Differences Between Ground, Apparent, and True Wind Direction

[TeddyDiver]

Vane on the mast senses only apparent wind. To get true (or whatever name) it got to be calculated from boat speed. For that we can use either in water sensor (log or transducer) or GPS data. The first one being True Wind as we normally understand it and the second one as Ground Wind. ?? Got it right now?

[s/v Jedi]

Originally Posted by TeddyDiver

Vane on the mast senses only apparent wind. To get true (or whatever name) it got to be calculated from boat speed. For that we can use either in water sensor (log or transducer) or GPS data. The first one being True Wind as we normally understand it and the second one as Ground Wind. ?? Got it right now?

hurray!! and that while some still think true wind uses true, instead of magnetic compass degrees

I know I'm repeating but: apparent and true wind are written as degrees from the bow, i.e. 40 degrees starboard, while ground wind uses a compass reading, mostly in 10 degree steps, i.e. 10 degrees is a wind coming from just east of north.

cheers,
Nick.

[FSMike]

Just think, I gave up tax accounting for the simple life of a sailor.
Obviously I didn't know which way the wind was blowing.

(Would Bob Dylan have written "You don't need a gps to know which way the wind blows"?)

[Ex-Calif]

Originally Posted by Dockhead

Well, I don't think we are solving for that. Meteorologists sit on land and we sit in water. Their point of reference is one thing and ours is another. Hence True Wind is different for us, than it is for them. Their True Wind is our Ground Wind --

that is, if we accept the instrument makers' definitions. There's no law against talking like a weatherman, if you so choose.

But another argument for the idea that our True Wind is not their True Wind is that we express True Wind in reference to our bow, not the compass. The weather reports, of course, talk about compass directions. Like our Ground Wind.

What does compass corrections have to do with it?? That is not at all the difference between meteorologists' True Wind, and ours. The difference is that we reference True Wind to water, not to ground. So if the water is moving, there will be a difference between our point of reference, and theirs.

The reason you need to understand meteorological tws is to oerform accurate passage planning as I said in my long post. With the instrument on the boat which can only read apparent wind you must have boat displacement from a gps and variation and deviation to derive met tws.

I said already. If you don't want to refence and interface with met reports then you can ignore variation. But I certainly wont. In fact if you get a weather report at sea (from a met) guy he's gonna give you met true wind. If you don't care to reconcile that with your present conditions that's fine. But with your boat tws referenced from your bow you will not be able to compare it to the forecast winds.

If the yachties want to define the ground wind = met tws then we have created definition that is redundant and not needed and actually not accurate. The definition of gws above is simply apparent magnetic corrected for boat displacement using gps. It still is not reference to true bearing and cannot be referenced back to the met report until it is corrected for mag var and deviation.

However the reason yachties created ground wind speed is because yachties have been using "magnetic apparent wind corrected for boat speed and referenced to bow" and calling it tws. It is no way a "true" wind speed. At best it is "corrected apparent"

The only reason I am staying with this thread is that In this day of point and shoot navigation using a gps, the "proper" way to plan and navigate gets lost in the shuffle.

If I am doing a 5 day passage and lay out my expected way points the only place to start is with the weather forecast, lay in the desire rhumb line and then calculate whether my boat can meet the performance need to make this passage. I need to know my wind angles based on my boat performance , I will need to sail those performance angles later but before that I need to estimate current and leeway. If current is against leeway it could net zero or it could be additive.

Why do I care about the plan? On a reaching course probably don't but if I have to beat to my destination and determine that I will not make it in one tack I can decide where along the route it may be more advantageous to do that tack. It may be advantageous to sail the tack first or maybe last. Without a plan it doesn't matter. But if you are going to plan and then track the plan you need to be able to reference back to the met report.

[Dockhead]

Originally Posted by Ex-Calif

The reason you need to understand meteorological tws is to oerform accurate passage planning as I said in my long post. With the instrument on the boat which can only read apparent wind you must have boat displacement from a gps and variation and deviation to derive met tws.

I said already. If you don't want to refence and interface with met reports then you can ignore variation. But I certainly wont. In fact if you get a weather report at sea (from a met) guy he's gonna give you met true wind. If you don't care to reconcile that with your present conditions that's fine. But with your boat tws referenced from your bow you will not be able to compare it to the forecast winds.

If the yachties want to define the ground wind = met tws then we have created definition that is redundant and not needed and actually not accurate. The definition of gws above is simply apparent magnetic corrected for boat displacement using gps. It still is not reference to true bearing and cannot be referenced back to the met report until it is corrected for mag var and deviation.

