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Old 25-12-2011, 19:30   #61
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Re: Differences Between Ground, Apparent, and True Wind Direction

I wish I had displays of my gadgets to show some of these effects. I am suprised no one has mentioned the solar wind. In the tropics when you lick your finger and point toward the sun and look up at it, if that doesn't make you dizzy, then look down on your chart. You see a big blue spot, then it's time to sit down and think about it. Your vision will slowly come back. It's time to realize the ferocious solar wind is howling somewhere up there and the only remedy is cold beer. Warm beer requires 2.8 times the amount of cold beer.
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Old 25-12-2011, 19:51   #62
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Originally Posted by s/v Jedi

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.
North sail racing seminars are still teaching managing current. Like November. I am not sure that we arent agreeing. If on a windward leward course with a right to left current the preferred tack is port into the current.

Most of these conversations are among folks that do understand all this but for each ten people posting there are many lurking for which it is not obvious.

And perhaps your prior comment was simplistic or I didnt absorb it but you said something like shoot straight across a current. If by that you mean sail the rhumb line heading it wont work. You will sail a french curve. If that is not what you meant then sorry but that is what I received.

Also in general I try to avoid phrases like, you are clearly confused or you dont understand this. The mor obvious reason is the one or both parties are not communicating well and if you dont understand what I am saying I presume that it is my failure of communication. I dont presume you dont understand the concept or sailing.

Cheers!
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Old 25-12-2011, 20:12   #63
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Re: Differences Between Ground, Apparent, and True Wind Direction

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Originally Posted by Ex-Calif View Post
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.
But we never disagreed about that! Passage planning, understanding weather charts, understanding the difference between true and magnetic compass, was never the topic here!

What you are talking about is what many of us -- and what our instrument makers -- call Ground Wind. Believe me, we all understand it! If the wind is blowing from the West according to the Met Office, this is not something which confuses us! It is a very straightforward concept! And I daresay we all know that the Met Office quotes in true degrees, not magnetic!

There are two levels of complexity between what happens on our decks and the Met Office reports. The next level of complexity is the water in motion. So, we very well understand that if the the wind is blowing at 15 knots out of the West, and we head South across the Channel at springs, then it's going to seem like 20 knots for six hours, then suddenly it's going to be like 10 knots for the next six hours, as we are swept back and forth by the tide, towards the wind, then away from it. Anyone who does much crossing tidal bodies of water understands this phenomenon.

The next level of complexity is the wind we feel on deck, which is influenced by our speed through the water. If we're sailing upwind, our boatspeed will magnify the wind; downwind and it will diminish it.

The three winds known to our instrument systems -- Apparent, True, and Ground -- correspond to these three levels of complexity -- wind directly measured, wind corrected for STW, wind corrected for COG and SOG.


I do continue to think that you have confused "true" as in true wind with "true" as in true north, two different things. Your phrase "magnetic apparent wind", which I have never heard before, makes me think that you think you derive true wind from apparent by applying deviation and variation (rather than boat speed). It don't work that way, however you look at it. But I don't exclude that I am simply ignorant of something -- it has happened many times before, and I am always eager to have my ignorance dispelled. Maybe you could explain to us what "magnetic apparent wind" is?
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Old 25-12-2011, 20:56   #64
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Ex calif,

Is that North as in the sail maker? They should tend their sewing machines then

Seriously, I never heard of French curves other than in romantic sense

So here goes my attempt, please try to follow me now

If a current sets you away from a buoy which is upwind fom you, what is the best tack?

Answer: the best tack is the one that creates the shortest route, ignoring effects from current.

Now the explanation; the current is a constant. It does not make you sail a curve, it only changes your COG so many degrees, but it will be a straight line. Just like leeway. Your COG is constant.

