The Equinox, celestial mechanics & pesky “True Wind”

Ben Ellison

Ben Ellison

Panbo editor, publisher & chief bottlewasher from 4/2005 until 8/2018, and now pleased to have Ben Stein as a very able publisher, webmaster, and editing colleague. Please don't regard him as an "expert"; he's getting quite old and thinks that "fadiddling fumble-putz" is a more accurate description.

48 Responses

  1. Johan Hackman says:

    You are funny, Ben! I am honoured that I could inspire you to write this fantastic article by providing so many confusing ideas about true wind in the forum. I am also looking forward to see how many readers will support the idea that us sailors should not be allowed to influence what is True Wind really is. I think the reactions will reflect the number of visitors to this site that are power-boaters more than anything else and they are not so interested in wind, are they? My guess is also that wind instrument manufacturers will have some doubt about whether to start selling wind instruments that show Ground Wind to sailors. But with your influence in the marine electronics world I should perhaps stock up with with instruments of the current type? 😉
    I am not opposed to different types of True Wind and I am clearly stating this in the forum discussion. My only aim was to explain the different mathematical concerns around it. If certain things are never discussed people will never understand how their wind instruments work in relation to the wind they use for sailing. They will think that you can substitute boat speed from the paddle-wheel with SOG from the GPS and get True Wind Over Ground. I think I explained that quite well in the forum.
    I actually think wind and wind instruments should be further discussed. I would love to go from a True Wind discussion (for sailors who use wind instruments) to an Apparent Wind discussion. There is so much more to say about how to use your wind instrument if you are a sailor.
    As a minor note, you can never judge the tone of voice in a typed text. We all know that, but I have feared that my input to the forum could be perceived as the voice of a Besserwisser or a person who wants to prove other people wrong. That is not at all the case. I am just forwarding conclusions I have made after years of studying, sailing, thinking, studying, thinking and steering a lot more. And I am very humble about other people knowledge and opinions. And I know I should learn to use smileys. 😉
    Johan Hackman
    Stockholm, Sweden
    Hanse 342 #306 S/Y Emilia

  2. Ben Ellison Ben Ellison says:

    Thanks, Johan, and not to worry, I think we’re almost all Besserwissers on this bus (psychedelic era reference here: )
    At any rate, I’m glad that you acknowledge that there are different types of True Wind. And I used to be comfortable that that’s the way it is. But, damn, the confusion about it never ends, and if our forebearers could figure out stuff like celestial navigation, can’t we work out boating terms that are precise?
    So if there were to be only one solid definition of what True Wind means would be referenced to ground, True North, etc or to the water? My opinion is clear but I don’t think the answers would break down quite as neatly as you think.
    * For one thing there are many boaters like me who are both sail and power.
    * For another even lots of sailors have no idea how their True Wind is calculated, if they have it, and don’t much care.
    * I can almost guarantee you that traditional sail folks (who don’t usually uses instruments) would be aghast that something called True would not be referenced to earth like all the other Trues in navigation.
    And that’s all the issue is, terminology. Wind corrected for boat motion only is obviously a valuable tool, but it never should have been called True. I like the term Sailing Wind that pro navigator and author Mark Chisnell uses ( ) but I’d definitely settle for ‘FartWind’.
    Incidentally I think it would be great if instrument manufacturers put the formulas for how they calculate values like True Wind in their manuals.

  3. Dan Corcoran (b393capt) says:

    Funny funny funny. Just spent the last three hours at a North Sails sail trim seminar where “True Wind” was uttered twice a minute.
    IMHO True, as in True Wind, is perfectly good terminology.
    There is the fake wind the observer experiences while the boat is propelled over the water, appearing on instruments as a FALSE reading of the actual wind, then there is the ‘TRUE wind’, as a calculated value of the wind measured if the boat wasn’t propelled.

  4. Jesse Deupree says:

    Happy anniversary to you both, Ben, and congratulations.
    But what is up with 60 knots per minute?:-)

  5. Anonymous says:

    I’ll make you a trade. I’ll get all my sailing mates to stop referring to Sailing Wind as True Wind, if you can stop the power boating world referring to their craft as “yachts”. Very confusing for those of us raised to believe a yacht is a wind powered vessel.

