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Old 05-01-2023, 11:35   #16
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Re: True wind from GPS?

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Originally Posted by El Pinguino View Post
GPS does not give you heading.
Oh yes, I guess I meant GPS+compass sensor. Electronic though.
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Old 05-01-2023, 11:38   #17
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Re: True wind from GPS?

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We are over thinking this problem. The instrument vendors are simplistically taking incremental GPS data over time to calculate both heading and boat speed (yes it is speed over ground, leeway is ignored for simplicity). Then the heading and boat speed are used to calculate true wind from apparent wind measured by the sensor.
So they are ignoring leeway and current then (?) since the true and apparent wind are relative to water, not ground.
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Old 05-01-2023, 11:45   #18
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Re: True wind from GPS?

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So they are ignoring leeway and current then (?) since the true and apparent wind are relative to water, not ground.
Correct
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Old 05-01-2023, 12:01   #19
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Re: True wind from GPS?

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Originally Posted by Dockhead View Post
...It is true that valid STW data is really really hard to get, and without valid STW data, your true wind calculations are useless. That's why many sailors give up and use SOG. But with any significant currents, the true wind you get using SOG is pretty much useless too.
I don't quite understand why valid STW is perceived as really hard to obtain.

We have had B&G propellor and now B&G (airmar) paddle wheel boat speed sensors for over 30 years. We've managed to maintain accurate boat speed readings during that time (at least as far as we can tell).

Boat speed needs to be recalibrated frequently but it is not difficult. I have a setting for when the bottom paint is fresh and clean, and then I make adjustments to the calibration numbers once or twice a year. If your bottom has gotten quite foul then the speed will be incorrect, and after a bottom clean it will need further re-adjustment.

Remember that calibration can be done with comparison to GPS, (a one way run if no current is present, or a two way run if current is present). I simply measure the distance on my chart plotter that we've travelled for 1 or 2 minutes, calculate the speed, and adjust the speedo calibration to match the speed observed. It takes about 5 minutes total. I have a spreadsheet.

A key factor is to remove the speed sensor when not in use. I don't mean daily, but if you are not underway for more than a week, pull out the paddle wheel. When you are racing it should be pulled and cleaned with a soft brush before each regatta.
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Old 05-01-2023, 12:09   #20
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Re: True wind from GPS?

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Originally Posted by Dockhead View Post
Ground wind is completely useless for sailing in this situation. Or even 6 knots against 2 knots of current, or with the current.
I disagree with the word “useless” .

These are a few examples where ground wind is better than true wind: only ground wind will tell you if the forecast conditions are accurate, predict which anchorages will be sheltered, or help you judge the likely wind shift when sailing around a headland where the current is expected to change. All valuable information, especially when combined with the apparent wind data.

Having said that, I accept that given a choice, accurate true wind rather than ground wind is preferable, but the problem is that on cruising boats it is rarely accurate. It bounces around and varies with tack, angle of heel and sailing direction. For example, sail around in a wide circle in zero current and most cruising yacht instruments will erroneously report a significant change in the true wind strength and TWD. Try this same test using SOG and the results are rock steady. Accurate ground wind is preferable to inaccurate and often misleading true wind.
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Old 05-01-2023, 16:31   #21
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Re: True wind from GPS?

[QUOTE I got aquestion in a quiz, basically asking if one had proper calibration and sufficient sea trials, if one could expect to get a reasonably accurate true wind from a GPS? I thought it would be "no" since the GPS only knows SOG, but I got it wrong. (?) What am I missing?[/QUOTE]

I would like to see the exact wording of the original question, because given the information the OP presented, he is correct. You cannot turn on your GPS and get any indication of the wind. Period. Actually, you can get a "reasonably accurate" true wind speed and direction by standing in the cockpit, or anywhere else out of doors. So what, exactly and specifically, did the original question ask? Were there other factors, such as a given scenario or situation? To me, this all points to a growing and regrettable lack of "sea sense," and the increasing reliance on black-box electronics instead of intelligence, training, and experience for navigation and seamanship.
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Old 05-01-2023, 21:24   #22
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Re: True wind from GPS?

I'd have to re-take the quiz to get the exact wording, but it was a really simple question, that was true/false. Can you get true wind using GPS and including calibration and/or sea trial data? So I assume it is using some form of the equation for leeway based on true wind and accounting for other known characteristics of a particular vessel.

Some form of

leeway = K * heel / speed squared
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Old 06-01-2023, 08:21   #23
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Re: True wind from GPS?

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Originally Posted by noelex 77 View Post
I disagree with the word “useless” .

These are a few examples where ground wind is better than true wind: only ground wind will tell you if the forecast conditions are accurate, predict which anchorages will be sheltered, or help you judge the likely wind shift when sailing around a headland where the current is expected to change. All valuable information, especially when combined with the apparent wind data.

Having said that, I accept that given a choice, accurate true wind rather than ground wind is preferable, but the problem is that on cruising boats it is rarely accurate. It bounces around and varies with tack, angle of heel and sailing direction. For example, sail around in a wide circle in zero current and most cruising yacht instruments will erroneously report a significant change in the true wind strength and TWD. Try this same test using SOG and the results are rock steady. Accurate ground wind is preferable to inaccurate and often misleading true wind.

Useless for sailing. Completely.


But I do agree that ground is not altogether useless. It is good for the purposes you cite. Just not for sailing, if there is any significant current.
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Old 06-01-2023, 08:26   #24
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Re: True wind from GPS?

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Originally Posted by wingssail View Post
I don't quite understand why valid STW is perceived as really hard to obtain.

Maybe because I don't have your skills?