Here's a good diagram; the red boat should have had the magical advantage as it's lee bowing, but it's clear that their relative positions are exactly the same current or not.


ciao!
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Old 25-12-2011, 21:43   #65
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Re: Differences Between Ground, Apparent, and True Wind Direction

OK, I've long thought that the tidal changes, where there are significant tides, cause wind due to the rising and falling of the water level. This is when there is no wind. Once when I was using the tide to draw out of an anchorage with a dead engine, there was a breath of wind that I used to move out into the bay, but it died once I was out there. Now I think the tide current creates an apparent wind of sorts as it moves your boat through the still air. I think we moved 6 miles in 14 hours. The novice helmsman got dismissed after losing 2 miles. The captain later learned the engine wouldn't work because the deadman's key was pulled out, lying on the cockpit sole.
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Old 25-12-2011, 22:05   #66
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Re: Differences Between Ground, Apparent, and True Wind Direction

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Originally Posted by Ex-Calif View Post
North sail racing seminars are still teaching managing current. Like November. I am not sure that we arent agreeing. If on a windward leward course with a right to left current the preferred tack is port into the current.

Cheers!
Maybe you gave a bad example here, but I have to disagree. Assuming uniform current and wind this example is a skewed course situation. Picture a one tack beat. On one tack you're lifted and slowed on the other headed and faster. Doesn't matter which one you do first. The one tack will be the same angle, speed, and time spent on it at one end of the course as the other, and that is true of the other tack. You'll arrive at the weather mark at the same time. You'll spend more time on one tack and so will have a long and a short tack. In addition my 2000 copy of North U. agrees.

Tactically there are several reasons to sail one leg over the other, but that's a different story.

John
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Old 25-12-2011, 22:23   #67
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Re: Differences Between Ground, Apparent, and True Wind Direction

Ahoy Jedi,
Sorry for my aside, but in your diagram, if the wind direction is from the committee boat at anchor and you are drifting in the current before the start, your true wind would seem to be coming from to port and the port tack would be best to take to the mark. If however you are sitting in irons before the start and seeing the wind coming directly from the mark, and you know you are being set to port, the starboard tack would be best.
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Old 25-12-2011, 23:42   #68
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Quote:
Originally Posted by s/v Jedi
Ex calif,

Is that North as in the sail maker? They should tend their sewing machines then

Seriously, I never heard of French curves other than in romantic sense

So here goes my attempt, please try to follow me now

If a current sets you away from a buoy which is upwind fom you, what is the best tack?

Answer: the best tack is the one that creates the shortest route, ignoring effects from current.

Now the explanation; the current is a constant. It does not make you sail a curve, it only changes your COG so many degrees, but it will be a straight line. Just like leeway. Your COG is constant.

Here's a good diagram; the red boat should have had the magical advantage as it's lee bowing, but it's clear that their relative positions are exactly the same current or not.

ciao!
Nick.
Maybe I did not explain clearly but your diagram proves my point. You boats start at opposite ends of the chart.

If both boats started beside each other on starboard tack, as they would in a race, with red upwind of green. Immediately after the start the red boat tacks to port. Red clearly wins. The vector lines for the red and green boats prove that. You and dockhead are putting words on my mouth by stating they I believe either boat has a Performance advantage (i.e. lee bow) I never claimed that. Because the green. Oat vector is shorter and higher green will spend less time in the adverse current.

That is what North is teaching. If your chart is a windward leward course anyone you long tacks left will lose.

Whether the mark is a race mark 1 mile away or a harbor 1000 miles away the boat with the shorter vector line pointed higher will get there first.

The comment about french curve is - if you sail a constant heading perpendicular to the current you will be set, if you do not sail a course above the rhumb line thereby creating a cog along the rhumb line you will eventually have to tack or as we all see beginning sailors do they will see they are being set, start to pinch and eventually run out of ability to head up and then they are forced to tack. I know you know this.

I doubt there is any disagreement about the basics.
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Old 26-12-2011, 00:08   #69
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I do continue to think that you have confused "true" as in true wind with "true" as in true north, two different things. Your phrase "magnetic apparent wind", which I have never heard before, makes me think that you think you derive true wind from apparent by applying deviation and variation (rather than boat speed). It don't work that way, however you look at it. But I don't exclude that I am simply ignorant of something -- it has happened many times before, and I am always eager to have my ignorance dispelled. Maybe you could explain to us what "magnetic apparent wind" is?
No confusion at all.