  6. Sandy Daugherty says:

    Once upon a time, a long,long time ago
    latitude and longitude were painted on the seas.
    But now the lines are all messed up, due to ebb and flow.
    The lines curl south, and wiggle west and east
    and some just sit and spin around
    a sadly plastic feast.
    And over all the winds do blow
    and greatly change the show
    together wind and water carve
    a new face on the beach.
    All boats must go from here to there,
    places set on land;
    that do not move with wind and wave
    except for piles of sand.
    Wise mariners choose not to follow
    the ancient painted lines,
    or trust the wind’s direction;
    they derive and follow fainter signs.
    They halter both to reach their goal
    and need to know which way each go
    to find their path to journey’s end
    So he who chooses not to know
    one, or the other,
    may find himself lost and alone
    upon some hidden obstruction.

  7. Larry Brandt says:

    Ben, given that at the time of writing you were about 33-40N, wouldn’t the sun have been about 56 degrees above the horizon at noon, not 66 degrees? Or are you using the antique French 400 degree circle?

  8. Ben Ellison Ben Ellison says:

    Congratulations to Jesse Deupree and Larry Brandt for winning the secret Panbo Addled Editor Equinox Contest!
    Meanwhile Sandy Daugherty picked up a Panbo Excellence in Commenting Award. I’d like to send a handsome can-not-be-bought-anywhere long-billed sun hat to each of you gentlemen. Please send your mailing address to ben (at) ASAP. I serious about the hats.

  9. NoelC says:

    Hi Ben,
    There is only one True Wind definition!
    There is much journalistic confusion regarding True Wind, Boat Wind, Apparent Wind, Wind relative to ground, Wind relative to course through water.
    Physics 101 teaches about relative motion.
    Motion is a vector with both speed and direction.
    When one body is in motion relative to another, there is relative motion and phenomena are seen differently by the observer in motion in relation to the stationary observer.
    Air (wind) in motion relative to a boat which is also moving, has the boat seeing the air velocity altered relative to the crew on the moving boat.
    The air seen by a masthead transducer of a moving boat is called the Apparent Wind.
    This Apparent wind is the Relative Velocity of the wind as seen by the moving observer observer on the boat.
    The Apparent Wind is the velocity of the wind with respect to the observer on the moving boat.
    It is the only wind experienced while in motion. It is never the true wind unless the boat is stopped in a no wind scene with a zero current, totally motionless over the ground.
    The Apparent Wind (AW) is the vector sum of;
    Velocity of the Wind (TW) -MINUS- the Velocity of the Boat (VB)
    or AW = TW – VB ( Vector sum)
    Many books on sail trim refer to a vector addition and use the plus sign while christening the boats forward velocity as the “headwind”, to force usage of the plus sign. This expalains why you mistakenly described the True Wind below as;
    And today true True Wind is relatively easy to calculate, being the difference between the COG/SOG vector and the Apparent Wind vector corrected to True Heading
    If there is a current then the boat’s speed is influenced by it and the effective boat velocity is the velocity over the ground, which is is the is the boat velocity over ground (as measured by gps) and which automaticaly incorporates current effects.
    Using the boats speed through the water as seen by padlle wheel log and compass + plus the current vector is largely irrelevant as the gps does the actual reckoning correctly.
    You can download from USNO a calculator to do this calculation and more from
    Might I suggest that is a good link for thorough Astonomical Navigation principles and calculation.

  10. Ben Ellison Ben Ellison says:

    Noel, I wish your main argument was true, but alas it is not. I can almost guarantee you that the “True Wind” Dan heard twice a minute at a North sail trim seminar is not defined the way you do. Performance sailors only want to correct the boat’s velocity through the water, not it’s velocity over the ground (GPS). It’s a perfectly valid calculation but it should not be called True.
    And check out this forum thread over at YBW in the UK, where they fiercely thrash out the multiple definitions of True Wind:
    Also, I don’t think there’s anything mistaken about my saying that True Wind is “difference between the COG/SOG vector and the Apparent Wind vector corrected to True Heading.”
    The only way I know how to solve vector problems is the old school maneuvering board way illustrated above. You just lay out the two known vectors and the difference is the answer. Of course everything has to be in the same units of measurement, and since Apparent Wind Angle is measured relative to the bow is has to be corrected to a True direction.

  11. Ben Ellison Ben Ellison says:

    Ockam Instruments has a good explanation of the two True Winds:
    Even Ockam puts the performance sailing Water relative version below the more generally used Ground relative True Wind.

