OpenCPN Manual on Ground Wind, True Wind, Apparent Wind

[Dockhead]

On a long passage between Greenland and Iceland, I re-read the OpenCPN manual (and as usual learned a number of interesting things).


I noticed, however, that in the Terminology section, which I had never erad before, the discussion of the differences between Ground Wind, True Wind, and Apparent Wind is a bit of a hash.


I might suggest a clearer set of definitions, something like this:


Apparent Wind: Wind speed and direction relative to the boat. Apparent Wind direction is expressed in angle to the bow of Ownship.


True Wind: Wind speed and direction relative to the surface of the water. True Wind direction is typically expressed as angle to the bow of Ownship. Note that mariners use the expression “True Wind” differently from people on land, including meteorologists. For them, there is no water surface to deal with, so True Wind = Ground Wind. Sailors sail in the interface between air and water, therefore mariners' True Wind is relative to water and different from landspeople’s true wind.


Ground Wind: Wind speed and direction relative to the ground (same as landspeople’s true wind). Ground Wind direction is expressed in compass direction.

[CarCode]

Originally Posted by Dockhead

On a long passage between Greenland and Iceland, I re-read the OpenCPN manual (and as usual learned a number of interesting things).


I noticed, however, that in the Terminology section, which I had never erad before, the discussion of the differences between Ground Wind, True Wind, and Apparent Wind is a bit of a hash.


I might suggest a clearer set of definitions, something like this:


Apparent Wind: Wind speed and direction relative to the boat. Apparent Wind direction is expressed in angle to the bow of Ownship.


True Wind: Wind speed and direction relative to the surface of the water. True Wind direction is typically expressed as angle to the bow of Ownship. Note that mariners use the expression “True Wind” differently from people on land, including meteorologists. For them, there is no water surface to deal with, so True Wind = Ground Wind. Sailors sail in the interface between air and water, therefore mariners' True Wind is relative to water and different from landspeople’s true wind.


Ground Wind: Wind speed and direction relative to the ground (same as landspeople’s true wind). Ground Wind direction is expressed in compass direction.

Which angle do you have between True Wind and Ground Wind?
I'm really interested...

[Dockhead]

Originally Posted by CarCode

Which angle do you have between True Wind and Ground Wind?
I'm really interested...

I don't understand the question. Are you asking about the relationship between True Wind and Ground Wind? The difference is the motion of the water in relation to ground.


Long discussion about it here:


http://www.cruisersforum.com/forums/...ion-73563.html

[Alan Mighty]

I will just note, to reinforce the authority of Dockhead's definitions and not to suggest that further debate or discussion is needed, that:

1. ground wind has been defined in the manner described by Dockhead in the wind energy industry since as early as 2013. See for instance Uwe Ahrens et al., Airborne Wind Energy, p. 413 (see attached graphic).

and

2. apparent wind and true wind have been defined in the manner described by Dockhead since as early as 1790. See for instance Leonhard Euler, A complete theory of the construction and properties of vessels … , trans. Henry Watson; where Euler defines true wind as the wind speed and direction calculated by vector subtraction of the velocity of the sail from the velocity of the apparent wind.

The only modification I could suggest would be to give two definitions of true wind (note that multiple definitions are a feature of many good dictionaries).

Definition 1 (the primary definition) could be exactly the text suggested by Dockhead.

Definition 2 might be something on the lines of "an approximation of true wind, derived through computation using instrument data such as compass heading, GNSS-derived Course over Ground (COG), and Speed Through the Water (STW)"

[rgleason]

Dockhead wrote:

I noticed, however, that in the Terminology section, which I had never erad before, the discussion of the differences between Ground Wind, True Wind, and Apparent Wind is a bit of a hash.

I might suggest a clearer set of definitions, something like this:

Dockhead,

I am responsible for creating the "hashed" Terminology Page.

The definitions you wish to change (and the others) were reviewed and discussed extensively during the process in the Tactics Plugin & Dashboard Plugin thread and several other threads by a number of involved users. They are a work in process and we would like to have them as accurate and useful as possible.

I have not read or followed the thread you reference, and will try to find some tme to do so. However I do not believe we should change them without further review and discussion, nor have I checked to see exactly what the differences are.

Additionally, some of these terms are tied to OpenCPN plugins and features, so we do not want to diverge from those standards to suit some meterologist's definition or whatever. Without getting into this in detail we really shouldn't change anything yet, IMHO, as it may conflict with Dashboard or Tactics, which do have some differences in terminology.

If someone originally involved or interested wishes to look into these proposed changes, making clear what the differences are and advise further that would be very helpful to focusing this discussion on the changes necessary.

Thank you. Also Dockhead, thank you for taking the time and having the interest to bring this to our attention.