However the reason yachties created ground wind speed is because yachties have been using "magnetic apparent wind corrected for boat speed and referenced to bow" and calling it tws. It is no way a "true" wind speed. At best it is "corrected apparent"

The only reason I am staying with this thread is that In this day of point and shoot navigation using a gps, the "proper" way to plan and navigate gets lost in the shuffle.

If I am doing a 5 day passage and lay out my expected way points the only place to start is with the weather forecast, lay in the desire rhumb line and then calculate whether my boat can meet the performance need to make this passage. I need to know my wind angles based on my boat performance , I will need to sail those performance angles later but before that I need to estimate current and leeway. If current is against leeway it could net zero or it could be additive.

Why do I care about the plan? On a reaching course probably don't but if I have to beat to my destination and determine that I will not make it in one tack I can decide where along the route it may be more advantageous to do that tack. It may be advantageous to sail the tack first or maybe last. Without a plan it doesn't matter. But if you are going to plan and then track the plan you need to be able to reference back to the met report.

I think you are profoundly confused about how the word "true" is being used, and I would like to help.

Let's slow it down.

Apparent Wind is not "magnetic". Its quality as "apparent" has nothing to do with true or magnetic compass directions. It is apparent because it is what you feel on your moving boat. If you are sailing with the wind ahead of the beam, the wind will seem stronger and will seem to come from further forward, than it will feel if you are stopped and drifting. That's Apparent Wind -- the subjective wind you feel on a moving boat, with the motion of the boat either adding or subtracting to the actual wind speed, depending on whether you are sailing into the wind, or away from it.

And as Nick explained, Apparent Wind is never expressed as a compass direction anyway -- it is expressed as a certain number of degrees from your bow.

OK so far? Now True Wind is the wind you would feel if you were drifting in the water with no boat speed. It's the direction and speed of the wind, again relative to your bow, with any effect of boat motion subtracted out. So if you have 5 knots of boat speed, your Apparent Wind will be exactly 5 knots greater than your True Wind if you are motoring directly into the wind, and exactly 5 knots less than True Wind if you are motoring or sailing directly away from the wind.

Your instruments calculate True Wind by taking the Apparent Wind directly measured by your wind instrument, and factoring in your speed through the water as measured by your log impeller. And so your instruments will tell you your True Wind in knots and degrees from your bow -- not as any compass direction, neither true, nor magnetic. "True Wind" is NOT true as in true versus magnetic. These having nothing whatsoever to do with each other. True Wind is true in the sense of without the effect of boat speed.


Your weather forecast will give you wind in terms of a compass direction. Naturally, if you are sailing to a magnetic compass (as I do!), the wind direction you get in your weather report is going to be different from your magnetic compass by local deviation and variation. I can't imagine that any weather forecasted wind will be so accurate that these few degrees are ever anything to worry about. In my area, if the wind is forecast as NW and it is in fact neither N nor W but somewhere NNW NW or NWW I consider that to be an extremely accurate forecast. The difference between true and magnetic compass bearings is completely washed out.

But anyway, the main thing to keep in mind is that True Wind has nothing whatsoever to do with true compass bearings!! They are entirely different concepts!! And there is no such thing as magnetic wind!!

By the way, Merry Christmas!!

[Cheechako]

I'm just not sure why we want to take current into account when discussing wind at all. It's more of a nav issue than a wind issue. We dont revise current speed taking into account wind speed do we? or maybe some of you do....? If I call someone onthe radio and say.." how much current are you seeing up there in the narrows?" I dont expect them to say "5 knots of current minus 20 knots of wind ... So I'm seeing -15 knots of current up here..."

[Dockhead]

Originally Posted by Cheechako

I'm just not sure why we want to take current into account when discussing wind at all. It's more of a nav issue than a wind issue. We dont revise current speed taking into account wind speed do we? or maybe some of you do....? If I call someone onthe radio and say.." how much current are you seeing up there in the narrows?" I dont expect them to say "5 knots of current minus 20 knots of wind ... So I'm seeing -15 knots of current up here..."

Exactly.

One more reason why water-referenced true wind is appropriate for us.

On the other hand, I can say from experience that it is striking what a big difference current can make. Crossing the English Channel you get six hours of current one way, then six hours the other way. At springs the current can be 4 or 5 knots (even more close to France). So if the current is running East at 5 knots and you have an 18 knot West wind (typical conditions), it feels like 13 knots (it will be True Wind of 13 knots, to speak precisely). Then when the tide changes, all of a sudden you've got 23 knots and wind against tide to boot -- an entirely different kettle of fish, to say the least. So there are cases where you need to pay attention to it.