  12. Johan Hackman says:

    I must say I am a bit disappointed.
    I spent hours and hours writing about True Wind in the forum trying to make sure that the intricacies in the relation between air, water and boat were expressed to best of my ability – and the only result is that people start arguing about the term! Is there not anyone who is interested in the usefulness of their instruments?
    If I were a wind instrument manufacturer reading this I would think that it would not be worth the effort designing a sophisticated wind instrument. I would print sticky labels saying “True Wind” and put them on an instrument that just showed random numbers. And again, I am not discussing the term here. I am discussing the features of the instrument. Let it show Ground Wind to make everybody happy. It will not be useful for me personally but I am looking forward to reading all the posts that will show up in the forum where people not only will explain how useful it is, but also why.
    Personally, I want to helm my boat keeping an eye on my wind instrument and when it tells me that wind has changed I want it to be because the wind has changed and not the boat’s speed over ground. If it is the boat’s speed over ground that has changed and not the wind, I want the wind readings to stay the same.
    Johan Hackman

  13. Ben Ellison Ben Ellison says:

    Sorry, Johan, but if the “True Wind” you’re talking about was called Sailing Wind, Boat Wind, Water Wind, or anything else but True Wind, I would not have brought it up in this entry. Terminology is very important, and confusion around “True Wind” is major.
    I fear that your last paragraph suggests some confusion about what is real. If you’re sailing in a steady breeze but suddenly enter a strong current, the Water Relative True Wind on your instrument is going to change. That’s because your apparent wind will change but not necessarily your boat speed. A sail change or trim may be necessary because the actual wind across your deck apart from boat motion has changed.
    But The Wind did not change. A boat near you but not in the current will still have the same Water Relative True Wind you had before entering the current. On both boats Ground Relative True Wind will remain the same until The Wind changes. Are you with me?

  14. Johan Hackman says:

    Ben, I admit to have conceived an example that was not so great. But yes, I am with you. What you are saying is exactly what I am talking about in the forum thread.
    I will give you a real-life example that will show you both the difference between the wind across water and the wind relative to ground and the usefulness of a wind instrument on a sail boat.
    We were headed North in the Baltic Sea towards Copenhagen. These waters are not tidal so you don’t really expect currents. The wind was Southerly, i.e. from behind so I had my eyes on the wind instrument to avoid a gybe. It was a windy day so a crash gybe would not have been nice. I therefore looked at the wind angle (I dare not say True Wind Angle so let’s say I was looking at the Apparent Wind Angle). The wind was steady but was forecast to increase quickly so I looked at the (..oh, I will have to say it..) the TRUE wind speed so I would detect the wind shift when it came.
    When we got in the vicinity of Copenhagen we entered the narrow channel between Amager and Peperholm which constitutes one of two entrances to Öresund. At that point the wind speed dropped according to my wind instrument and the pressure of the sails went down. This was contradicting the forecast but gave me some room to go below for a pee!
    Now, what happened is that we had got a strong current from behind. The entire Baltic “breathes” through Öresund. Our SOG had increased a lot, boat speed was almost the same and as we were now were moving faster away from the wind there was also a decrease in the wind speed.
    The ground wind surely did not change at this moment but the wind that I don’t dare to call true (even if it is printed on the instruments) did change. It would be wonderful to have had information about both winds displayed to confirm this but when it comes to usefulness of the it was the wind information (True Wind Speed) on my instruments that told me that I could take a leak.
    I don’t see why you need to change the meaning of True Wind only because you have found something that you believe is more true. Find another word for it! However, I don’t care so you can rename it. BUT I want to see the entire world use the metric system. Tomorrow! Can you make sure this happens, please! 😉
    Johan Hackman

  15. Dave says:

    I agree with Ben , the term “True Wind” has been appropriated by instrument makers. True wind has historically been the estimate of the real wind “out there” and written into logs for 100s of years.
    Instrument makers wished to display true wind, because all they had was STW, they used that, incorrectly calling it “True Wind”. In practice there is little difference on most sailing between True wind and “apparent wind at zero boat speed through the water hence my new moniker AWZSTW. Are you listening Raymarine.
    You never sail or trim to the True Wind, you always sail and trim to the Apparent Wind.

  16. NoelC says:

    Ben said:
    “Performance sailors only want to correct the boat’s velocity through the water, not it’s velocity over the ground (GPS)”
    This is ambiguous and misses the entire point as getting from point to point can’t ignore any physical phenomenon active and to do so ignores basic physics.
    Performance sailors in racing and cruisers also, in a moving boat influenced by wind and current, going to a mark fixed to the ground, as racing marks are, should realize that the speed and course over the ground is the only parameter that is relevant to shaping a course toward the fixed mark.
    Velocity through the water ignoring current is an artificial construct and is irrelevant in real world scenarios.
    The quickest way to the mark is determined by boat speed and direction along with the current speed and direction, which is the course over ground as indicated by gps.