In navigation you have magnetic headings and bearings and true heading and bearings. True heading and bearings are referenced to true north on a chart. Magnetic headings and bearings are compass refenced and need to be corrected for variation and deviation to obtain true bearings and headings.

When measuring wind the meteorological office will reference wind direction from true north bearing and the call it true wind.

When measuringnwind sailors will measure apparent wind refenced from the bow. They will due a calculation for boat speed vector and adjust the apparent wind refernced off the bow and call this true wind.

The sailor uses the exact same term words "true wind" to describe something completely differernt than what the met office calls true wind.

You, Nick I and many others are not confused at all.

I know exactly what you are saying. Unfortunately I unable to communicate to you what I am saying.

Those that say I don't know what compasses, variation and deviation have to do with wind do not have a complete picture of the wind, weather forecasts and passage planning.

The reality today is simple. The gps gives you sog, cog and cts. I am 100 percent convinced the "average" sailor is adjusting heading to maintain a cog as described by a gps plotter by trial and error method.

That is if the rhumb line is 090 and after an hour sailing 090 the boat is set to left and the cog is 080 the skipper "tries" 110 for a while to get back to the rhumb line and then upon reaching the rhumb line "tries" 100 to stay on it. I have watched them do it! And when I show them the cts page and ask them what they think of that they look at me like the rca victor dog. Huh? What is cts? I didn't read that far in the gps manual.

That's what I call point and and shoot passage making. I don't care if everyone is doing it, machts nichts to me. But when the plotter takes a hike you oughta be able to calculate a cts to stay on the rhumb line from best and last known information including forecasts.

Sorry for the pilot reference but when I did my pilot license I had to plot the passage including cts to arrive at waypoints on time and on target. A pilots circular slide rule is used and true winds, true headings and all the numbers had to be complete for each leg before departure. No computer. Using a met report that references true wind bearings at multiple altitudes.

And once again - the title of the thread is about true wind and apparent wind. In the most simple sense we all know that apparent wind is wind measured by the windex and indexed to the bow. True wind is adjusted for speed vector. But until you talk about "the other true wind" I dont think the convsationis complete.

The only conflict I still have that the ground wind proponents can help me with is the statement that ground wind = meterological true wind. I am not getting it.

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Old 26-12-2011, 00:16   #70
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Quote:
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Ahoy Jedi,
Sorry for my aside, but in your diagram, if the wind direction is from the committee boat at anchor and you are drifting in the current before the start, your true wind would seem to be coming from to port and the port tack would be best to take to the mark. If however you are sitting in irons before the start and seeing the wind coming directly from the mark, and you know you are being set to port, the starboard tack would be best.
Thank you...

I think this is the theory that is being debunked. If there was zero wind. The boat true wind in 5 knots current (assuming Nicks chart is north oriented) is 270@5 - you better be on port tack!

The boat doesn't care what is causing the true wind. If the wind on the committe boat (anchored) is dead calm and the current is running 5 knots the race boats will feel 5 knots true. And if you are a super sailor and can eke out a knot or two of forward speed the apparent wind will move forward, of course...

Nowif the real wind picks up to 180@10. Port tack is still favored.
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Old 26-12-2011, 00:50   #71
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Re: Differences Between Ground, Apparent, and True Wind Direction

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Originally Posted by s/v Jedi View Post
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.
I'm not sure if this is a question meant to test someone else's understanding, but in case you don't already know the answer, the apparent wind angle measurement is in neither true or magnetic degrees. It is in degrees relative to the bow of the boat, where the bow is 0, starboard is 90, the stern 180, and port is 270 (or port can be -90). Sometimes the numbers run from 0 to 180, with "P" or "S" appended to indicate side.

This is the case for apparent wind and boat-referenced true wind.

The ground wind, or ground-referenced true wind as given by the forecasts is given in true degrees referenced to true north. You can of course adjust this to magnetic if that makes it more convenient for you, but it is reported in true (north) degrees.

I think the term "ground wind" was invented to avoid the confusion between sailing "true wind" and meteorological "true wind".
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Old 26-12-2011, 00:51   #72
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Re: Differences Between Ground, Apparent, and True Wind Direction

You have to do the other tack on a windward/leeward course.