[cal40john]

So far we have 3 if not 4 definitions of true wind going on here and as many people sure that there is only one way to define it. Until you get Bowditch updated, or an organization like RYA or ISAF to define the other terms or redefine true wind, you're going to argue about it. Even if you win your argument you now have only one more person with your POV, just the rest of the world to go. Seems to me come up with a concise set of definitions you use for your boat, and let anyone you communicate with know what they are.

John

[noelex 77]

Originally Posted by Cheechako

I'm just not sure why we want to take current into account when discussing wind at all. It's more of a nav issue than a wind issue. We dont revise current speed taking into account wind speed do we? or maybe some of you do....? If I call someone onthe radio and say.." how much current are you seeing up there in the narrows?" I dont expect them to say "5 knots of current minus 20 knots of wind ... So I'm seeing -15 knots of current up here..."

It is not the current reading that is of concern, but the wind reading. The current reading is not influenced by the wind, but the "true wind" reading is influenced by the current.

Most sailing instruments, when displaying true wind will report the wind speed and direction that would be felt on a boat that was drifting in the current. This is different from the wind that would be felt at anchor, when tied to dock,or on land, which, for want of better term, has been called ground wind in this thread

[Ex-Calif]

Originally Posted by Dockhead

Exactly.

One more reason why water-referenced true wind is appropriate for us.

On the other hand, I can say from experience that it is striking what a big difference current can make. Crossing the English Channel you get six hours of current one way, then six hours the other way. At springs the current can be 4 or 5 knots (even more close to France). So if the current is running East at 5 knots and you have an 18 knot West wind (typical conditions), it feels like 13 knots (it will be True Wind of 13 knots, to speak precisely). Then when the tide changes, all of a sudden you've got 23 knots and wind against tide to boot -- an entirely different kettle of fish, to say the least. So there are cases where you need to pay attention to it.

Dockhead - I am not profoundly confused at all. I have been reading meterological reports since around 1976 when I started pilot training. I understand exaclty how true and apparrent are used on boats. I have been race sailiing for a number of years. When passage planning on a boat you are dealing with vehicle speeds of 5-10 knots and I see tons of sailors describe a french curve when driving to a mark. This is becuase they do not understand leeway and current and plan poorly. In an ariplane you are dealing with vehicle speeds in hundreds of knots and if you are under atc control they expect you do drive a straight line to a waypoint.

You and everyone who wants to ignore the met report that't fine. Of course I will allow that we have mixed definitions in regards to tws. The boating tws is clearly different than met tws.

The question in you example of above raises my point completely. Until you go out in the boat the tws you are quoting comes from a met rerport and has nothing to do with the bow of your boat or your windex. So sitting in the cockpit planning a passage you do care about current, you should understand what tack is favored in order to minimize passage time. Once you get on the boat you want to compare actuaal conditions to the met report so you can see if you plan will work. If you cant fence back to foreecast conditions you are lost and will likely drive a french curve on the way to France.

In February we are doing a 60 mile ocean race. There are 3-4 knot currents all around Singapore. The winner is the person who underestands passage planning, currernts and will adjust his pkan based on actual vs. forecast conditions.

Merry Christmas!

[Ex-Calif]

Originally Posted by Cheechako

I'm just not sure why we want to take current into account when discussing wind at all. It's more of a nav issue than a wind issue. We dont revise current speed taking into account wind speed do we? or maybe some of you do....? If I call someone onthe radio and say.." how much current are you seeing up there in the narrows?" I dont expect them to say "5 knots of current minus 20 knots of wind ... So I'm seeing -15 knots of current up here..."

If you are on a passage with 000 rhumb line, true wind 000 and going to pass through a period of 090 degree cross current. Which tack is favored 045 or 135?

[s/v Jedi]

Originally Posted by cal40john

So far we have 3 if not 4 definitions of true wind going on here and as many people sure that there is only one way to define it. Until you get Bowditch updated, or an organization like RYA or ISAF to define the other terms or redefine true wind, you're going to argue about it. Even if you win your argument you now have only one more person with your POV, just the rest of the world to go. Seems to me come up with a concise set of definitions you use for your boat, and let anyone you communicate with know what they are.

John

John,

I think that all the electronics manufacturers agree on the term as described by Dockhead and me. The rest of organizations we don't care much about how they think, but if us, users of instruments, want to understand how they work and use them well, the terms as described by us are the way to get there.