  17. Ben Ellison Ben Ellison says:

    How about if boaters stop using “True Wind” altogether! I’m serious. The definition is perhaps forever messed up, as shown on this thread and many other places like that YBW forum ( ). And it’s just going to get worse as more and more boats have the ability to calculate both types of “True Wind”. Here’s what I propose instead:
    Apparent Wind: what you feel on a moving boat, what your wind sensor reports, definition not in doubt. Apparent Wind Angle generally expressed on instruments as relative to bow.
    Water Wind: what you feel on a drifting boat, often called True Wind by performance sailors, generally calculated by removing Speed Through Water (STW) from Apparent Wind vector. Water Wind Angle is generally expressed on instruments as relative to bow.
    Ground Wind: what you feel on an anchored boat; often called True Wind by mariners, meteorologists, and many others; generally calculated by removing Speed Over Ground (SOG) and COG from Apparent Wind vector. Ground Wind Angle is generally expressed on instruments as relative to True North.
    I don’t get to tell anyone what to do, but wouldn’t this work? And doesn’t it make sense to put the major reference in the name? Like Air Speed and Ground Speed in the aviation world?
    These are the terms I’m going to use henceforth on Panbo, and I’ll also try to program Gizmo’s displays this way where possible (so far only Maretron I think). And I’ve got a handsome Panbo hat for the first instrument developer who adopts Apparent, Water, and Ground Winds as their defaults.

  18. Dan Corcoran (b393capt) says:

    Your incorrect Dave and NoelC. Those of us who understand True Wind (relative to water) use it.
    Not just for racing, but for cruising also. Both Johan and I use the feature on our autopilots to steer to true wind relative to water.

  19. Dan Corcoran (b393capt) says:

    I follow the logic. Ground Wind = Wind Relative to Ground; Water Wind = Wind relative to Water.
    But, “Water wind”, thats going to throw people who didn’t read this entry.
    How about?
    TWrw = True Wind relative to water
    TWre = Ture Wind relative to earth

  20. Ben Ellison Ben Ellison says:

    Dan, those names just get shortened and the confusion continues. You yourself are proof of that, as discussed on the Forum. Airmar consistently refers to the Ground Wind coming from the PB200 as “True Wind” and the Water Wind it can output (with help from a speedo) as “True Wind relative to water”.
    Yet you, who are very familiar with the PB200, had convinced yourself that Airmar primarily thinks of True Wind as Water Wind. AIRMAR DOES NOT. MANY OF US DO NOT. Sailor’s “True Wind” is not True, it is a form of apparent wind as illustrated by Johan above (when his “True Wind” changes while real wind does not).
    Most of all, Sailor’s “True Wind” is an affront to True North, which is the very foundation of navigation, astronomy, time, etc. That’s what I was trying to get at in this entry. There’s a reason that the True compass rose is such a potent symbol, and that the Equinoxes and Solstices are so revered across cultures and history.
    Slowly but surely the ancients figured out a few things that are True about our planet. No one argues the truth of the poles and the equator, right? And then they built a system of geolocation and direction around those truths. Everything we do in navigation — tracking ourselves, charting the globe, predicting the motion of celestial bodies, tracking weather phenomenon like hurricanes and wind — is locked to True North.
    So “True Wind” relative to water — which sounds a hell of a lot like True North, but is not related to True North — was a mistake in the first place, and has been causing confusion ever since. (Especially since True Wind relative to ground is locked to True North and is viewed as the True Wind by scientists, merchant mariners, and many others.)
    Finally, it’s rather amazing how some sailors have absolutely convinced themselves that “True Wind” relative to water is the one and only True Wind when that is obviously not true.
    So I think “True Wind” as a boating term is ruined. We need a real distinction and a balanced way to get there. Let’s all give up “True Wind”. Water Wind and Ground Wind work, just like STW and SOG. We can do this.

  21. madscientist says:

    “Your incorrect Dave and NoelC. Those of us who understand True Wind (relative to water) use it.
    Not just for racing, but for cruising also. Both Johan and I use the feature on our autopilots to steer to true wind relative to water.

    From my perspective, many people use wind relative to the water. But Im also one of the people that also agrees with Ben, that True Wind is wind referenced to the Earth. The term has been mis-applied by boat instrument makers.
    Wind referenced to the water should not be called True Wind, thats the kernal of the argument here, not what you use on the boat for this or the other

  22. SheltieJim says:

    Ben, this is a great article, one that both amused and educated. Thanks.
    I know that you know better, but I thought I’d point out that “you’re still going a neat 60 knots per minute” means that you’re accelerating far better than most jet airliners! Of course, you meant “you’re still going a neat 60 nautical miles per minute”.