Over my sailing career, STW has been a real challenge, but I was never as diligent with my paddlewheel transducers as you are. I admire your approach, and your diligence


The best STW data I had was when my previous CS4500 ultrasonic transducer was fairly new. When that died, I replaced it with a modernized N2K version of that which never worked well. Now I have the DX900+. We'll see how that works.
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Old 06-01-2023, 10:01   #25
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Re: True wind from GPS?

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Originally Posted by Dockhead View Post
Useless for sailing. Completely.


But I do agree that ground is not altogether useless. It is good for the purposes you cite. Just not for sailing, if there is any significant current.
I think most people would agree that ground wind is always superior on a cruising boat (because of the inherent accuracy) when sailing in areas of negligible current.

When sailing in areas of current, I agree that true wind is superior if you can achieve accurate STW (something I feel is not common on a cruising boat), but I still feel it is erroneous to call ground wind completely useless for sailing in current. Both true and apparant wind are only useful for the prevailing conditions. They are of little use predicting the sailing conditions in areas of changing currents.

Awareness of how the real atmospheric wind is changing (or not) is helpful, especially when sailing in and out of areas of high current.
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Old 06-01-2023, 10:21   #26
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Re: True wind from GPS?

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Originally Posted by noelex 77 View Post
I think most people would agree that ground wind is always superior on a cruising boat (because of the inherent accuracy) when sailing in areas of negligible current.

When sailing in areas of current, I agree that true wind is superior if you can achieve accurate STW (something I feel is not common on a cruising boat), but I still feel it is erroneous to call ground wind completely useless for sailing in current. Both true and apparant wind are only useful for the prevailing conditions. They are of little use predicting the sailing conditions in areas of changing currents.

Awareness of how the real atmospheric wind is changing (or not) is helpful, especially when sailing in and out of areas of high current.

We are violently agreeing.


Yes, of course. Ground Wind is the "weather wind" -- ESPECIALLY useful in strongly tidal waters, so you know what will happen when the tide changes, and in general, as you say, this is the longer-term wind. Ground Wind typically displaced as a compass direction -- and this is important.



I also agree that not that many cruising boats have decent STW. That does not however make Ground Wind a substitute! Even a knot of current will profoundly throw off your True Wind, if you are calculating it with SOG and displaying it with reference to the bow.
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Old 06-01-2023, 10:41   #27
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Re: True wind from GPS?

Is this not why mathematicians invented vector algebra? To help the poor sailors that attempt to describe both wind and boat velocity with a single number, and fail?

True wind is a vector; ground speed and a direction; but we can't get a direct measurement of that.
boat velocity thru water is also a vector; speed thru the water and direction.
apparent wind is still a vector, obtained by the vector addition of both true wind vector and boat velocity vector.
And, as been submitted before, boats dont' have a way to easily measure velocity thru the water, since we can't tell if our direction of travel is our heading. The paddle wheel responds to water movement in the fore/aft direction. That is, it will read zero if we are slipping 100%. My dash reports same value for apparent wind speed (scalar magnitude) vs true wind speed scalar, if the paddlewheel is not spinning. So, that fact tells us there is missing data for the wind computer. It is just doing the best it can..

Perhaps the ultrasonic transducer does a better job??
or, why not use GPS to get the accurate boat velocity vector? ie: is not the paddlewheel rather useless when GPS data is available?
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Old 06-01-2023, 11:33   #28
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Re: True wind from GPS?

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. . . or, why not use GPS to get the accurate boat velocity vector? ie: is not the paddlewheel rather useless when GPS data is available?
The problem is that in the sea, the water is almost never motionless. So, GPS-derived wind data will not give you the boat velocity vector which you need, for the jobs you need True Wind for. The jobs for which you read True Wind referenced to the bow of the boat, not to compass direction.

For compass direction wind problems, then the opposite is true -- you need the ground reference. All three kinds of wind (Apparent, True, Ground) have their own specific use cases.


To give an example, if you are sailing downwind in 10 knots of wind, with a 2 knot current on the beam, the Ground Wind will be 11.537 degrees different from the True Wind. That's a big difference, which could screw up your lay lines big time, or your sailing DDW, leading possibly to an accidental jibe.
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Old 06-01-2023, 12:00   #29
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Re: True wind from GPS?

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Originally Posted by Dockhead View Post
The problem is that in the sea, the water is almost never motionless. So, GPS-derived wind data will not give you the boat velocity vector which you need, for the jobs you need True Wind for. The jobs for which you read True Wind referenced to the bow of the boat, not to compass direction.


For compass direction wind problems, then the opposite is true -- you need the ground reference. All three kinds of wind (Apparent, True, Ground) have their own specific use cases.
I will have to look up the difference between true and ground wind. Never heard of it, but, I'm new at this. Fundamentally, apparent wind is easy to measure, and for the sail trim exercise, that and a paddlewheel are good enough. I rather disagree with the GPS vs boat velocity unknown. That is essential for accurate True wind calculation. GPS describes it accurately; what IS missing is the component of the Water Current Vector. And, the paddlewheel gives only an approximate solution. And for sail trim: is the current flow important info? For that matter, what value is true wind for sail trim?
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Old 06-01-2023, 12:06   #30
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Re: True wind from GPS?

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Boat speed needs to be recalibrated frequently but it is not difficult. I have a setting for when the bottom paint is fresh and clean, and then I make adjustments to the calibration numbers once or twice a year. If your bottom has gotten quite foul then the speed will be incorrect, and after a bottom clean it will need further re-adjustment.

What is the mechanism that causes a foul bottom to cause incorrect speed readings? Is it turbulence that causes the flow over the paddlewheel to no longer be representative of boatspeed?
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