Judging true wind direction

[webcraft]

.
It's getting worse.

True wind speed and direction has nothing to do with the movement of the water due to tide or current. A boat drifting on the tide and therefore moving over the ground is being affected by the apparent wind, while a boat at anchor and not swinging is being affected by the true wind. There is no such thing as 'ground wind', unless by that you actually mean 'true wind'.


- W

 

[flaming]

.
It's getting worse.

True wind speed and direction has nothing to do with the movement of the water due to tide or current. A boat drifting on the tide and therefore moving over the ground is being affected by the apparent wind, while a boat at anchor and not swinging is being affected by the true wind. There is no such thing as 'ground wind', unless by that you actually mean 'true wind'.


- W

I'm afraid you are just wrong there.

3 winds.

Apparent - the wind felt by the boat whilst it sails.
True - the wind felt by a drifting boat.
Ground - the wind felt by an anchored boat.

Understanding how the true wind will change as you sail out into stronger tide, or the tide changes, is a key part of race stratergy. It may be of limited interest to cruisers, but that doesn't stop the definitions being there, and being important to some.

 

[webcraft]

I'm afraid you are just wrong there.

You are making terms and definitions up. True wind only means one thing. When you up anchor and drift on tide the apparent wind changes, not the true wind.

I believe your confusion may arise because older instruments used the log (i.e. boat speed through the water) in combination with the masthead wind instrument to calculate true windspeed. In this case the distinction you make is relevant though not immediately helpful to the helmsman. However, modern instruments use GPS-derived COG and SOG to calculate true wind speed.

Your dispagaging remarks about cruisers - who generally have far wider experience and clock up higher mileages in a wider variety of waters than round the cans merchants - are really not helpful and add nothing to the discussion.


- W

 

[flaming]

You are making terms up. True wind only means one thing.

- W

Yes - true wind is the wind felt by a boat drifting with the tide.

Which is DIFFERENT from the wind felt by an anchored boat. Which we call ground wind.

Can you not see that?

 

[Deleted member 36384]

.... recognise the effect that tide will have on the true wind, ....

So the true wind has nothing to do with an air / water interface where the ground wind is dragged to a different direction. Its to do with how the boats will drift in current and thus affect the ground wind angle i.e. true is really an apparent wind before any effect of yachts headway is taken into account.

 

[flaming]

So the true wind has nothing to do with an air / water interface where the ground wind is dragged to a different direction. Its to do with how the boats will drift in current and thus affect the ground wind angle i.e. true is really an apparent wind before any effect of yachts headway is taken into account.

Correct.

Your boat can only use energy that results from the difference in speeds of the water and air. If there was a 10 knot tide and a 10 knot wind running in the same direction the true wind would be 0 and your boat is unable to move through the water.

The ground wind of 10 knots is completely irrelevent as you cannot use it to move your boat.

 

[webcraft]

Can you not see that?

It is nothing to do with 'not seeing' anything. I understand what you are saying but you are using terms up that are not in common useage and presenting the issue in an over-complicated and confusing way. I realise that this might not seem important to those who do not teach, but believe me it is.

Apparent wind is the wind you feel, the wind a boat sails by, and is a factor of the boat's motion through the air mass, whether caused by tide, propulsion or a combination of both. If you choose to differentiate between the apparent wind vector caused by the boat drifting on the tide and the apparent wind vector resulting from the boat's movement through the water then you are free to do so, but there is only true wind and apparent wind.

- W

 

[flaming]

but there is only true wind and apparent wind.

- W

But there is also ground wind!

Earlier in the thread one poster even stated that Raymarine now give you an option to output apparent, true, or ground.

So just saying "but I only deal in true and apparent" and then defining true as what others define as ground is only going to cause confusion.

 

[webcraft]

But there is also ground wind!

Earlier in the thread one poster even stated that Raymarine now give you an option to output apparent, true, or ground.

So just saying "but I only deal in true and apparent" and then defining true as what others define as ground is only going to cause confusion.

