Differences Between Ground, Apparent, and True Wind Direction

[s/v Jedi]

It's always amazing to find how different other, experienced, sailors look at things.

Let's try to get agreement over the following points, and see where exactly we think the same or different about... because it's not clear to me at all:

1. A marine meteo wind direction is always referenced to True headings, i.e. geographical north instead of magnetic north.

2. A marine meteo wind speed is always referenced to the ground, at the height of 10 meters above sea level.

3. On a boat, apparent wind speed and direction is always referenced to the boat. The bow is pointing to 0 degrees so we can have S10 for 10 degrees at starboard or P140 for a nice broad reach with the boom at starboard.

Your instrument looks something like this:

Now, in the digital LCD part you see an indication "APP" which means apparent. This meter shows that the wind direction is P50 at 16.4 kts (can't read unit but assume it's kts)

4. If, on this display, you press the APP/TRUE button, the wind direction and speed will be corrected by a vector calculation which uses your log (SOW speed over water).

Oh my, I'm curious

cheers,
Nick.

[s/v Jedi]

Originally Posted by Dockhead

But every sailor who crosses the English Channel, with six hours of current sweeping you East, then six hours sweeping you West

For us Dutchies it is the Lowestoft <> IJmuiden crossing. It is 100 nm and straight east or west. The current is right on the beam switching sides every 6 hours. You always steer your boat either straight east or straight west, without compensating for the current.... assuming it takes you the average 12 hours to cross.

ciao!
Nick.

[s/v Jedi]

Ah, for those who never saw the "Ground Wind" expression as being used by sailing instruments, here is a picture; right under the clock, upper left corner:



ciao!
Nick.

[Ex-Calif]

Originally Posted by s/v Jedi

It's always amazing to find how different other, experienced, sailors look at things.

Let's try to get agreement over the following points, and see where exactly we think the same or different about... because it's not clear to me at all:

1. A marine meteo wind direction is always referenced to True headings, i.e. geographical north instead of magnetic north.

2. A marine meteo wind speed is always referenced to the ground, at the height of 10 meters above sea level.

3. On a boat, apparent wind speed and direction is always referenced to the boat. The bow is pointing to 0 degrees so we can have S10 for 10 degrees at starboard or P140 for a nice broad reach with the boom at starboard.

Your instrument looks something like this:

Now, in the digital LCD part you see an indication "APP" which means apparent. This meter shows that the wind direction is P50 at 16.4 kts (can't read unit but assume it's kts)

4. If, on this display, you press the APP/TRUE button, the wind direction and speed will be corrected by a vector calculation which uses your log (SOW speed over water).

Oh my, I'm curious

cheers,
Nick.

Agree 100%

Dockhead and everyone else. I am not trying to be argumentative or change anyone's mind. My only point is that if you are going to do passage planning you better know that everything on the boat is mag referrenced and everything the met gives you is true referenced. That boat wind true or apparent is ferenced to the bow is simple math to get mag bearing for wind so that is kind of a red herring. It is patently obvious though that the one critical confusing report is true wind speed. Same words, two meanings. If you choose not to know how to interpret met charts that's cool.

The error for anyone new or lurking is to look at a met report, 180@20, get on the boat and drive 090 mag in at 10 knots and -14*var and get to the bar and say, "wow. Those weather guys are stupid. I was driving 090 at 10 knots and the was on a run not a reach."

By the way, yes, christmas morning math 315* is the alternate tack. And it does matter.

The idea to shoot 'straight' across the current guarantees, sailing further than you need to, a french curve or a tack to the harbor at the other end.

[noelex 77]

Originally Posted by s/v Jedi

It's always amazing to find how different other, experienced, sailors look at things.

Let's try to get agreement over the following points, and see where exactly we think the same or different about... because it's not clear to me at all:

1. A marine meteo wind direction is always referenced to True headings, i.e. geographical north instead of magnetic north.

2. A marine meteo wind speed is always referenced to the ground, at the height of 10 meters above sea level.

3. On a boat, apparent wind speed and direction is always referenced to the boat. The bow is pointing to 0 degrees so we can have S10 for 10 degrees at starboard or P140 for a nice broad reach with the boom at starboard.