  23. Ben Ellison Ben Ellison says:

    Thanks for the comment, SheltieJim, on both counts. As Jesse Deupree pointed out, I also got my math wrong. Way wrong as a matter of fact. Here’s what I was trying to point out.
    The Equator is the only line of latitude that is a great circle like all the meridians of longitude which pass through both poles. Latitude is measured north and south along meridians and is the same everywhere on earth; one minute of latitude always equals one nautical mile. The circumference of our planet equals 360 x 60 = 21,600 nautical miles.
    So the Equator is the only place where minutes of longitude = minutes of latitude. Therefore, when you’re on the equator going zero knots SOG you are also going 21,600/24 nautical miles per hour or 900 knots (because the earth rotates once per 24 hours).

  24. Eric says:

    It’s all rubbish if your instruments can’t be calibrated… try calibrating “true wind angle” on most instruments and you’ll find that feature missing. Uh oh.

  25. Larry Brandt says:

    Thanks for the Panbo hat, Ben. It just arrived and I will take it to the boat tomorrow. I will wear it with *elan* if you don’t mind. My M-W says élan = assurance of manner, brilliance of performance, or liveliness of imagination. As much elan as I can rustle up, anyway.

  26. Ben Ellison Ben Ellison says:

    Excellent. And hip hip for the U.S. Postal Service. That hat was mailed “cheapest rate” (a little over $2) from South Carolina on Saturday at about 11 am.

  27. Taniwha says:

    I’d bet that Dave in his Mar 22nd comment is correct about why racing sailors and wind instrument makers call Water Wind “true Wind”: because before GPS the instruments couldn’t correct to the real true wind because they only had STW & heading info, not SOG. So to make life simple, they called that “True Wind”, and it that’s what all us sailors came to know it as.
    Then when SOG became available through GPS, they had a whole generation of sailors calling Water Wind “True Wind”, and all the inertia in their development and sales/marketing teams thinking that way, and they just couldn’t bring themselves to change it.
    My personal opinion, with a science and sailing background, is that I agree with Ben that the water wind should not be called “true wind” – it just isn’t. But it is VERY useful.
    I use all 3 winds whilst sailing, for different things:
    Apparent: To get the best out of the boat in “this instant”,
    Water: To understand how the boat will react in the immediately coming instances (esp when tacking or gybing), and how to best plan a short-term route to where I want to go,
    Land: To know what the real conditions are so that I don’t make a total hash of things when dramatically changing direction or speed and understanding whether we’re putting the boat into a dangerous situation. Weather forecasts work off the land wind.
    My 2 cents.

  28. Kris Amundson says:

    Hi Ben,
    I’m stuck on the hard with miles of ice in front of me, but it’s Panbo and SA that keep me sane until I return home. If you want to see the Equinox from someone at the South Pole, I took some pictures this weekend.
    Saturday we had our “Sunset Dinner”, where we celebrate the departure of light for 4 months and the arrival of stars, the Milky Way, and auroras for 24×7.

  29. Johan Hackman says:

    I agree, nautical terms need to be updated. Here’s my contribution to the new dictionary:
    Performance sailors – a small but irritating group of boaters that not only own a sail boat but also hoist their sails. They are irritating because COLREGs give them right to way despite the fact that they don’t seem to be able to sail a straight course, which powerboaters always do.
    Arrogant sailors – those in the previous group who not only understand what their wind instruments are used for but also are aware that their are many different types of wind that interact – apparent wind, head wind (or boat wind, aka ”fartvind” in Swedish), wind across the water, wind according to height over water surface, wind across land, upwash and downwash (that play with the accuracy of the readings), etc.
    Powerboaters – the good citizens that support the oil industry. This is good because they make economies across the world prosper, but bad because the exhaust from their engines (and their cars) adds to global warming.
    Global warming – a misleading term. It should be ”global average warming” as a raise in the average world temparuture results in warmer climates in some places and colder climates in other. People generally have a problem believing in global warming if they freeze.
    Wind – this term traditionally means ”mass of air in motion”. It has been used as a source of proplusion since the beginning of time but with the technological advantages of today it shold be modernized and divided up in two:
    Hobby wind – the source of propulsion used by the first group (performance sailors) and supervised by the arrogant sailors through their wind instruments as an aid to better performance. One of the chacteristics of Hobby Wind is that you can never control the direction or the speed of it. There is no electronic gadget that will help you do it so you will have to live with the fact that this wind might be strong, light or even non-existant. You will also have to live with the fact that it will be right on the nose when you head home on Sunday afternoon.
    True Wind – as wind always has been referred to the source of propulsion the ”New True Wind” should refer to the fuel that makes it possible for powerboaters go from one place to another. In other words, ”True Wind” means diesel or petrol! It is easily obtained by just drilling deep holes in the desert or the sea bed deep in the ocean and then transport it to a refinery and then ship it around the world. You will find it in a pump nearby. What sailor can say the same about their ”hobby wind”?
    Johan Hackman
    Hanse 342 #306 S/Y Emilia
    Stockholm, Sweden
    PS. I had a ball writing the above. I hope no one got offended. 😉

