

If you don't wish to appear rude why not make a little more effort?

Go check out the definitions of 'true wind' I posted. I was not aware that Raymarine - or any other instrument manufacturer - was in the business of producing dictionaries, nautical or otherwise.

- W

 

[The engineer]

Apologies post deleted.

 

[flaming]

The statement that the 'power' that is available to move the boat is the difference between air speed and water speed seems to be a little simplistic, but I can see where you are coming from. Again though I do not find this helpful, either as a thought exercise or practically.

I can understand where the rest of your post comes from - even if I disagree, and am surprised that you don't have a name for the most useful measure of wind.

But I'm not sure why this is simplistic? In what other way does your boat garner power, other than the difference between those two velocities?

Oh, and I could reference any number of racing guides as sources supporting my definitions of ground wind, true wind and apparent wind, but for some reason they don't offer their content up for free on the internet! Must be some sort of money making weeze. If you make it to Southampton, grab a copy of pretty much any racing guide and have a look.

 

[webcraft]

Quick question for those who think true wind is irrelevant - how do you estimate what the wind is going to do on the next leg of a passage?

I think people said true wind was irrelevant when sailing the boat . . . you are talking about passage planning, which is a different thing altogether whether done in advance or on the fly.

- W

 

[Woodlouse]

No-one disagrees that this is what yachting instrument manufacturers label 'true wind'. That stems IMO from the historical fact that boat speed was all that was available to add to the computation in the past. True wind ('true' true wind) has now apparently been renamed 'ground wind' by them. That does not however make it right.



- W

Maybe it doesn't make it right. But like it or not the definition of 'true wind' has changed from what you understand it to be. Language has evolved again and there are new words, terms and phrases to learn.

If you get on a boat that has apparent wind, true wind and ground wind as options you won't go calling the ground wind display true wind any more than you'll start calling the true wind reading the tide affected wind or what ever term you think right. Neither will anyone else and lo and behold true wind will be the wind as felt by a stationary free floating object and ground wind will be as felt by a geostationary object.

What shore stations call true wind is irrelevant. They only have one measure of wind to worry about so to them any wind is just 'wind'.

 

[webcraft]

Maybe it doesn't make it right. But like it or not the definition of 'true wind' has changed from what you understand it to be. Language has evolved again and there are new words, terms and phrases to learn.

Apparently it has also changed from everyone else's definition including all dictionary definitions and that used by the merchant navy among others. Truly strange.

I guess so long as we all understand what we are talking about when we use a certain term it doesn't really matter . . . what really amazes me is the fact that an apparently simple matter has proved to generate as much heat (and lack of light) as the average ColRegs or anchoring thread.


- W

 

[grumpy_o_g]

I can understand where the rest of your post comes from - even if I disagree, and am surprised that you don't have a name for the most useful measure of wind.

But I'm not sure why this is simplistic? In what other way does your boat garner power, other than the difference between those two velocities?

Oh, and I could reference any number of racing guides as sources supporting my definitions of ground wind, true wind and apparent wind, but for some reason they don't offer their content up for free on the internet! Must be some sort of money making weeze. If you make it to Southampton, grab a copy of pretty much any racing guide and have a look.

Surely the force produced by the sails and boat's windage is a function of the delta between the boat's speed and air speed and the angle between the airflow and the durfaces (angle of attack), and nothing to do with the water velocity?

I'm intrigued by this new definition of true wind. Some of the instrument makers also make aircraft instruments and they'd be in serious trouble if they tried to claim True Wind - usually written as Wind (True) in my flying days - was anything other than the wind speed and direction relative to a fixed point on the Earth's surface. There'd be a few pilots walking back to base as well, if they were lucky.

Raymarine with their Raytheon background and Garmin should be especially ashamed of themselves; whether it is accepted or not now is irrelevant - when they started doing it there was absolutely no question of the definition of True Wind and they were showing you something completely different.

Having said that I got told off for calling COG or CTS or CMG or one of them the Track. It worked for a few hundred years but apparently it's now very confusing.

 

[flaming]

Surely the force produced by the sails and boat's windage is a function of the delta between the boat's speed and air speed and the angle between the airflow and the durfaces (angle of attack), and nothing to do with the water velocity?

I'm sorry, it must be early, but I don't really understand that. The available power to your sails has a lot to do with water velocity, since your boat "grips" the water not the land.

So if you are on a reach, but the tide is pushing you directly to windward, you will feel an increase in windspeed from the ground wind, and be able to sail through the water faster than if there was no tide.
And if the tide was taking you downwind, there would be less wind than ground wind and you would be sailing slower.

 

[bbg]

I'm intrigued by this new definition of true wind.

As I said in an earlier post, this is not a 2011 definition. Every instrument system I've ever seen calculates true wind speed/angle/direction using the wind instruments + log (boat speed through the water). Until reading this thread I didn't know any manufacturers offered the option of calculating any wind data based on GPS.

So for many, many years TWS, TWD and TWA has meant the wind with reference to the surface of the water.

I have to say that when I first read flaming's first post in this thread, I thought, "you're normally right, but you've got this one wrong, mate". After reading more, I agree with him about what true wind speed/direction/angle is. What is new to me is the concept of ground wind, and how it can be used.

So thanks to flaming for that - if I make it onto the podium in Bahia D, , yeah, right!) there's a virtual beer in it for you.

 

[The engineer]

So ground wind is a new piece of information in addition to true wind as indicated by marine instruments for decades.
The dictionary definition of true wind is land based and has never been calculated by onboard instrtuments. What has been called true wind on yachts
as read from an instrument has never been true wind as per the dictionary.
This seems to be the discrepancy.

 

[grumpy_o_g]

I'm sorry, it must be early, but I don't really understand that. The available power to your sails has a lot to do with water velocity, since your boat "grips" the water not the land.

So if you are on a reach, but the tide is pushing you directly to windward, you will feel an increase in windspeed from the ground wind, and be able to sail through the water faster than if there was no tide.
And if the tide was taking you downwind, there would be less wind than ground wind and you would be sailing slower.

And that tide alters the velocity of the boat, which is what is making the difference, not the tide itself. The actual water velocity may affect the boat's velocity but it's the boat's velocity relative to the airflow that will affect the force produced. Given that different boat's will be affected differently by the current in the real world because of keel depth, inertia, etc. I'd say that's quite an important distinction.

 

[flaming]

And that tide alters the velocity of the boat, which is what is making the difference, not the tide itself. The actual water velocity may affect the boat's velocity but it's the boat's velocity relative to the airflow that will affect the force produced.

Technically, I suppose you're correct, but that's not a very helpful way of thinking about it, given the reality of sailing.

Given that different boat's will be affected differently by the current in the real world because of keel depth, inertia, etc. I'd say that's quite an important distinction.

All boats are affected equally by tide.

 

[grumpy_o_g]

All boats are affected equally by tide.

OK, what I was trying to get at was something like a lift-keeler could tuck in to the shallows and avoid the the current where a deep fin keeler would be stuck out in the worst of it. You're right, I didn't word it well.

I've always tried to keep an image of drift in my head when sailing. When planning I'd calculate drift and then just apply that to True Wind (as in against a fixed point on land) to get the apparent wind with no way on. Add in the estimated boat speed through the water and the heading and you get the estimated apparent wind. Done at the same time as planning the route. Mind you we didn't win many races...