Your instrument looks something like this:

Now, in the digital LCD part you see an indication "APP" which means apparent. This meter shows that the wind direction is P50 at 16.4 kts (can't read unit but assume it's kts)

4. If, on this display, you press the APP/TRUE button, the wind direction and speed will be corrected by a vector calculation which uses your log (SOW speed over water).

Oh my, I'm curious

cheers,
Nick.

I don’t want curiosity to eat away at you Nick so I will have a go.

1-3 look right to me

4 Is probably right, but it depends on the capability of the instrument and how it is set up. I strongly suspect the entry level Furuno equipment will only display " true" wind, not ground wind.

I know you feel ground wind should never be displayed as angle from the bow, which I suspect is the reason behind the question, but there is no reason why it cannot be displayed in any form we like..
Personally I prefer a dial display which is permanently set to apparent wind, with digital displays of both apparent and ground wind. Others a different combination is better, which is fine. If you understand the difference you are doing better than most.

[cal40john]

Originally Posted by s/v Jedi

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.

I think everyone's response to my last post proves my point.


Yes I know a little about current sailing and I used to race here. 99% of our sailing is against or with the current not across it. Last trip home from the San Juans I never saw less than 8 knots over the bottom from Rosario Straits down to south of Admiralty Inlet. The tide ebbs south in the San Juans and north in Admiralty Inlet. Catch the change right in the middle and never slow down. Yes I trimmed my sails to my apparent wind regardless of which direction and how fast my GPS said I was going. Mostly I use my GPS to confirm I caught the currents and am making good time. Heading north I used the GPS to work my way east in the channel until I got to the higher current in Haro Strait, was doing 10-12 knots over the bottom there.

Most interesting feeling of something didn't look right is making windway tacking up Seymour narrows with around 5 knots of current going my way. Seymour narrows can get up to 14 knots.

I didn't say any of you were wrong. If you accept the definition of the poster in their own post they've made sense, what you're arguing is semantics and which version makes the most sense to use. And after telling me you're not doing it, you've continued.

John

[Dockhead]

Originally Posted by Ex-Calif

Agree 100%

Dockhead and everyone else. I am not trying to be argumentative or change anyone's mind. My only point is that if you are going to do passage planning you better know that everything on the boat is mag referrenced and everything the met gives you is true referenced. That boat wind true or apparent is ferenced to the bow is simple math to get mag bearing for wind so that is kind of a red herring. It is patently obvious though that the one critical confusing report is true wind speed. Same words, two meanings. If you choose not to know how to interpret met charts that's cool.

The error for anyone new or lurking is to look at a met report, 180@20, get on the boat and drive 090 mag in at 10 knots and -14*var and get to the bar and say, "wow. Those weather guys are stupid. I was driving 090 at 10 knots and the was on a run not a reach."

By the way, yes, christmas morning math 315* is the alternate tack. And it does matter.

The idea to shoot 'straight' across the current guarantees, sailing further than you need to, a french curve or a tack to the harbor at the other end.

If you have 14 degrees variation where you sail, then of course the consequences of confusing magnetic with true North are more serious. Where I sail the variation is about a degree and a half and decreasing every year, so absolutely negligible where wind direction is concerned (although I have to say, that where I sail, no wind forecast is accurate to within 14 degrees, either).

But we were talking about whether True Wind should be water or ground referenced. Not whether to use magnetic or true compass bearings. It's quite a different subject, I think.

[Surfer Girl]

These sorts of arguments about 'who is right' always come when professionals from different fields 'look' at things from a different perspective. So, we have professional mariners, meteorologists & (computer aided) instrument makers....

And never the twain shall meet! But does it really matter?

You guys need to get back into a sailing dinghy without any instruments again! A good driver, sailor, pilot relies on their senses and their trained brain to make constant decisions regarding the environment to steer a course, make adjustments and keep everyone safe. Things like True wind, Apparent wind, set, boat speed, current (and more) are automatically taken into account and the resultant action is a hand moving a tiller! Instruments and electronic gadgets are 'nice' to have and aid the driver, sailor, pilot - but trusting your senses and applying your skill accordingly is always the most important thing.

We are getting dazzled by computer generated information that we are being told is 'better' and will make our lives 'easier'. This is folly! If we keep dumbing down and not developing our very clever brains and motor skills we're going to end up like the humans in the movie 'Wally" .....

