I have confirmed that at least with the NMEA 2000 Garmin units attached to a GMI10 display you have the option of selecting your speed / heading inputs from the following (assuming they are all present) magnetic compass, paddle wheel, and GPS....
Logic?
- Thread starter Albanach
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Now, THAT makes sense.
It would be helpful if someone did a test and see whether SOG or STW works for true wind!
It would be helpful if someone did a test and see whether SOG or STW works for true wind!
Another example
Here's another example: would you use SOG or STW?
boat heading due north, current at three knots running due north, apparent wind three knots from dead ahead
figure the boat's just drifting north on the current
Calculate the true wind
Here's another example: would you use SOG or STW?
boat heading due north, current at three knots running due north, apparent wind three knots from dead ahead
figure the boat's just drifting north on the current
Calculate the true wind
OK, wise guy!
Without all the discussion about definitions, let's keep it simple: true wind is defined as the vector that shows the real wind direction as compared to the apparent wind direction.
OK?
Without all the discussion about definitions, let's keep it simple: true wind is defined as the vector that shows the real wind direction as compared to the apparent wind direction.
OK?
Ok, I say 3 knots true wind. By my definitions, to which others might object.OK, wise guy!
Without all the discussion about definitions, let's keep it simple: true wind is defined as the vector that shows the real wind direction as compared to the apparent wind direction.
OK?
You might not think so, but I do. If you won't define the term "true wind", I am free to use my definition.don't think so, try again.
true wind = "the movement of the air relative to the water"
So in your case with a boat drifting on the water, with no speed through the water, showing an apparent wind of 3 knots from the north. The "true wind" is 3 knots from the north.
Give me a different definition of "true wind" and I will calculate a new answer for you.
This is exactly my problem with this term, some want to define relative to the water and some want to define it relative to land.
Please help me understand why the answer wouldn't be zero. It's exactly analogous to someone riding a bike down the street when there's no wind.
Witzend, yes that is correct. You just need to know what the person that you are talking to means. Not everyone uses the same meaning.
Stu, I defined the term "true wind" above. By that definition, I have a true wind of 3 knots on that river. If I defined "true wind" as the the wind felt as a person standing on the riverbank, it would be zero.
I am not sure where the disconnect in our communication is, but it might be that you are not using my definition, but steadfastly holding to a different definition. I don't really know where to go next to explain myself.
Well, another thought, modifying your analogy of a bicycle. A different analogy would be a person standing on a sliding sidewalk. He will feel a wind. It is the wind over that moving surface ( the river ) that I refer to as "true wind".
True wind is defined as the direction relative True or Magnetic (land based) that the wind blows to. So you have a true wind of 234 deg true or magnetic depending on which you use as your LAND reference.
I'm thinking that someone who would use an angle measuring system that is based on moving water would get all kinds of weird and arbitrary values of "true wind" as that system does not have any consistent way of being measured. Using the later definition all boats in a race would have a different "true wind"
I'm thinking that someone who would use an angle measuring system that is based on moving water would get all kinds of weird and arbitrary values of "true wind" as that system does not have any consistent way of being measured. Using the later definition all boats in a race would have a different "true wind"
With that said the vector equation for true wind is:
True wind - boat speed vector = apparent wind
or
True wind = apparent wind + boat speed vector
This is where you get into the discussion, the boat speed vector can be SOG and true heading where direction and speed are relative to the land or boat true heading and STW. the true heading accounts for current as if my bearing is 360 @ 1 knot and the current is at 180 at 1.5 knots the true heading is 180 and at a speed of 0.5 knots. But the STW is 1.0 knots. So which would you use the actual speed in the actual direction or the direction the boat is pointed (but not moving in ie current and leeway...) and the STW?
I know my RayMarine wind instruments use the GPS SOG and True heading not the SPW and true bearing. Which is why I say
True wind = (SOG/true heading vector) + apparent wind
True wind - boat speed vector = apparent wind
or
True wind = apparent wind + boat speed vector
This is where you get into the discussion, the boat speed vector can be SOG and true heading where direction and speed are relative to the land or boat true heading and STW. the true heading accounts for current as if my bearing is 360 @ 1 knot and the current is at 180 at 1.5 knots the true heading is 180 and at a speed of 0.5 knots. But the STW is 1.0 knots. So which would you use the actual speed in the actual direction or the direction the boat is pointed (but not moving in ie current and leeway...) and the STW?
