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Old 03-02-2018, 01:19 PM   #1
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Airmar 220wx

Hi All,

I think I'm being confused by semantics here. Can someone explain this information (from the Airmar website) that is thoroughly confusing me:

"Virtually all mechanical and ultrasonic anemometers report apparent wind speed and direction. The Airmar WX Series is unique because it calculates both theoretical and apparent wind speed and direction. These wind readings are the same if the unit is mounted in a fixed location. However, if the WX Series is mounted on a moving vehicle, the apparent wind is the wind you would feel on your hand if you held it out the window while going down the highway. Since the WX Series has a built in GPS and compass, it calculates the theoretical wind based upon the apparent wind, speed of the vehicle, and compass heading. "

So far, so good. As a long-time sailor, I'm intimately familiar with the concept of true (never heard it called "theoretical" wind before!) and apparent wind, and the instrumentation and calculations necessary to determine one from the other. However, later in the same document from Airmar, they state:

"True Wind: True wind is the same as above BUT relative to True (or magnetic) North. In the case of a moving vehicle, True wind is not relevant because the vehicle will (almost) never be aligned to True (or Magnetic) North. In a mobile application True wind is a meaningless value."

Ehhh???? I be confused. True wind (and I believe Airmar's definition of Theoretical Wind) is the absolute direction on the surface of the earth from which the wind originates, and has nothing to do with the alignment and/or the speed of the vehicle (or boat in our case). AND it can be calculated and reported in true bearing, OR magnetic. So what the heck does Airmar mean by "...In a mobile application True wind is a meaningless value."???

Regards,

Pete
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Old 03-02-2018, 01:40 PM   #2
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I'm not sure what they mean either. But, we have an Airmar 200wx on Blue Sky and love it. It has many features, some of which we don't use such as rate of return.

But the most useful stuff is the wind info. The wind speed is true, and the wind direction is true, after the effects of moving relative to the planet are calculated and cancelled out by the instrument. Very helpful.

One odd thing though; when we had the WX on a boat equipped with Furuno, the true wind direction was shown relative to the boat's heading. When we moved to a Simrad system the true wind direction was shown relative to true north.
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Old 03-02-2018, 02:23 PM   #3
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The airmar stuff makes no sense.

I think we all understand apparent wind.

And I think ground wind is clear. Itís the wind speed and direction relative to the ground. Using vector math, itís apparent wind minus COS/SOG

True wind seems to have different shades of meaning depending on who you talk to. In general terms itís the wind speed and direction relative to the sea surface. Or at least thatís my understanding. Given current, the sea surface can be moving relative to the ground. But the specifics seem to vary by implementation. Technically speaking, itís apparent wind minus HDT/STW.

I think true wind exists because before GPS, all anyone knew was their movement relative to the sea surface. With the advent of GPS itís easy to calculate ground wind.

Where things get fuzzy is that I have seen devices calculate true wind when there is no STW. Go figure. All I can guess is that they are using HDT instead of GOG, but still using SOG. So it really neither of the two.
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Old 03-02-2018, 03:02 PM   #4
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I agree, the Airmar technical writer doesn't have a good grasp of apparent and true wind, expressed in either compass direction or true direction. As a lifelong sailor, wind was all important and one really did not need to know the true direction since we sailed and trimmed according to the apparent wind direction.
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Old 03-03-2018, 03:58 PM   #5
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Peter: I'm no expert here but I have the earlier version (PB 200) of this instrument and I really like it. It's connected to the Furuno FI-501 weather display and also directed to the Maretron monitor as well as the computer via N2K. So I get wind speed and vector showing up on the Coastal Explorer chart. I'm in the process of figuring all of this out right now, but I get minor (2-3 kt) discrepancies between the 501 display and the Maretron that may be true/apparent issue. Not sure.

Jim
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Old 03-03-2018, 05:49 PM   #6
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I get a difference between maretron which I have set to ground wind, and CE which displays true wind. But I have no STW, so an example of true wind that isn't really true wind.
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Old 03-03-2018, 07:10 PM   #7
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could it be that "true wind" is just the direction of the wind base on true north, rather than magnetic north ? I would assume this since it says "the vehicle will never be aligned to true north" Compass navigation is obviously based on mag. north, so true north is not a factor.

Whatever they mean, they could have found a much clearer way to say it!!
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