Show me one recognised authority that uses the term 'ground wind' and defines it as a separate entity from 'true wind'

- W

 

[pyrojames]

Yes - true wind is the wind felt by a boat drifting with the tide.

Which is DIFFERENT from the wind felt by an anchored boat. Which we call ground wind.

Can you not see that?

I can see that you a defending a limited definition of "true wind" used by a small (very small) group, which is obtuse in its definition and bears little relation to reality, or the use by a wider section of the community.

I doubt anyone outside you unique collection of racers will use it, and I would suggest that if you said that the "true wind" is only a term that sailors use, to pilots, you would get a good laugh.

As far as manouvring a vessel goes, typically people take into account "wind and tide", but I guess you only take into account "true wind" as it already accounts for the tide?!

 

[webcraft]

I can see that you a defending a limited definition of "true wind" used by a small (very small) group, which is obtuse in its definition and bears little relation to reality, or the use by a wider section of the community.

Quite correct - and by insisting that this narrow, obscure and utterly unnecessary definition is the right and only one, and disparaging others, the OP is doing a disservice to inexperienced sailors seeking an understanding of this rather important issue.

- W

 

[The engineer]

This has been an educational thread to me as I had never heard of ground wind before. I now firmly believe it exists!

Maretron seem to sum it up well:

Ground wind is the speed and direction of the wind referenced to a compass rose, which is different than apparent and true wind which are referenced to the bow of the boat. The apparent and true wind are interesting to see how the wind is effecting or pushing the boat (i.e., when sailing or docking), the ground wind is important for understanding the weather (i.e., where is the low pressure).

And they know a thing or 2 on the subject IMHO

 

[Deleted member 36384]

From my post with the links, the second link: -

The true wind is generically defined as a vector wind with a speed referenced to the fixed Earth and a direction referenced to true north. The meteorological definition of true wind (Table 1) references the direction from which the wind is blowing (Huschke 1959), while oceanographers often reference the direction to which the wind is blowing (Hosom et al. 1995). The Merchant Marine utilizes two true wind definitions: one identical to the meteorological and the other with the true wind direction reported relative to the bow of the ship (Bowditch 1984). The authors' experience with WOCE data indicates that the lack of a standard true wind definition or documentation of a specific definition is partially responsible for large discrepancies found in automated true wind data and in bridge measurements reported primarily by Volunteer Observing Ships (VOS; Pierson 1990; Wilkerson and Earle 1990; Kent et al. 1993).

I do like the "Ground Wind" definition. I dont see any issue with teaching true wind / apparent wind to introduce the concepts. The introduction of ground wind at a more advanced stage would be reasonable to identify the various vectors that apply.

 

[Woodlouse]

I can see that you a defending a limited definition of "true wind" used by a small (very small) group, which is obtuse in its definition and bears little relation to reality, or the use by a wider section of the community.

I don't think you can call racing sailors a very small group. I can also see the need in racing for recognising the difference between true wind and ground wind. Just because you don't race, and don't have a need for the terms doesn't mean you have to argue against them. From what I've seen Flaming write in the past he seem a fairly accomplished racing sailor who doesn't often talk b*llocks. He wouldn't be defending himself in this way if he wasn't certain of his facts.

 

[webcraft]

. . . true wind . . . referenced to the bow of the boat

Maretron (an obscure instrument manufacturer) are the only people AFAIK who use this definition. No sail training or teaching organisation uses it, nor does any yachting writer I am aware of.

It is unnecessary, wrong and confusing. If you want to use it then fill yer (old) boots, but please don't seek to confuse or mislead others on a public forum.

- W

 

[Lucky Duck]

Maretron (an obscure instrument manufacturer) are the only people AFAIK who use this definition. No sail training or teaching organisation uses it, nor does any yachting writer I am aware of.

It is unnecessary, wrong and confusing. If you want to use it then fill yer (old) boots, but please don't seek to confuse or mislead others on a public forum.