Sorry for getting a little off topic...

[s/v Jedi]

The difficulty is that not everything is magnetic referenced aboard. Your GPS is feeding variation information into the network and most displays can switch between magnetic and true. Normally, motorboats prefer COG, SOG and true headings etc. while sailboats use water referenced.

On airplanes there is no water referenced info. There isn't even a wind instrument in the cockpit, I don't understand how being pilot makes one understand how it's done on boats. I know it probably wasn't supposed to sound like that but it came across as such to me. Aero navigation is very different, although many of the principles are shared. The problem is the differences.

I hardly dare to ask anymore.... but the meter I linked in my earlier post... the reading is apparent wind there... are the degrees it shows true or magnetic?

*hides behind tree*

Merry Christmas!
Nick.

[Ex-Calif]

Originally Posted by Dockhead

If you have 14 degrees variation where you sail, then of course the consequences of confusing magnetic with true North are more serious. Where I sail the variation is about a degree and a half and decreasing every year, so absolutely negligible where wind direction is concerned (although I have to say, that where I sail, no wind forecast is accurate to within 14 degrees, either).

But we were talking about whether True Wind should be water or ground referenced. Not whether to use magnetic or true compass bearings. It's quite a different subject, I think.

Im ok with you or anyone narrowing the definition to only where mag var is 0.

Again limiting to the boat environment as well means you can throw away the met report and stick your finger in the air and use all mag bearings.

I would venture that a complete discussion of wind should include met reports and variation. Particularly for people who are not just sailing locally. My perspective in this thread has always been passage planning and passage making.

I have no argument with daysailing and 80% of my sailing is day sailing. But for passage making and racing proper planning and maximizing efficiency matters a lot.

Point and shoot at 10 knots works - you'll get there eventually. For a pilot (and I know we dont want to go there) it doesn't work at 200kts airspeed and 60 knots wind speed.

However even at 3 knots current and 8 knots boat speed it should matter to you what direction to head the boat on a 1500 mile crossing. A mag heading on the rhumb line could put you miles and miles downwind and down current.

By the way crossing a current especialy one you will drive through and leave behind, you should head into the curreent in general. The current acts like a lift into it and a header away from it. In other words you will be in the current longer if you head away from it at an angle.

[Dockhead]

Originally Posted by Ex-Calif

Im ok with you or anyone narrowing the definition to only where mag var is 0.

Again limiting to the boat environment as well means you can throw away the met report and stick your finger in the air and use all mag bearings.

I would venture that a complete discussion of wind should include met reports and variation. Particularly for people who are not just sailing locally. My perspective in this thread has always been passage planning and passage making.

I have no argument with daysailing and 80% of my sailing is day sailing. But for passage making and racing proper planning and maximizing efficiency matters a lot.

Point and shoot at 10 knots works - you'll get there eventually. For a pilot (and I know we dont want to go there) it doesn't work at 200kts airspeed and 60 knots wind speed.

However even at 3 knots current and 8 knots boat speed it should matter to you what direction to head the boat on a 1500 mile crossing. A mag heading on the rhumb line could put you miles and miles downwind and down current.

By the way crossing a current especialy one you will drive through and leave behind, you should head into the curreent in general. The current acts like a lift into it and a header away from it. In other words you will be in the current longer if you head away from it at an angle.

We're really talking about different things. Of course you are entirely right about not confusing magnetic with true compass bearings. I don't think anyone would disagree. Even with only 1 1/2 degrees of variation, I am careful to keep it straight. I have my navigation gear set up to read in magnetic, so everything is consistent with my compass. If I do plots on a paper chart, I am carefuly to use the magnetic compass rose on the chart, and not the lat/lon lines. So we don't disagree at all there.

But where true wind is concerned, the compass is simply not part of the equation at all. The question is -- I have 20 knots of wind 60 degrees off my starboard bow Apparent, and I'm travelling at 8 knots through the water -- what is the True Wind? Your instruments will subtract the vector from your STW from the Apparent Wind data from your wind instrument and will tell you something like -- you have 15 knots of true wind at 70 degrees off your port bow. There is no magnetic or true north here. The compass doesn't enter into the discussion at all. Some people think that -- in order to be consistent with the way weathermen use the term -- we should be subtracting our COG vector instead of STW vector, so that True Wind is referenced to the ground, instead of to the water. Again, no magnetic here anywhere, and no true north. It's a question of these many or those many degrees off the starboard bow. Whether your variation is 0 degrees or 30 degrees makes not the slightest difference.


