tag:blogger.com,1999:blog-714718709601620866.post484648857153337025..comments2024-03-28T04:33:15.006-07:00Comments on David Burch Navigation Blog: True Wind from Apparent Wind –– Revisited.David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.comBlogger26125tag:blogger.com,1999:blog-714718709601620866.post-63822137413705045702024-02-07T09:49:19.212-08:002024-02-07T09:49:19.212-08:00My script is more a collection of the formulas and...My script is more a collection of the formulas and a simple class calculating the missing parts from whatever you supply. It's python, so runs everywhere python runs. You cannot use it directly but you can integrate in your software. I came across this topic when fixing the SailInstrument plugin https://github.com/quantenschaum/Sail_Instrument for AvNav.<br /><br />BTW do you know AvNav? Have a look https://www.wellenvogel.net/software/avnav/docs/beschreibung.html?lang=enquantenschaumhttps://www.blogger.com/profile/08179992963680967550noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-35469250052489868752024-02-07T09:34:10.580-08:002024-02-07T09:34:10.580-08:00Correction to the above comment. I did *not* call ...Correction to the above comment. I did *not* call it "water wind" in that article. It was called just "true wind" as is usually the case where the distinction is made. Somewhere we do have discussion of the proposed name "water wind" as opposed to "ground wind" but this is frankly not a name we support. In settings where this matters, ie sailboat routing programs that work with polars, we use the name true wind and ground wind. The excellent free program qtVlm, for example, lets users distinguish between the two, using true wind for performance analysis and ground wind to compare with grib forecasts. Expedition does the same. David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-23881219030965205712024-02-07T09:25:35.952-08:002024-02-07T09:25:35.952-08:00Thanks for the note. Yes I agree that the "tr...Thanks for the note. Yes I agree that the "true wind" used in polar diagrams is what we have called "water wind" in this article: https://davidburchnavigation.blogspot.com/search?q=ground+wind which should have been referenced in this one. I will add it. Does your py script run on a mac? and what libraries would be needed if so?David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-70144848189980037872024-02-07T06:03:03.848-08:002024-02-07T06:03:03.848-08:00Hi David, IMHO this is not entirely correct but on...Hi David, IMHO this is not entirely correct but on the right track. You have to distinguish between ground wind and true wind (relative to water).<br /><br />I just came across this problem and wrote down the formulas. You may want to have a look. https://github.com/quantenschaum/mapping/blob/master/coursedata.pyquantenschaumhttps://www.blogger.com/profile/08179992963680967550noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-32124976050448380692020-08-24T01:01:01.743-07:002020-08-24T01:01:01.743-07:00>David, the formula TWD = ATAN ( u / v ) will m...>David, the formula TWD = ATAN ( u / v ) will move from 0 deg to 90 deg and then make a hyper >critical jump to -90 deg and moves back to zero. If you add 180 deg to the equation when v >changes sign then the result makes more sense. Here is my spread sheet formula used for >testing:<br />>=IF(v>0,180+DEGREES(ATAN(u/v)),DEGREES(ATAN(u/v)))<br /><br />I agree with the above comment - the formula was not working for me until I applied this correctionNBhttps://www.blogger.com/profile/10427270046554994037noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-67703973620632477122019-08-19T09:44:52.179-07:002019-08-19T09:44:52.179-07:00This comment has been removed by the author.David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-86001173548259427442019-04-27T10:20:33.539-07:002019-04-27T10:20:33.539-07:00In the third edition of our text Modern Marine Wea...In the third edition of our text Modern Marine Weather (https://www.starpath.com/catalog/books/1886.htm) there is an Appendix 9 called Nuances of True Wind Computations, which covers, with numerical examples, all the points raised above and a few more, such as heel angle affecting AWS and AWD as well as AWS measurements as a function of mast height.David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-33397038984072538262015-02-02T14:43:30.993-08:002015-02-02T14:43:30.993-08:00Thanks Bob, we will look into this. Thanks Bob, we will look into this. David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-54265602381261170112015-02-02T14:42:38.428-08:002015-02-02T14:42:38.428-08:00Albert, just a late note to thank you for the comm...Albert, just a late note to thank you for the comment. we do not own or have a copy yet of the NMEA 4 spec but are still looking. Other work here makes us even more interested in what they are doing with these terms.David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-68364420878011943932015-02-02T00:23:00.468-08:002015-02-02T00:23:00.468-08:00David, the formula TWD = ATAN ( u / v ) will move...David, the formula TWD = ATAN ( u / v ) will move from 0 deg to 90 deg and then make a hyper critical jump to -90 deg and moves back to zero. If you add 180 deg to the equation when v changes sign then the result makes more sense. Here is my spread sheet formula used for testing:<br />=IF(v>0,180+DEGREES(ATAN(u/v)),DEGREES(ATAN(u/v)))Anonymoushttps://www.blogger.com/profile/02514965658014702968noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-13819305860743603122014-02-11T17:57:18.752-08:002014-02-11T17:57:18.752-08:00Do you have a reference for that citation you ment...Do you have a reference for that citation you mention. I will have a look. i am puzzled by the term "theoretical wind"... or a better put, gobsmacked.David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-82427758233523240082014-02-11T14:19:27.239-08:002014-02-11T14:19:27.239-08:00Oddly enough, I read more carefully the NMEA 0183 ...Oddly enough, I read more carefully the NMEA 0183 spec yesterday, and saw a definition of true wind that I'd never seen before. Don't know about other types of sailor, but in navies around the world, true wind has always been defined as wind speed and direction with respect to true North, while stationary with respect to ground. And relative wind is defined as wind speed and direction relative to ship's heading, i.e. the bow defines 000 degrees relative.<br /><br />Version 4.0 of the NMEA 0183 spec defines something they call "theoretical wind," which seems to be what others here called "water wind." Even though older NMEA specs called this "true wind." Aaargh. I think the NMEA spec needs to create a third category, or we're going to be seriously confused here. If true wind is not wrt true North, any use of the NMEA 0183 MWV sentence in navy ships will be a risky proposition.Albert Manfredinoreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-35469214414528900442014-02-07T10:47:00.701-08:002014-02-07T10:47:00.701-08:00OK. the downloadable exe (windows only) works fin...OK. the downloadable exe (windows only) works fine for finding apparent from true. Just drag around the dots to set your values. I am not sure how well the java app works anymore, but it might work online as well, but it is old code.David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-43065223835471624932014-02-07T10:32:56.713-08:002014-02-07T10:32:56.713-08:00Actually, i did not read your question carefully e...Actually, i did not read your question carefully enough. I apologize. You want apparent from true. we have that calculator in our weather trainer live program, but let me check to see if it is in the package i referenced and i will get back. <br />David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-57689720644423010692014-02-07T09:57:43.215-08:002014-02-07T09:57:43.215-08:00The formulas are given above, but if you want thes...The formulas are given above, but if you want these formulas using law of sines and cosines, please see www.starpath.com/freeware and then download the information package there. It includes the theory and several calculators. Or you can get your answer immediately at www.starpath.com/calc in the true wind computer part of the weather computers.<br />David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-12027054199501972822014-02-06T17:06:30.478-08:002014-02-06T17:06:30.478-08:00Sir David Burch, may i know the formula for this p...Sir David Burch, may i know the formula for this problem : vessel is in course 135 degrees, speed 18 knots, true wind speed 24.5 knots, apparent wind direction 40 degrees. what is the speed of apparent wind?jaime cortes(jaime.cortes16@yahoo.com)http://davidburchnavigation.blogspot.com/2013/04/true-true-wind-from-apparent-wind.htmlnoreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-88156494758626114152013-09-05T16:59:05.128-07:002013-09-05T16:59:05.128-07:00Relative bearings are also used routinely in radar...Relative bearings are also used routinely in radar applications in the head-up mode on all vessels, large and small.David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-69912235013075739552013-09-05T16:57:30.438-07:002013-09-05T16:57:30.438-07:00This comment has been removed by the author.David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-59728448919392748532013-09-05T16:54:30.954-07:002013-09-05T16:54:30.954-07:00I would check the definition of Relative. Often R ...I would check the definition of Relative. Often R is used on commercial and military vessels as an alternative to true or magnetic bearings, namely bearings relative to the bow. ie 090R is starboard beam, 180R is astern, 270R is to port. This would apply to any bearings, wind or nav targets. This would not depend on the motion of the boat at all. I do not know of any directions convention that depends on the speed of the vessel.David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-19009737228891434882013-09-05T12:41:41.719-07:002013-09-05T12:41:41.719-07:00I am currently investigating the possibilities of ...I am currently investigating the possibilities of adding true wind calculations to our range of NMEA multiplexers and I am running into the same confusion.<br /><br />The NMEA standard defines the MWV sentence for true and apparent wind (marked by an 'R') and also the MWD sentece for the wind across the earth's surface.