In my diagram I've laid out the lifted and headed tacks, and tacked on the laylines. See that the distance and angle sailed on port tack is the same whether you do it at the top of the beat or the bottom? And the same is true of the starboard tack? So the distance sailed is the same in each case, but the course is skewed making for a long and short tack.

John

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Maybe you gave a bad example here, but I have to disagree. Assuming uniform current and wind this example is a skewed course situation. Picture a one tack beat. On one tack you're lifted and slowed on the other headed and faster. Doesn't matter which one you do first. The one tack will be the same angle, speed, and time spent on it at one end of the course as the other, and that is true of the other tack. You'll arrive at the weather mark at the same time. You'll spend more time on one tack and so will have a long and a short tack. In addition my 2000 copy of North U. agrees.

Tactically there are several reasons to sail one leg over the other, but that's a different story.

John
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Maybe I did not explain clearly but your diagram proves my point. You boats start at opposite ends of the chart.

If both boats started beside each other on starboard tack, as they would in a race, with red upwind of green. Immediately after the start the red boat tacks to port. Red clearly wins. The vector lines for the red and green boats prove that. You and dockhead are putting words on my mouth by stating they I believe either boat has a Performance advantage (i.e. lee bow) I never claimed that. Because the green. Oat vector is shorter and higher green will spend less time in the adverse current.

That is what North is teaching. If your chart is a windward leward course anyone you long tacks left will lose.

Whether the mark is a race mark 1 mile away or a harbor 1000 miles away the boat with the shorter vector line pointed higher will get there first.

The comment about french curve is - if you sail a constant heading perpendicular to the current you will be set, if you do not sail a course above the rhumb line thereby creating a cog along the rhumb line you will eventually have to tack or as we all see beginning sailors do they will see they are being set, start to pinch and eventually run out of ability to head up and then they are forced to tack. I know you know this.

I doubt there is any disagreement about the basics.
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Old 26-12-2011, 05:29   #73
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@Paul,

Yes, I was asking to test and isn't it amazing how many people read it but don't feel sure enough to answer?! The thing is that there is a group who is dependent on seeing their boat move over a chart on a plotter and we all (rightly so) start calling on paper charts, learning to navigate etc. But there is also this large group of old salts who know how to navigate, often incl. sextant, but have no clue what these modern instruments actually show on their dials. I hate to admit that I'm starting to be an old salt, but my career in electronic engineering drives me to learn and understand every aspect of the new electronics coming on the market. It also allows me to open my mind for new ideas, technologies and methods.

@John,

Thank you for the diagram. I fear it's gonna be a futile enterprise though. A long time ago some well known sailors came up with this lee bowing effect as secret advantage and as often the case, scientific proof of it being a myth is pushed aside without engaging the detailed proof that is put on the table. There is never gonna be a diagram and/or math in response.

All that the discussion now shows is regular layline tacking; the myth of presenting the current your lee bow for an extra boost is clearly debunked in the diagram I linked, where the red boat does this. The blue boat, according to the theory, would have had a penalty for not lee bowing, but the diagram shows that the current has zero effect other than an equal offset for everybody. And they all have to make the short tacks somewhere.
I got this from young Olympic sailors blogs, so the new generations will fix this

cheers!
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Old 26-12-2011, 06:00   #74
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Re: Differences Between Ground, Apparent, and True Wind Direction

OK, if you are racing to the windward mark, on either tack you would prefer to have current lifting your boat. The prefered tack is toward the current. Just like tacking on wind shifts, a long windward beat in a tidal current could have the prefered tack shift when the tide turns.
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Old 26-12-2011, 08:29   #75
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Re: Differences Between Ground, Apparent, and True Wind Direction

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No confusion at all.

In navigation you have magnetic headings and bearings and true heading and bearings. True heading and bearings are referenced to true north on a chart. Magnetic headings and bearings are compass refenced and need to be corrected for variation and deviation to obtain true bearings and headings.

When measuring wind the meteorological office will reference wind direction from true north bearing and the call it true wind.

When measuringnwind sailors will measure apparent wind refenced from the bow. They will due a calculation for boat speed vector and adjust the apparent wind refernced off the bow and call this true wind.