Also, current.. is of much importance for sailing wind-wise. Current can make you sail straight into the wind when both course and wind over-ground is used so the only way to trim your sails right and steer the right angles is by using a wind direction referenced to the water instead of to ground... try it out !

cheers,
Nick.

[martinjrichter]

I learned my meteorology as an airplane pilot. That being said, we relied on the 500 millibar chart (about 7500 ft altitude), known as the winds aloft chart. The direction of wind at that level differs from "Ground Wind" due to friction with the surface features and mountains. Unequal heating of the earths surface also effects the "ground wind" which isn't apparent on the ocean surface. Water gains and losses heat much slower than earth which is why you see a directional change in the wind as you approach a harbor entrance for example. Usually there is approximately 30 degree directional difference but that varies dependent on location.

[Dockhead]

Originally Posted by cal40john

So far we have 3 if not 4 definitions of true wind going on here and as many people sure that there is only one way to define it. Until you get Bowditch updated, or an organization like RYA or ISAF to define the other terms or redefine true wind, you're going to argue about it. Even if you win your argument you now have only one more person with your POV, just the rest of the world to go. Seems to me come up with a concise set of definitions you use for your boat, and let anyone you communicate with know what they are.

John

Who's arguing? Everyone is entitled to use the term as they like.

I personally find the definition of True Wind as used by Raymarine, Furuno, etc. -- that is, referenced to water rather than ground -- to be absolutely logical and perfectly useful, and that's the way I think about it.

You are perfectly entitled to translate what your instruments tell you into something else -- into True Wind as meteorologists use it. You will have to make an additional calculation to get your style of True Wind since your instruments will not tell you how many degrees off your bow the wind is blowing, true, ground based. Nor do I quite understand how, exactly, that information will be useful to you. But -- if it makes you happy, no one is going to talk you out of it! You are the captain of your own ship!

I think we have not 4 or 5, but only two coherent definitions of True Wind -- one referenced to water, and another referenced to ground. I personally don't find anything at all troubling about the fact that True Wind may be calculated with different reference points for different situations -- one way to someone on land, and another way for someone on water. In fact, I think it is inevitable that different reference points will be used for different purposes. The issue arose when instruments were invented which were precise enough - and had the computing power -- to make these measurements and calculations. The instrument makers thought about it, and the consensus was that they should use water as a reference point, and they came up with the term Ground Wind for ground-referenced true wind. For all the reasons I have laid out, I agree with them. You are perfectly entitled to disagree with them, and with me, and think about the wind in whatever way you find most useful to you!

Merry Christmas, everyone!


P.S. -- Bowditch's calculations of True and Apparent wind do not work if True Wind is ground-referenced. They must be water-referenced to work (or it must be assumed that there is no current, so that water-referenced and ground-referenced True Wind are the same) -- because there is no correction in his formulas for current set or drift. See page 197, Bowditch, American Practical Navigator. This problem -- how to calculate your point of sail after a tack -- is an excellent example of why sailors need water-referenced True Wind. Most sailors, anyway.

[Dockhead]

Originally Posted by Ex-Calif

If you are on a passage with 000 rhumb line, true wind 000 and going to pass through a period of 090 degree cross current. Which tack is favored 045 or 135?

I'm not sure why you would take a 135 heading if you're trying to get to a way point bearing 000. Did you mean 315?

If the weather reported wind is 000 -- someone may call it Ground Wind, someone may call it True Wind (but in any case it's not the True Wind reported by our instruments, which is not expressed as any compass direction) -- then we get to the old question about lee bowing, which as far as I can see has nothing whatsoever to do with the OP's original question.

Lee bowing is a fascinating topic. Some people think that if you favor that tack which has you heading into the current, you will get there faster since you will travel a lesser distance over ground. I think that this has been proven to be a myth, since your boat travels through water and not on ground. So lee bowing may have some limited application in case your choice of tack improves the wind speed (not your example, since the wind direction is perpendicular to the current). But every sailor who crosses the English Channel, with six hours of current sweeping you East, then six hours sweeping you West, knows that the fastest way across is to maintain a constant compass heading (calculated using tidal vectors) and let the current do its worst. So your track on your chart plotter -- you path over ground -- is a big "S" curve. If you fight to maintain a constant COG towards your destination, you sail as much as 30% or 40% further through water, and it takes you correspondingly longer. English Channel Crossing 101. First thing you learn.