Rick

[rgleason]

Some of the discussion threads (only portions)
http://www.cruisersforum.com/forums/...d-44087-2.html
http://www.cruisersforum.com/forums/...ml#post2315045
http://www.cruisersforum.com/forums/...ml#post2324847

These threads are quite long so I am having trouble digging the exact discussion out.

From "Terminology"
------------------------

Apparent Wind (AW) – Apparent wind is the wind as it would be perceived if one were standing on the deck of a boat that was subject to movement by; a) The movement of the boat within a body of water. (caused by some form of propulsion; sails, engine etc) and b) The movement of the body of water itself (caused by currents, tides etc.). Apparent Wind is relative to the boat.

Apparent Wind Angle (AWA) – Wind angle relative to the bow in degrees to port or starboard of the Heading, as measured by the onboard wind instrument.

Apparent Wind Speed (AWS) – Wind speed relative to the boat as measured by the onboard wind instrument

True Wind (TW) - True Wind is the wind that would be perceived if there was no movement of the boat through the water. It represents the wind perceived by an object stationary in the water, but moving with the tide or current. The wind felt by a stick floating down a river for example.

True Wind Direction (TWD) - Compass direction from which the wind is coming over the surface of the water. Sometimes called “Ground” Wind, not relative to the boat.

True Wind Angle (TWA) - Wind angle relative to the bow in degrees to port or starboard of CRS
(some instrument systems do not consider leeway in the calculation of TWA and in this case TWA is the wind angle relative to the bow in degrees to port or starboard of HDG).

True Wind Speed (TWS) - Wind speed over the surface of the water.


Ground Wind - (GW) Ground Wind the wind as it would be perceived if one were standing on land. For those on the water, it might be considered as the wind that is perceived when anchored or moored. Ground Wind readings assume that there is no movement of the boat in relation to the earth. Ground Wind is used in weather forecasts and reports.

Ground Wind Direction (GWD ) or (TWD)- Compass direction from which the wind is coming over the surface of the earth. Commonly known as True Wind Direction in the US.

Ground Wind Speed (GWS) or (TWS) - Wind speed over the surface of the earth. Commonly known as True Wind Speed in the US.

[rgleason]

First reading:

Apparent Wind "Wind speed and direction relative to the boat."

- This is a good way to start in each case, setting the plane of reference.

I have no problem with the clarification
"Apparent Wind direction is expressed in angle to the bow of Ownship."

and other similar clarification for True wind direction, and Ground wind direction, which is more specific to Opencpn.


I think we can improve these definitions, but would like further discussion.
Perhaps we simplify the wind speed and direction acronyms by including them below the 3 definitions.

[Dockhead]

Originally Posted by rgleason

------------------------

Apparent Wind (AW) – Apparent wind is the wind as it would be perceived if one were standing on the deck of a boat that was subject to movement by; a) The movement of the boat within a body of water. (caused by some form of propulsion; sails, engine etc) and b) The movement of the body of water itself (caused by currents, tides etc.). Apparent Wind is relative to the boat.

That is the heart of the matter.

Originally Posted by rgleason

– Wind angle relative to the bow in degrees to port or starboard of the Heading, as measured by the onboard wind instrument.

Apparent Wind Speed (AWS) – Wind speed relative to the boat as measured by the onboard wind instrument

Originally Posted by rgleason

- True Wind is the wind that would be perceived if there was no movement of the boat through the water. It represents the wind perceived by an object stationary in the water, but moving with the tide or current. The wind felt by a stick floating down a river for example.

Correct, but you are dancing around the heart of the matter -- that True Wind is the wind speed and direction RELATIVE TO WATER.

Originally Posted by rgleason

- Compass direction from which the wind is coming over the surface of the water. Sometimes called “Ground” Wind, not relative to the boat.

Wind relative to water and wind relative to ground are two different things. This definition is a real hash -- because "compass direction from which the wind is coming OVER THE SURFACE OF THE WATER" is NEVER called "Ground" Wind. "Ground Wind" is a term only used by mariners, and it is ground-referenced wind which people on land call "True Wind", which is what causes the confusion.



"True Wind Direction" is itself a land concept -- we express True Wind as an angle to the bow, and not in compass direction.

Originally Posted by rgleason

- Wind angle relative to the bow in degrees to port or starboard of CRS
(some instrument systems do not consider leeway in the calculation of TWA and in this case TWA is the wind angle relative to the bow in degrees to port or starboard of HDG).

And here you don't talk about reference at all. Wind angle relative to the bow -- WHICH wind? Relative to water or relative to ground? Relative to water is the correct answer.

Originally Posted by rgleason

- Wind speed over the surface of the water.

Correct.