<br /><br />It appears to me that it all boils down to using the right terminology:<br />True wind: the wind speed and direction across the earth<br />Relative wind: the wind speed and direction measured on your boat, not moving<br />Apparent wind: the wind speed and direction measured when your boat is sailing/moving.Meindert Spranghttp://www.shipmodul.comnoreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-26279953515589955812013-07-06T18:11:47.191-07:002013-07-06T18:11:47.191-07:00Sorry for the long delay. I will be looking into y...Sorry for the long delay. I will be looking into your references very shortly. In passing, we also have strong currents in Pac NW racing waters. On some popular courses there are places (ie Admiralty Inlet) it can reach over 4 kts, not uncommonly. Also, needless to say, sailors do race in the Gulf Steam, where there is longer time to deal with these subtleties in such strong currents. <br /><br />My first impression is with calibrated instruments well placed, we should be able to compute all the factors and then use whatever is wanted.<br /><br />This question might be more a matter of terminology choices. The terms true wind, true wind speed, and true wind direction have very precise and unambiguous meanings. So that leaves us to define other terms as best meets the needs at hand.<br /><br />There are other subtleties for larger vessels that have the anemometers located what we would call a boat length off the center line. These do not qualify as "well placed" instruments. Also some commercial vessels do not use the center line as the apparent wind direction reference, so they have even more to unfold in the computations.<br /><br />David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-33088593211647357522013-05-08T10:34:39.085-07:002013-05-08T10:34:39.085-07:00I would agree with you that most racing sailors do...I would agree with you that most racing sailors don't think about this detail at all in the context of true wind angle. But when it comes to true wind speed, I would disagree if your with sailors that sail in current like we have hear on Long Island Sound and it's connected bays and harbors. Here we have currents exceeding 1.25 knots four times a day.<br /><br />I have learned in recent conversations that most sailors understand true wind speed to be water wind speed, and lack clarity of what true wind angle is. <br /><br />Focusing on true wind speed, when I first learned to sail, I learned true wind speed as water wind speed. When I advanced into navigation classes far beyond what most people learn, the angle of the wind relative to earth became important to me, and it became frustrating that the value was not available to me thru my instruments while under sail (e.g. leeway, etc.). Those advanced classes had me viewing true wind angle as ground wind and almost forgot the importance of water wind. <br /><br />When I started racing my big cruising boat I became reoriented to water wind. I learned the equivalent of the links below. They are an example of a tutorial provided to novice race sailors how to use water wind. I choose this one of many on google, partially because it was the first I found but also I like it because the writer is making a point that the sailor can succeed without fancy instruments and describes true wind like the (non racing) sailing class I took here on Long Island 12+ years ago.<br /><br />Here are the links<br /><br />http://l-36.com/polars.php<br />http://l-36.com/polar_tws.php<br /><br />Clearly the writer believes true wind speed is water wind speed.<br />Dan Corcoranhttp://www.servicespan.netnoreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-46286961978935288892013-05-07T17:33:31.347-07:002013-05-07T17:33:31.347-07:00The question i see from Ben is do i realize that m...The question i see from Ben is do i realize that most racing sailors think of this differently? Having been a racing sailor for very many years, i would have to say that *most racing sailors* have not thought about this detail at all, one way or the other. I can easily imagine that *some* racing navigators, sail makers, and yacht designers have addressed the point, but I really need to first check the several references Ben provided and then think on the basics myself some. I hope to get to that next week.<br /><br />Thanks for the reminder.<br /><br />Also i can easily imagine that wind instruments do this in a variety of ways. For example, there were wind instruments computing true wind before they even had the option of doing it with cog and sog. And in fact my own recent interest in the subject stems from discussions with a colleague when he was telling me how the AirMar system did it. I will check these things and get back shortly.<br /><br />An example that comes to mind is the wind instruments on a round drift buoy.David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-29194594727815156822013-05-07T13:49:29.670-07:002013-05-07T13:49:29.670-07:00David, a short note to let you know I too am curio...David, a short note to let you know I too am curious about the answer to Ben's question, as perhaps other readers of Panbo. Look forward to your reply.Dan Corcoranhttp://www.servicespan.netnoreply@blogger.comtag:blogger.com,1999:blog-714718709601620866.post-5203756843359423212013-04-23T20:53:25.604-07:002013-04-23T20:53:25.604-07:00Thanks Ben. You have a good treatment of this subj...Thanks Ben. You have a good treatment of this subject at that link, with many cross references. I will have to look into this some more. I know for sure that many instruments do it that way (ie compute "water wind" when they want true wind), regardless of the type or purpose of the boat they are used on. What I have to learn more of is the logic behind a value to the "Water Wind" to sail performance analysis. It escapes me for the moment. More soon.David Burchhttps://www.blogger.com/profile/13413234790483329339noreply@blogger.com