  30. Sandy Daugherty says:

    Aaaahh; the hat! I feel like I’ve just been Knighted! When I put it on my head, I began to dance around….

  31. Sandy Daugherty says:

    Aviation Forecasters refer to this as surface wind. It is referenced to a compass (not a heading, so it must be a ) froming.
    Unless you are on a small lake, the water around the boat is probably moving. It is a factor in your sailing conditions. It would be nice to know how the water’s movement affects your voyage. All else being equal, If the current is more than trivial, he who understands its effect gets to finish first. Just ask a Bay racer, or someone crossing the Gulf Stream. In light air its critical to know what current is doing to you. Lumping it into “true wind” is a mistake. THAT should be called “Not really true wind”.
    I have a question: Are our state-of-the-art instruments up deriving current effects? HOW?

  32. SheltieJim says:

    Gee, Kris, just rub it in, why don’t you? ;^)
    I would absolutely love to overwinter at the South Pole. Heck, I’d love to just visit Antarctica at any time! Not quite sure I’m up to sailing across the Southern Ocean to get there, though.
    Thanks for the photos!

  33. NoelC says:

    Dan Corcoran (b393capt) in reply to Dave at March 24, 2013 10:55 PM said
    “Your[sic] incorrect Dave and NoelC. Those of us who understand True Wind (relative to water) use it.”
    This makes an incorrect assertion, while declaring others wrong.
    There is no true wind relative to water as the water is itself moving, either as a current or tidal stream and the boat, being on the water is being moved along with the water as well as moving through it.
    “Both Johan and I use the feature on our autopilots to steer to true wind relative to water.”
    An autopilot configured to follow a course relative to the wind will take data from a wind instrument, which generates only Apparent Wind. Any autopilot I have seen only steers to the Apparent wind as it is the only wind felt onboard.
    A NMEA bus or a proprietary bus can take the any of boats Speed and Course values present on the bus and add these to the Apparent wind speed and direction on the bus to get a version of “True” Wind relative to the speedo and a compass heading, while remembering that the Apparent wind direction is usually stated relative to the bow and must be corrected relative to North, which can be problematic as discussed below.
    The only way to get True Wind is to take GPS SOG and COG and add this to the Apparent Wind corrected for North, to get the absolute True Wind.
    I have been on a yacht that has two fluxgate compasses, one for the autopilot and one for the masthead instrument and neither give, to within 10 degrees, the same course indication on their displays, despite swinging them to eliminate “error”. Which do you believe?
    The solution is to use the course indication from the gps, which is absolute and relative to the ground and North.
    Polar charts for sail trim need to be carefully reviewed if “water speed” is used but as SOG and COG is experienced, there will be errors in interpretation if there is a current or tidal stream.

  34. Ben Ellison Ben Ellison says:

    Kris, thanks so much for commenting. I love your photo of the Geographical South Pole and it’s exactly the sort of True reference I tried to conjure up in this entry.
    But, do tell, how the heck are you online browsing web sites at the South Pole? Iridium OpenPort satellite?

  35. Dan Corcoran (b393capt) says:

    NoelC, I entirely disagree.
    I will make my statement below using the terms Ben layed out (e.g. Water Wind: what you feel on a drifting boat, often called True Wind by performance sailors, generally calculated by removing Speed Through Water (STW) from Apparent Wind vector. Water Wind Angle is generally expressed on instruments as relative to bow)
    Note, the Water Wind is relative to the bow and calculated from apparent wind – STW. It requires no fluxgate or other heading source.
    The US Sailing website provides a Sailing Wind calculator, it takes as three inputs AWA, AWS, and STW.
    The following article authored by Stan Honey explains how to steer to Water Wind (he calls it true wind)
    In regards to autopilots, I am perplexed why you doubt me. The Raymarine S1G Autopilot, standard equipment on my 2006 Beneteau cruising sailboat, is an example of an autopilot with a true wind option. Following Stan Honeys article above, if I know that the Water Wind Speed is 12 knots, and for my boat the best angle to steer to a point upwind in 12 knots of Water Wind is 41 degrees, I can select 41 degrees (starboard or port) in my Raymarine Autopilot and go. Yes … unlike autopilot steering to apparent wind I cannot cleat my sheets, I need to trim them constantly, but I will reach my upwind destination sooner than steering to apparent wind and enjoy trimming the sheets along the way.

