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Old 29-12-2012, 09:06   #1
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Acronym TWS and TWD ?

Hello All,

In the Dashboard, the instrument "Wind history" has two acronyms:
- TWS
- TWD

Does that mean they.

Thank you in advance

Gilletarom
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Old 29-12-2012, 09:19   #2
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Re: Acronym TWS and TWD ?

I believe that would be True Wind Speed and True Wind Direction. As opposed to Apparent wind speed and direction which is influenced by the speed and direction of your boat's movement.
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Old 29-12-2012, 09:22   #3
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My guess would be True Wind Deviation (direction) and True Wind Speed
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Old 29-12-2012, 09:52   #4
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Re: Acronym TWS and TWD ?

Here are the wind measurement acronyms. I assume OpenCPN is using them in this way:

AWS = Apparent Wind Speed (as measured from your boat under sail)
AWA = Apparent Wind Angle (relative to your bow, as measured from your boat under sail)

TWS = True Wind Speed (the speed that would be measured if your boat was motionless on the water).

TWD = True Wind Direction (the compass heading from which the wind is coming, as measured from your motionless boat). This is the wind direction the way a land-based weather station reports it.

TWA = True Wind Angle (the wind angle, relative to the bow, after removing the "boat motion" component.

When your boat is motionless in the water, TWS = AWS, and TWA = AWA.

Wind Angles can be displayed in three ways:

Bow = 0
Starboard beam = 90, or 90, or 90 Starboard
Stern = 180
Port beam = 270, or -90, or 90 Port

There are subtleties in the True Wind measurements. Traditionally, the frame of reference is the water's surface, so currents can affect the TW numbers. Some instrumentation uses GPS-derived boatspeed for the Apparent-to-True calculations, and this gives a ground-based, rather than a water-based reference. This gives different numbers in the presence of current.

To be useful, TWD, in my opinion, should be ground-referenced, since this is how the GRIBs and weather charts provide it. It can be referenced to true or magnetic north.
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Old 29-12-2012, 10:57   #5
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Re: Acronym TWS and TWD ?

Gilletarom,

TWS = True Wind Speed
TWD = True Wind Direction

as Paul Elliot already described in the previous post.

For a description, please have a look at post #10 in "OpenCPN Beta Version 3.1.1224 Released" :
OpenCPN Beta Version 3.1.1224 Released

Thomas
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Old 29-12-2012, 11:03   #6
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Re: Acronym TWS and TWD ?

Quote:
TWS = True Wind Speed (the speed that would be measured if your boat was motionless on the water).

TWD = True Wind Direction (the compass heading from which the wind is coming, as measured from your motionless boat). This is the wind direction the way a land-based weather station reports it.
Nope thats Ground Wind.
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Old 29-12-2012, 11:13   #7
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Re: Acronym TWS and TWD ?

Whatever the exact definition is, the "wind history" instrument reads the NMEA sentences MWD and VWT.
And our other dasboard instruments call the MWD sentence "True Wind Direction", and VWT is "True Wind Speed".

MWD --> TWD --> True Wind Direction
VWT --> TWS --> True Wind Speed


Thomas
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Old 29-12-2012, 11:20   #8
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Technically speaking these are anagrams not acronyms. But that's not important now.

Everyone keeps looking at and configuring this open cpn software and I don't know why it is so much in demand. I understand and respect everyone's choices here but not the attraction.

I use is os device with navionics marine app. It costs very little indeed and permits me endless and free downloads for every chart possible.

I can't fault it especially considering the cost
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Old 29-12-2012, 11:32   #9
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Re: Acronym TWS and TWD ?

Quote:
Originally Posted by tom_BigSpeedy View Post
Whatever the exact definition is, the "wind history" instrument reads the NMEA sentences MWD and VWT.
And our other dasboard instruments call the MWD sentence "True Wind Direction", and VWT is "True Wind Speed".

MWD --> TWD --> True Wind Direction
VWT --> TWS --> True Wind Speed


Thomas
The Op was asking what does it mean, TWS and TWD are derived from STW and Heading, Ground wind uses SOG & COG

Dave
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Old 29-12-2012, 11:38   #10
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Re: Acronym TWS and TWD ?

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Originally Posted by Leonjennings View Post
I use is os device with navionics marine app. It costs very little indeed and permits me endless and free downloads for every chart possible.

I can't fault it especially considering the cost
Well, at least you're interested in O, otherwise you wouldn't follow this forum ...
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Old 29-12-2012, 11:49   #11
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Re: Acronym TWS and TWD ?

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Originally Posted by goboatingnow View Post
The Op was asking what does it mean, TWS and TWD are derived from STW and Heading, Ground wind uses SOG & COG

Dave
You're right. And sometimes I'm annoyed that my instruments use STW instead of SOG. Whenever we run down the waves with a gennaker, my true wind display is wrong, as the instruments don't take the difference between STW and SOG into account...

Thomas
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Old 29-12-2012, 13:14   #12
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Re: Acronym TWS and TWD ?

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Originally Posted by goboatingnow View Post
Nope thats Ground Wind.
This is from the NMEA spec for the MWD sentence:
Quote:
MWD – Wind Direction & Speed
The direction from which the wind blows across the earth’s surface, with respect to north, and the speed of the wind.
It appears thar OpenCPN displays MWD as TWD. For what it's worth, so does NavMonPc. If MWD isn't available, NavMonPc attempts to calculate TWD from the available inputs. By the way, the MWD sentence includes both direction and speed.

The whole "ground wind" vs "true wind" issue is confusing, since the distinction wasn't necessary until GPS made ground-referenced measurements practical. Until then, everything was water-referenced, since that's what we had to work with. For most sailing needs, the water-referenced measurements are more useful.
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Old 29-12-2012, 13:52   #13
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Re: Acronym TWS and TWD ?

Quote:
Originally Posted by Paul Elliott View Post
The whole "ground wind" vs "true wind" issue is confusing, since the distinction wasn't necessary until GPS made ground-referenced measurements practical. Until then, everything was water-referenced, since that's what we had to work with. For most sailing needs, the water-referenced measurements are more useful.
There have been a few threads on this in CF and there is certainly no consensus.
Personally I prefer ground based wind.

One big difference for a cruising boat is the GPS is always in perfect calibration. Unlike the log which is easily thrown out with a bit of fouling.

If Program's can be written to incorporate ground based wind (using SOG) I think quite a few cruising sailors will prefer this option when they have tried it.
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Old 29-12-2012, 13:59   #14
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Re: Acronym TWS and TWD ?

Quote:
Originally Posted by Leonjennings View Post
Technically speaking these are anagrams not acronyms. But that's not important now.

Technically speaking you're out in left field. They certainly are acronyms.
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Old 29-12-2012, 15:44   #15
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Re: Acronym TWS and TWD ?

Hello All,

Fantastic ... I ask a naive question (which I had the answer in front unknowingly). And I received a dozen replies in return!

I have a right to a "guerre pichrocholine" between advocates and defenders of word anagram and word acronym.

In any case, thank you all for your responses.

Gilletarom
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