Concerning getting a lift from the tide: I think you are talking about the lee bow effect. This is generally considered to be a myth, nowadays. The classic argument against the lee bow effect is this one:

"THERE IS NO LEE-BOW EFFECT - Dave Perry

"One of the most fascinating and timeless controversies in our sport is over what effect current has on how we sail and race our boats. Beginning in early 1979, Peter Isler and I filled hours of time debating the effects of current, and it wasn't until mid-1980 that he finally parted my clouds and shook me loose
from years of misconceptions and incorrect assumptions. Here then is my understanding of the effects of current, substantiated by several of my more mathematically-clever friends.

"Assuming that we're sailing in constant current direction and strength, No! As we've determined, the direction and strength of the current created wind is the same no matter at what angle the boat is aiming or at what speed it is moving. The presumption of the lee-bow effect is that if you are sailing directly into
the current you can pinch slightly, putting the current on your leeward bow, and the current will push you up to weather. This is obviously false because the only direction the current can move you is in the direction it is going (the stick on the river).

"The presumption of those who believe that in current a boat will have a different apparent wind direction and strength on opposite tacks, is that on one tack the boat will be slowed more by the current than on the other. The extreme example is when port tack takes you right into the current, and starboard tack takes you across it. The illusion is that on port tack it would seem that the boat is still going forward toward the wind, but that on starboard the boat is being swept away from the wind by the current.
Therefore, the apparent winds must be different on the two tacks.

"The fallacy here, though, is that the judgment of going toward the wind and being swept away are made in reference to fixed objects such as the mark, an anchored boat, or land. In reality, both boats are being affected equally by the current and the wind "sees" both boats in the same way. In other words, if you were following the race in a motorboat and were in the ocean where you couldn't see any land for reference, the boats would look identical on either tack, and in fact you would have no clue that there even was current unless you knew from charts or perhaps from the surface condition of the water. Put
another way, if you're sailing on a boat with apparent wind strength and direction instruments, they'll read the same on both tacks because the boat is affected in the same way by the current on either tacks (the stick in the river again). "

-- Excerpt from Winning in One-Designs by Dave Perry,
http://www.ussailing.org/member/library/wiodcurrent.htm

cited in LEE BOW EFFECT - Yachts and Yachting Online Forum - Page 1

[TeddyDiver]

I believe that the most confused discussion of the terms comes from the fact that (old days?) sailors (me anyways) didn't bother to think what true wind means, just it's true enough. Besides during dinghy races the true has allways been reported from a steady land based vane. In the oll days not even the million dollars maxies had gaskets to show "true wind" and just made their best quess (or calc). So my point is "true wind" originally was as true as should be. Just the those noobies making instruments messed it up..

[Ex-Calif]

Originally Posted by Dockhead

We're really talking about different things. Of course you are entirely right about not confusing magnetic with true compass bearings. I don't think anyone would disagree. Even with only 1 1/2 degrees of variation, I am careful to keep it straight. I have my navigation gear set up to read in magnetic, so everything is consistent with my compass. If I do plots on a paper chart, I am carefuly to use the magnetic compass rose on the chart, and not the lat/lon lines. So we don't disagree at all there.

But where true wind is concerned, the compass is simply not part of the equation at all. The question is -- I have 20 knots of wind 60 degrees off my starboard bow Apparent, and I'm travelling at 8 knots through the water -- what is the True Wind? Your instruments will subtract the vector from your STW from the Apparent Wind data from your wind instrument and will tell you something like -- you have 15 knots of true wind at 70 degrees off your port bow. There is no magnetic or true north here. The compass doesn't enter into the discussion at all. Some people think that -- in order to be consistent with the way weathermen use the term -- we should be subtracting our COG vector instead of STW vector, so that True Wind is referenced to the ground, instead of to the water. Again, no magnetic here anywhere, and no true north. It's a question of these many or those many degrees off the starboard bow. Whether your variation is 0 degrees or 30 degrees makes not the slightest difference.