The sailor uses the exact same term words "true wind" to describe something completely differernt than what the met office calls true wind.

You, Nick I and many others are not confused at all.

I know exactly what you are saying. Unfortunately I unable to communicate to you what I am saying.

Those that say I don't know what compasses, variation and deviation have to do with wind do not have a complete picture of the wind, weather forecasts and passage planning.

The reality today is simple. The gps gives you sog, cog and cts. I am 100 percent convinced the "average" sailor is adjusting heading to maintain a cog as described by a gps plotter by trial and error method.

That is if the rhumb line is 090 and after an hour sailing 090 the boat is set to left and the cog is 080 the skipper "tries" 110 for a while to get back to the rhumb line and then upon reaching the rhumb line "tries" 100 to stay on it. I have watched them do it! And when I show them the cts page and ask them what they think of that they look at me like the rca victor dog. Huh? What is cts? I didn't read that far in the gps manual.

That's what I call point and and shoot passage making. I don't care if everyone is doing it, machts nichts to me. But when the plotter takes a hike you oughta be able to calculate a cts to stay on the rhumb line from best and last known information including forecasts.

Sorry for the pilot reference but when I did my pilot license I had to plot the passage including cts to arrive at waypoints on time and on target. A pilots circular slide rule is used and true winds, true headings and all the numbers had to be complete for each leg before departure. No computer. Using a met report that references true wind bearings at multiple altitudes.

And once again - the title of the thread is about true wind and apparent wind. In the most simple sense we all know that apparent wind is wind measured by the windex and indexed to the bow. True wind is adjusted for speed vector. But until you talk about "the other true wind" I dont think the convsationis complete.

The only conflict I still have that the ground wind proponents can help me with is the statement that ground wind = meterological true wind. I am not getting it.

Peace on earth good will to men....
Ground Wind = Meteorologist's True Wind simply means that what our instruments call "Ground Wind" is the same thing as true wind for someone on land. It is ground-referenced true wind instead of water-referenced True Wind. Some people are uncomfortable with the idea that True Wind on our instruments is not calculated according to the definition used by the Met Office. Calculated using a different reference point. They say "meteorologists own the term 'true wind'" and don't like the fact that we use the term differently.

CTS - course to steer -- is of course an essential concept for decent navigation -- you are right. You can, in fact, navigate to a waypoint with perfect efficiency by adjusting the autopilot until COG equals bearing to waypoint -- IF the current is constant over the entire leg. If the current changes, then you have to calculate a course to steer based on your expected set hour by hour over the leg. That's the only way to traverse efficiently a piece of ocean with different currents in it. In changing currents, sailing by COG and crabbing the rhumbline can increase passage time by 30% -- 40% or more. And "point and shoot navigation" -- well, we've all seen it, haven't we?

I do not think that my electronic navigation can calculate a course to steer -- maybe I also didn't read that far in the manual. But the concept is absolutely fundamental to good navigation. We do the calculation by hand. You take a tidal atlas and sketch out your intended passage hour by hour. Then you calculate the set over each hour, and calculate the net set. Then you calculate a course to steer which compensates that aggregate net set. Then you add a little more to be sure you arrive a bit uptide of your destination -- you want to err on that side, especially sailing across to Normandy where the tides really rip close to the French coast (actually I don't "add a little" -- I simply set the waypoint half an hour's sailing uptide of the real destination).

The really useful electronic navigation function for this is x-track error. This allows you to check your calculations in real time. At any given moment the x-track error should equal the cumulative set up to that moment. If there is a big difference you know you made a mistake.

Of course the other problem with this kind of passage is that you can't always know what your speed is going to be. If you arrive an hour earlier or later than you planned, that's an hour less or more of set in that direction -- and you can end up in the wrong place (God forbid downtide). So I always calculate three scenarios - based on 7, 8, and 9 knots of average speed (if the wind is decent), producing three different courses-to-steer. A few hours into the passage you get a feel for which scenario is closest to reality, and you can adjust the course to steer if necessary. I usually run all the numbers over again in the middle of the passage just to double check.
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