  36. Ben Ellison Ben Ellison says:

    Thanks for using “Water Wind”, Dan, much appreciated.
    I was surprised to learn that your autopilot can steer to Water Wind because I thought that was a high-end feature only seen on fast, skittish boats like Open 60’s.
    But here’s a description of the Wind Vane mode on a Raymarine p70 control head:
    “Autopilot engaged steering to maintain a selected apparent or true wind angle, activated from the Mode menu, or by pressing AUTO and STANDBY together.”
    Of course, Raymarine means “water wind angle” not “true” but maybe they’ll come around 😉

  37. Kris Amundson says:

    We’re actually spoiled down here, albeit our access is slow.
    We get 7 hours of GOES-3 (old, old NOAA sat), 4 hours of TDRS F5 (NASA), and we’re testing a newly installed setup called Skynet 4B (old UK military sat).
    When we don’t have sat time we have a bank of 12 Iridium sailor unit modems multilink-PPP’d together for just email.
    I suggested investigating OpenPort, which they did, but was cost prohibitive.

  38. dr. o says:

    The only wind that counts for real to any sailor is a fair wind.
    Fair winds to you all.

  39. Ben Ellison Ben Ellison says:

    I’m not surprised to learn that one my very favorite artist’s, Eric Hopkins, is another devotee of celestial mechanics. But I didn’t know until this morning how he starts his day:

    Every morning when he wakes up,
    whether on the island of North Haven
    or the Rockland mainland, artist Eric
    Hopkins performs a little personal ritual just
    to get himself located in the universe.
    He begins by facing East, taking a deep,
    cleansing breath, and raising his arms to the
    sky. He proceeds to make the same gesture in
    the other three cardinal directions—South,
    West and North. Next he bows toward the
    molten core of the Earth and stretches up
    toward deepest space. He revolves 360
    degrees with his arms outstretched and then
    places his right hand on his heart and his left
    on his head in the belief that his hands create
    a circuit between head and heart. He concludes
    this morning centering ritual by reciting
    a Penobscot Indian prayer:
    Woli, wani, kachini, wesch.
    “Thank you, Creator, for everything.”

    Full Edgar Allen Beem article PDF with wonderful Eric Hopkins art here:
    Unfortunately I was searching because I feared that the young man who died in car crash on North Haven early this week might be Eric’s son.

  40. Larry Brandt says:

    Ben, David Burch at Starpath Navigation recently posted a variation of this topic on his blog, but David’s discussion is limited to the use of True Wind as it bears on marine weather analysis.

  41. Ben Ellison Ben Ellison says:

    Thanks, Larry, very interesting. I think it’s noteworthy that a navigator and sailor as deeply experienced as David Burch doesn’t even seem to acknowledge Water Wind (sailor’s “True Wind” or whatever). But maybe I’m confused. I left a comment on his blog, so perhaps we’ll get David’s further opinion on wind definitions.

  42. Larry Brandt says:

    Ben, it’s important to appreciate that David’s blog post addresses *only* True Wind as used in the analysis of weather underway. The essence of his argument, I believe, is that accurate monitoring of True Wind direction and velocity (that is, the wind as referenced to the bottom beneath the sea) while underway, and the comparison of same to OPC observations and predictions, provides valuable weather information that can significantly assist the navigator.
    The obstacles to obtaining True Wind from observed “apparent wind” on a vessel are not trivial, as this thread’s discussion makes clear. My impression from reading the Panbo blog over recent weeks and David’s post is that you guys are somewhat in unison on the salient points.

  43. NoelC says:

    Larry, I can’t agree that David’s blog post is confined to true wind analysis relative to weather, or that the assertion that posts here of “you guys are somewhat in unison” with David’s reality.
    David’s blog is an analysis of the physics of the situation where a boat is underway with boat speed from its sails/motor and the influence of the wind and current on its motion additionally.
    It is a rigorous analysis of the real world motion of a boat with wind and current as distinct from the genuinely believed “facts” stated by many in this discussion.
    His avoidance of spurious wind names for items such as “water wind”, is 100%+ accurate.
    Study of David’s blog rather than partisan assertions of “fact”, however genuinely held, is warranted.

