"True wind" vs "ground wind"

[High Current]

While installing a wind sensor on my boat using Signal K (which is awesome BTW!), I was frustrated to find I couldn't get it to calculate true wind without a knotmeter because it requires speed through the water (STW). I was able to jump through hoops and make STW mirror GPS speed over ground, and along the way I found someone already created a GitHub feature request to make this easy, but it was rejected because it's "wrong." Why? I can't see any reason to use STW except as a relic of the days before GPS.

Where I sail, tidal currents are a thing, so STW is a pretty terrible indicator of how fast - or in which direction - my boat is actually moving. Not to mention the paddle wheel is always getting fouled and is only accurate at the start of the season (if it works at all). "True" wind, thus calculated, therefore tells me how fast the wind is moving relative to the tide. I actually need "ground wind" if I want to know how hard the wind is actually blowing, or from which direction, although unlike "true" wind it's relative to the compass, not the bow. Since the tide has no bearing on my sails, I fail to see why it should factor into windspeed calculation.

So...is there a purpose / value of "true" wind that I'm missing, or is it indeed just a relic? I feel like apparent and "ground" wind are the only things I should care about.

 

[thinwater]

Here we go ....

 

[PaulK]

Your question seems to assume that the instruments calculate the true wind information with such accuracy that a knot or two of current impacts the results a lot. Does it? How accurate are the results that actually come up on the screen? Has anyone tested them? Since the sails use the apparent wind to make the boat move does it really matter what the True or Ground wind speed and direction are?

 

[Synapse Navigation]

@PaulK, yes it matters to get the True wind and Ground wind, if you are looking for performance (note that the concept of performance can be very different for each of us, it can be racing, or it can be just enjoying a 'clean' leg well trimmed). The polars of your boat, the lay lines are refering to the True wind data. The weather forecast are 'Ground wind' data. If you sail in a high speed current area, the True wind (and then apparent wind) can be very different from the Ground wind given by the weather forecast.
I wrote a guide for these notions, the associated devices and their calibrations. It is available here : https://synapsenavigation.fr/les-guides-synapse-navigation/ (sorry it is in French ...)

 

[High Current]

@PaulK - no it doesn't matter. My question mostly arose from the Signal K developers' insistence that "true wind" needs speed through water. I want to understand why!

My interest is mostly academic. I sailed 5 seasons just fine without an anemometer (and one without even a windex), but I still didn't have a good "feel" for what 15 vs 20 vs 25 kt means because wind forecasts in ME are almost meaningless. Plus I'm a data junkie and want to know what I can realistically expect from my boat under what conditions.

 

[jssailem]

If I asked my Irish grandmother this question, she would have talked about spirits and wind fairies. The gremlins in the system and the leprechauns stealing your gold.

But you stated this is an academic query. From a mathematical perspective, how do you solve a problem with a missing variable? (highlighted in RED). This is a vector calculation. What substitute is there for Speed through Water (STW)
TWS=(AWS⋅cos⁡AWA−STW)²+(AWS⋅sin⁡AWA)²

 

[Davidasailor26]

Plus I'm a data junkie and want to know what I can realistically expect from my boat under what conditions.

That’s exactly where true wind is the most important. Here’s a simple case - Your polars or your historic data say you should go 4 knots dead downwind in 10 knots of wind. Now, one day you’re sailing downwind in 8 knots of ground wind speed, but against a 2 knot current. The only way to know your actual performance relative to your polars is by measuring the speed of the boat and the wind relative to the water. If you measure the speed of the boat relative to the ground it’ll be artificially lower because of the current holding the boat back. If you measure the speed of the wind relative to the ground you’ll think your boat speed should only match the 8 knot wind speed plot, but really it should match the 10 knot plot, because all the boat “knows” is the wind relative to the water.

 

[thinwater]

If you understand the questions, including the effects of variable currents on course made good and sea state, then you know the answers. If you don't understand the interactions, you won't understand the answers.

• If sailing in a large-area current, then the water is simply a moving frame of reference and the true wind over the water is the only one you will sense. Of course, your course over ground will be affected by leeway and the current.
• If the current varies (coastal, rivers) then the difference between ground and true wind may be apparent when you enter and leave the current. The sea state will also change for a mile or more, depending on wave size. Then, after a long period in the current, the waves accept the current as the new frame of reference, the period adjusting to the reference.

If the differences are not obvious, it's probably simpler to ignore them and consider it to be magic.

 

[Stu Jackson]

HC,

Whew, I'm sure glad you followed up your OP with this explanation, because when I first read your question I was aghast that a fellow C34 skipper could seemingly be missing the concept.

The answer is because that is what the definition of TRUE WIND is - for you data junkie. The reason is different and it is because you sail through water not over ground.

For someone who sails in coastal currents you are aware of the different speed over ground you make, for example, into or following a flood or ebb current. Sure, it affects the speed over ground and the wind you feel (apparent wind).

When Signal K programmed their instrument, they used the actual definition and required an input for the value.

I, too, sailed for years without instruments and have learned the terminology enough to understand the differences between true, ground and apparent.

If you do the vectors and two different situations, you'll see the affect: if the difference between boat speed and current are high SOG vs STW won't make much difference; if, however you're a 6 to 8 knot sailboat in 3-4 kt current, it sure does.

To understand why, just do the math. B&G has good manuals and tutorials, and I'm sure the equations are there to use.

Does this answer your question of why? If not, we can get deeper, conceptually, if you'd like. Iyt's an often misunderstood topic/subject.

Here's a recent long discussion from CF that covers the subject. It's interesting because of the discussion of true, apparent and ground. https://www.cruisersforum.com/forums/f90/true-wind-vs-apparent-wind-300979.html

@PaulK - no it doesn't matter. My question mostly arose from the Signal K developers' insistence that "true wind" needs speed through water. I want to understand why!

My interest is mostly academic. I sailed 5 seasons just fine without an anemometer (and one without even a windex), but I still didn't have a good "feel" for what 15 vs 20 vs 25 kt means because wind forecasts in ME are almost meaningless. Plus I'm a data junkie and want to know what I can realistically expect from my boat under what conditions.

 

[High Current]

OK I get it now. David's example helped a lot.

To rephrase: if I'm running at 5kt in 10kt of ground wind against 5kt of current, SOG is zero so my sails are seeing 10kt of apparent wind, which is the same apparent wind as I would normally see running at 5kt in 15kt with no current. So I would look at the 15kt line on the polars.

Thanks for the patient explanations!

 

[PaulK]

@PaulK, yes it matters to get the True wind and Ground wind, if you are looking for performance (note that the concept of performance can be very different for each of us, it can be racing, or it can be just enjoying a 'clean' leg well trimmed). The polars of your boat, the lay lines are refering to the True wind data. The weather forecast are 'Ground wind' data. If you sail in a high speed current area, the True wind (and then apparent wind) can be very different from the Ground wind given by the weather forecast.
I wrote a guide for these notions, the associated devices and their calibrations. It is available here : https://synapsenavigation.fr/les-guides-synapse-navigation/ (sorry it is in French ...)

Intéressant. Je vais m'y mettre pour voir pourquoi le vent réelle est tellement utile à savoir.