Concerning getting a lift from the tide: I think you are talking about the lee bow effect. This is generally considered to be a myth, nowadays. The classic argument against the lee bow effect is this one:

"THERE IS NO LEE-BOW EFFECT - Dave Perry

"One of the most fascinating and timeless controversies in our sport is over what effect current has on how we sail and race our boats. Beginning in early 1979, Peter Isler and I filled hours of time debating the effects of current, and it wasn't until mid-1980 that he finally parted my clouds and shook me loose
from years of misconceptions and incorrect assumptions. Here then is my understanding of the effects of current, substantiated by several of my more mathematically-clever friends.

"Assuming that we're sailing in constant current direction and strength, No! As we've determined, the direction and strength of the current created wind is the same no matter at what angle the boat is aiming or at what speed it is moving. The presumption of the lee-bow effect is that if you are sailing directly into
the current you can pinch slightly, putting the current on your leeward bow, and the current will push you up to weather. This is obviously false because the only direction the current can move you is in the direction it is going (the stick on the river).

"The presumption of those who believe that in current a boat will have a different apparent wind direction and strength on opposite tacks, is that on one tack the boat will be slowed more by the current than on the other. The extreme example is when port tack takes you right into the current, and starboard tack takes you across it. The illusion is that on port tack it would seem that the boat is still going forward toward the wind, but that on starboard the boat is being swept away from the wind by the current.
Therefore, the apparent winds must be different on the two tacks.

"The fallacy here, though, is that the judgment of going toward the wind and being swept away are made in reference to fixed objects such as the mark, an anchored boat, or land. In reality, both boats are being affected equally by the current and the wind "sees" both boats in the same way. In other words, if you were following the race in a motorboat and were in the ocean where you couldn't see any land for reference, the boats would look identical on either tack, and in fact you would have no clue that there even was current unless you knew from charts or perhaps from the surface condition of the water. Put
another way, if you're sailing on a boat with apparent wind strength and direction instruments, they'll read the same on both tacks because the boat is affected in the same way by the current on either tacks (the stick in the river again). "

-- Excerpt from Winning in One-Designs by Dave Perry,
http://www.ussailing.org/member/library/wiodcurrent.htm

cited in LEE BOW EFFECT - Yachts and Yachting Online Forum - Page 1

Life is too short.

I can see you will not recognize the need to passage plan or reference back to met charts. I can restate my points over and over. If you refuse to recognize the need for a compass to reference met charts so be it. Life is too short.

Lee bow will or will not be believed. We did the math at a north sails racing seminar. I have raced extensively. See you on the race course. By the way. I am not talking about boat speed or boat performance. You will be in the current longer heading away from it and be set farther. I agree 100% thay the boat has no idea it is in a current.

[s/v Jedi]

Originally Posted by Ex-Calif

Life is too short.

I can see you will not recognize the need to passage plan or reference back to met charts. I can restate my points over and over. If you refuse to recognize the need for a compass to reference met charts so be it. Life is too short.

Lee bow will or will not be believed. We did the math at a north sails racing seminar. I have raced extensively. See you on the race course. By the way. I am not talking about boat speed or boat performance. You will be in the current longer heading away from it and be set farther. I agree 100% thay the boat has no idea it is in a current.

Ex-Calif, I had thought it would be assumed we all know the importance of met charts and how to interpret them, and how to use compasses and such. But you keep stating the opposite, so I mention that anyway, just to let it be clear you're not the only regatta racer here and we all do passages etc.

Lee bow effect however is a myth. Only when the current is changing like with the turn of he tide, it's usable for tactics. If the current is constant, the optimal VMG is on the tack with the smallest angle to the waypoint, i.e. the shortest route. You can not do away with it with a statement like "believe it or not", because you can prove it with a drawing and some math, or look it up on-line. Ask your race tactician, he will confirm the myth.

ciao!
Nick.

[David M]

In simple terms, it's all with respect to what.

True wind is the speed of the wind with respect to the Earth.

Apparent wind is the speed of the wind with respect to the boat.

As far as direction goes, you can pick true north or magnetic north.