Originally Posted by rgleason

- (GW) Ground Wind the wind as it would be perceived if one were standing on land. For those on the water, it might be considered as the wind that is perceived when anchored or moored. Ground Wind readings assume that there is no movement of the boat in relation to the earth. Ground Wind is used in weather forecasts and reports.

and is, confusingly, called True Wind by people on land, including meteorologists, although it is different from True Wind as the phrase is used by mariners.

Ground Wind Direction (GWD ) or (TWD)- Compass direction from which the wind is coming over the surface of the earth. [/QUOTE] NOT TWD!

Ground Wind Speed (GWS) or (TWS) - Wind speed over the surface of the earth. Commonly known as True Wind Speed in the US.[/QUOTE] No, not indeed in the U.S.! On land! AND furthermore, certainly not TWS! You correctly defined TWS further up.


This is really a hash, and needs to be cleaned up.




We are speaking in general terms, but these are precise concepts.


Ground Wind is calculated using COG and SOG.


True Wind is calculated using STW and Heading.


They are different, and you must not muddle them up like this.


WHY we use water-referenced True Wind is because the boat sails in the interface between wind and water. So all calculations and plotting requiring True Wind need THIS. As explained on Ockam's site:




"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 meteorology 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 alone prevents the calculation of current.
  • When converting to water-true, say for next-leg apparent, the local current [] 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"

True wind - Ockam Sailing Instruments






And here is a pithy definition from Raymarine:

• Difference between Apparent Wind, True Wind and Ground Wind

Apparent Wind = the wind speed and direction in relation to your boat.


Requires - Wind Data


So in a dead calm motoring ahead at 10 knots, you would have a 10 knot apparent wind at 0 degrees.


True Wind = the wind speed and direction in relation to the WATER.


Requires - Wind Data, Speed through the water (STR)


In the example above, you would have Apparent Wind of 10 knots and True Wind of 0. Your instruments calculate True Wind by taking Apparent Wind (the only thing your wind instruments measure directly) and taking out the vector for speed through the water. Which is why you get no true wind readout if you have not got STW data in your network.

Ground Wind = the wind in relation to land, not water.

Requires - Wind data, COG, SOG


In the example above, if there is no current and no leeway, Ground Wind = True Wind. If you are motoring directly against a 5 knot current, however, then you have Apparent Wind of 10, True Wind of 0, and Ground Wind of 5. To calculate Ground Wind, your instruments take out the vector for SOG, rather than STW. Ground Wind is of no relevance to sailors unless we are sailing in a strong tidal current and are trying to anticipate what the wind will be like when the tide changes.

Before GPS, it was impossible to calculate Ground Wind. That is because the only speed data we had was STW. Possibly the idea of True Wind versus Ground Wind arose because instrument makers who suddenly had access to SOG data on the network needed some way to differentiate true wind calculated that way versus True Wind calculated using STW.

Don't be confused by the fact that meteorologists use True Wind to describe the difference between wind and land. Of course they do -- they are sitting on land, not on water. That in no way contradicts our usage of True Wind as being the difference between wind and water.



http://www.raymarine.fi/knowledgebas....cfm?view=7595




Precision in these definitions is essential because the concepts are PRECISE. They really should not be muddled up like this. The instruments don't give "maybe land, maybe water -- it's subjective."



The Dashboard plug-in -- the only part of O which uses True Wind AFAIK -- does not have any option for choosing COG and SOG for the calculation, so I presume that unless it is using True Wind calculated data provided by another instrument, true wind is ALWAYS water-referenced, anyway. All the more reason not to fudge these concepts.

[rgleason]

Dockhead, you will not get good results by yelling.
Have a good day.

[rgleason]

Dockhead, I've gone through the definitions, and have used your suggestions and cleaned up and hopefully clarified some of these definitions.

I had a problem with this structure, "True Wind direction is typically expressed as angle to the bow of Ownship." as there is TWD and TWA, two different things. So I have "unhashed" that as well as other things in the Terminology definitions, such as conflicts between GW and TW (do not know how that happened!).

I hope you will take a moment to look it over and advised.
There is one other source of Terminology and Definitions that we need to refer to in Tactics_pi which Thomas has created for his plugin. Tactics Terminology

I've reviewed it and it is consistent with the Terminology page but more focused on instruments. Also with the exception of the definition of the VMG (towards or aways from TWD) definition, these are consistent with the Dashboard.

Thank you for your interest and help.

[Dockhead]

Originally Posted by rgleason

Dockhead, you will not get good results by yelling.
Have a good day.

No yelling was intended, and I apologize if it was taken that way.

[rgleason]

Dockhead, it's ok. I got over it fast. Thanks for improving and making the terminology better.