I know my RayMarine wind instruments use the GPS SOG and True heading not the SPW and true bearing. Which is why I say
True wind = (SOG/true heading vector) + apparent wind
I know my RayMarine wind instruments use the GPS SOG and True heading not the SPW and true bearing. Which is why I say
True wind = (SOG/true heading vector) + apparent wind[/quote]
Well now I believe we have come full circle with this discussion, because for my Raymarine instrument St60+ W,S,D and C90W Chart Plotter setup all connected with sea talk there is no differencation between "True" and "Apparent" wind when water speed is not reported.
Bill if you know how to get the True Heading Vector and SOG data to substitute for the loss of STW, Albanach and I would be grateful. I'm going to look further into my Raymarine setup doc. to see it there are any options for this.
True wind = (SOG/true heading vector) + apparent wind[/quote]
Well now I believe we have come full circle with this discussion, because for my Raymarine instrument St60+ W,S,D and C90W Chart Plotter setup all connected with sea talk there is no differencation between "True" and "Apparent" wind when water speed is not reported.
Bill if you know how to get the True Heading Vector and SOG data to substitute for the loss of STW, Albanach and I would be grateful. I'm going to look further into my Raymarine setup doc. to see it there are any options for this.
wikipedia
Definition of apparent wind
The Apparent wind is the wind experienced by an observer in motion and is the relative velocity of the wind in relation to the observer.
Apparent wind velocity is the vector sum of the true wind and the headwind an object would experience in still air. The headwind velocity in still air is inverse of the object's velocity, therefore the apparent wind can also be defined as a vector subtraction: the Velocity of the wind minus the Velocity of the object.
A simple example
Suppose you are riding a bicycle on a day when there is no wind. Although the wind speed is zero (people sitting still feel no breeze), you will feel a breeze on the bicycle due to the fact that you are moving through the air. This is the apparent wind. On the windless day, the apparent wind will always be directly in front and equal in speed to the speed of the bicycle.
Now suppose there is a 5 mph wind coming directly from the north. If you pedal at 10 mph due north, you will feel an apparent wind of 15 mph from the north. But if you pedal 10 mph due south, you will feel an apparent wind of 5 mph from the south. The apparent wind is not only different in speed than the true wind, but also different in direction.
In these simple examples, the motion is parallel to the true wind which makes it easy to calculate of apparent wind. When the motion is not parallel to the wind, one must use vectors and trigonometry to calculate apparent wind.
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The example:
Velocity of the wind minus the Velocity of the object.
Wind "felt" (apparent wind) [3] minus velocity of the boat (moving north on the current) [3] = zero true wind
Definition of apparent wind
The Apparent wind is the wind experienced by an observer in motion and is the relative velocity of the wind in relation to the observer.
Apparent wind velocity is the vector sum of the true wind and the headwind an object would experience in still air. The headwind velocity in still air is inverse of the object's velocity, therefore the apparent wind can also be defined as a vector subtraction: the Velocity of the wind minus the Velocity of the object.
A simple example
Suppose you are riding a bicycle on a day when there is no wind. Although the wind speed is zero (people sitting still feel no breeze), you will feel a breeze on the bicycle due to the fact that you are moving through the air. This is the apparent wind. On the windless day, the apparent wind will always be directly in front and equal in speed to the speed of the bicycle.
Now suppose there is a 5 mph wind coming directly from the north. If you pedal at 10 mph due north, you will feel an apparent wind of 15 mph from the north. But if you pedal 10 mph due south, you will feel an apparent wind of 5 mph from the south. The apparent wind is not only different in speed than the true wind, but also different in direction.
In these simple examples, the motion is parallel to the true wind which makes it easy to calculate of apparent wind. When the motion is not parallel to the wind, one must use vectors and trigonometry to calculate apparent wind.
****************
The example:
Velocity of the wind minus the Velocity of the object.
Wind "felt" (apparent wind) [3] minus velocity of the boat (moving north on the current) [3] = zero true wind
You can easily observe apparent wind with a sock filled with corn starch and a short walk on a rather calm day. When you walk in the same direction as the wind the dust from the sock filled with starch will stay with you but when you walk into the wind the dust will blow away. Smoke does the same thing on a perfectly calm day smoke will rise straight up. If you carry a smoke source the smoke will trail out as apparent wind blowing it away from you.
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