- W

As stated above, Raymarine also use apparent/true/ground and I wouldn't count them as an obscure instrument manufacturer.

 

[webcraft]

As stated above, Raymarine also use apparent/true/ground and I wouldn't count them as an obscure instrument manufacturer.

Can you reference this please?

The Raymarine ST70 can show:
~ Apparent wind speed and wind angle
~ True wind speed and true wind angle
~ Ground Wind Direction (GWD)

. . . but there is no definition of 'ground wind' supplied, andof course there is only a direction measured, not a speed. The 'ground wind direction' display merely shows the true wind on a compass type dial with North up - i.e. it references it to the compass direction rather than the boat's heading. This 'ground wind direction' is not in any way different from the true wind direction, it just saves you having to do a sum where you add or subtract the boat's heading from the true wind angle.

If the true wind display used the mechanical log (boat speed through the water) for calculation rather than COG/SOG from the GPS it woujld behave as you describe, and wold be a different value, but I do not believe this is the case. 'Ground Wind Direction' appears to be just an extra function Raymarine have added to the display. It shows exactly the same data as the true wind display, it just references it to a different scale (i.e. a compass rose rather than thte boat's heading).
- W

 

[The engineer]

Maretron are not that obscure. Maybe not that well known in the UK but in the US, quite common in larger yachts.
Maybe it is an Americanism?
Yachting writers are far from the definitive source of all that is truth in the world IMHO.
I am assuming your knowledge of "sail training or teaching organisations" is extensive and worldwide to enable your sweeping statement to be made in such a definitive manner.
Your statement "It is unnecessary, wrong and confusing. If you want to use it then fill yer (old) boots, but please don't seek to confuse or mislead others on a public forum" must be based on some true conviction that you and only you are correct. I am therefore changing my opinion on this to "open minded" if that is OK.

 

[flaming]

Can you reference this please?

- W

From their website.

http://www.raymarine.co.uk/view/?id=264

Wind

Apparent wind speed and wind angle
True wind speed and true wind angle
Ground Wind Direction (GWD)
Cardinal
Beaufort scale
Wind speed and wind direction graph
Min/max apparent wind speed
Min/max True wind speed
Min/max Apparent wind angle
Min/max True wind angle
Close hauled wind

 

[The engineer]

Even that obscure company Garmin seem to reference it:

GWS 10
Wind Sensor

The GWS 10 uses a wind vane for detection of wind direction and an anemometer to register wind speed. These measurements enhance your boat's potential as you adjust your sail accordingly to harness the wind's strength. Besides delivering wind measurements, the GWS 10 captures barometric pressure and air temperature data. When combined with other sensors, the GWS 10 provides those on board the ability to gather specific wind data such as ground and true wind, wind VMG and track heading. Combining wind and weather measurement from a single, integrated sensing unit, the GWS 10 installs with ease – providing data output to your cockpit display via NMEA 2000 cable and bus, which powers the unit. The GWS 10 is designed to interface with Garmin's GMI™ 10 universal marine instrument and / or GPSMAP 750(s)/4000/5000/6000/7000 series. The GWS 10 is fully networkable, with a "smart" transducer for data output via NMEA 2000.

* Provides wind speed
* Direction data to compatible NMEA 2000 display
* Shows barometric pressure and air temperature data
* Provides data such as Apparent Wind Angle, True Wind Angle*, Wind VMG*, Tack Heading*, Ground Wind*, Pressure, Pressure Trend*, Air Temperature, Set*, Drift*
* Features 10Hz wind direction output to create the smoothest graphical needle movement on the market today
* Wind speed range: 0-99 knots
* Input voltage: 9-16V

Key Features

* Wind speed range: 0-99knots
* Reports barometric pressure and air temperature data
* Provides extensive data – apparent wind angle, true wind angle, wind VMG, tack heading, ground wind, pressure, pressure trend, air temperature and drift.
* Smoothest graphical needle movement on the market today (10Hz wind direction output)
* Input voltage: 9-16V via NMEA 2000
* NMEA 2000 compliant