  44. Ben Ellison Ben Ellison says:

    Please take it easy, Noel; you’re the one who’s getting partisan here. I agree 100% with David Burch’s “rigorous analysis of the real world motion” and I even agree with his definitions:
    TWS = True Wind Speed (relative to the fixed earth)
    TWA = True Wind Angle (relative to the bow)
    TWD = True Wind Direction (relative to true north)
    (assuming that “relative to bow” includes correction to true heading)
    The problem is that there are significantly different definitions in use. In “the real world” if you choose TWS on almost any instrument the default value will probably be True Wind Speed relative to the water surface, not the fixed earth. That’s a fact, and that’s a problem.
    I wish it hadn’t happened but it seems obvious that the term True Wind is now badly flawed around boats. Without further detail you can’t know what it means on an instrument and you can’t tell what another boater thinks it means.

  45. Mark Morwood says:

    I agree with the position Ben is laying out, that the term True Wind was expropriated by instrument manufacturers. Having said that, I’m not sure what we can do about it. My Garmin instruments at least attempt to recognize the 3 different wind measurements by providing screens to display three different types of Wind on a GMI10:
    Apparent Wind
    True Wind (I assume this is “water wind” but see comment below)
    Ground Wind
    However I suspect that behind the True wind name they have some confusion, as there is an option to select either “Water” or “GPS” as the speed source for wind calculations. I’m unsure if selecting GPS converts the True Wind display to display Ground Wind while still labeling it True Wind or does something else.
    As a full-time cruising sailor on a catamaran, I use all three winds:
    – When setting out (or the conditions change) I use Ground Wind to make sure I can sail to where I want to with reasonable comfort and speed. I use the Ground Wind to set the course and decide on my routing. (I separately look at currents if they are significant for my route – either tidal or general)
    – When setting out or changing course, I set the autopilot to sail the course I want and trim the sails to apparent wind but I don’t use the instruments for that, I look at the tell tales on the sails
    – Once the sails are set I will often then change the autopilot to sail to True Wind (Water Wind) so that the boat keeps sailing well in the right general direction but follows small shifts in direction without me changing the sails. (I will still have to adjust the sails if the wind speed changes much as the apparent wind will change as the wind and boat speeds change). I typically don’t set the autopilot to sail to the apparent wind as with our boat which can accelerate fairly quickly you can have the same apparent wind sailing slowly close hauled as you have sailing fast on a reach. It means the boat can happily wander way off my intended course without the apparent wind direction changing.
    I realize that racers (or cruisers racing) use the different winds differently, and that they probably have more impact on the decisions that the instrument manufacturers make than the average cruiser. Maybe the best we can hope for is to keep Apparent and True as they are generally understood by boat instruments today, and add in “Ground Wind” as a new name for what the rest of the world calls “True Wind”.

  46. Dan Corcoran says:

    Ben, What are your thoughts about Andrew Cross’s May 9th article about True Wind here:

  47. Ben Ellison Ben Ellison says:

    Thanks, Dan. I think it’s great that David Burch’s analysis is now in Bluewater Sailing. Maybe some more sailors will realize the confusion that has evolved around the “True Wind” term as used in boat instrumentation.

    But it’s odd that anyone but Burch is given a byline for the article as I think it’s entirely copied from David’s 2013 blog entry:

  48. Ben Ellison Ben Ellison says:

    I just stumbled on an excellent Raymarine FAQ called “Apparent Wind, True Wind and Ground Wind, and data required to calculate” written by their longtime ace tech forum administrator “Chuck”:

    “True Wind
    What Raymarine refers to as True Wind (TWA, TWS) is always wind-over-water, not wind-over-ground (more on this below.) We calculate this from AWA and AWS, plus speed-through-water (STW, from a paddle-wheel or equivalent.) It’s not possible to calculate proper True Wind from GPS speed (SOG.) If you’re looking at your wind instrument and you have AWA and AWS but TWS is showing as dashes (-.-kn) then you don’t have Speed data coming in. In addition to TWA and TWS, our recent multifunction displays and i70/i70S instruments also offer TWD (true-wind referenced to north rather than the bow.) This requires compass Heading as well as STW. We can’t use GPS COG for this because COG is the direction you’re moving, not the direction you’re facing (which is what Heading measures.) Lighthouse 2 MFDs also require Heading in order to display TWA/TWS.”

    Of course, I believe that this “True Wind” is better called Sailor’s True Wind or True Wind relative to Water, and apparently even Raymarine has jiggled with how it’s calculated.

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