True Wind Discussion Redux

May 25, 2012
4,333
john alden caravelle 42 sturgeon bay, wis
i disagree with his made up definitions. the true wind is the wind over the surface of the earth. always has been, let's keep it that way.
 
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May 25, 2012
4,333
john alden caravelle 42 sturgeon bay, wis
david, i will tell you, as i have mentioned many times before. you want all the answers to getting the most out of your vessel? buy Frank Bethwaite's book! read it 20 times! keep copies at home and on the boat. it's all about the wind and then how to sail in it to a top level.
analogy to these all this is an auto pilot for steering the vessel. it's always late, it's always behind on the shifts, it give a poor ride.
i am not a racer, never was, yet i won the biggest race on green bay. took little kids, beginners, out the first year from sailing camp and took a third.
not one so called racer from sturge has ever beat me on the distance races on the bay.

winners drive like Barry describes. tacticians scan the water. crews trim for speed.
i can sail, i'm a great helmsman, but it was Bethwaite that taught me how to get the very most out of my vessel for sailing from A to B.


you race to come in first. nothing else
 
Jan 11, 2014
11,323
Sabre 362 113 Fair Haven, NY
A few thoughts.

Because we can read or calculate windspeed, direction or speed to several decimal points, it doesn't make the information any more useful or accurate. Instrument calibration, measurement error, and rounding can give us a false sense of accuracy and value. There are just too many variables to put much stock in a calculated TWS or TWD. That doesn't mean the information isn't useful, it just means take it with a grain of salt.

The B&G Zues2 and 3 have a feature that can track TWS and TWD and graph it over a period of time. That information is really useful as it helps make navigation decisions, is the wind trending left? or Right? up or down? I don't use it much when out for a day sail, however, when crossing the lake at 30+ miles it is very useful.

I also find TWS useful for calculating my eyeballs, it is a simple process. I look at the sea state, estimate the wind speed, and then check my estimate against the instruments.

AWS and AWD are far more useful when it comes to sail trim and reefing decisions.
 
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Nov 26, 2012
1,653
Hunter 34 Berkeley
Well, that argument has been made and rejected by sailors and I understand the reason why (not saying you're not a sailor! ;)). After reading the article that @DrJudyB provided, it's clear that the sailor definition of "True Wind" is in relation to the boat's movement through water and it is distinct from "Ground Wind", which is what you are defining as true wind. I accept the definitions as they are accepted by sailors, that there is a distinction between True Wind and Ground wind. Afterall, it's just semantics and I'll recognize the distinction.
I'm going to have to go ahead and "disagree". There are only two types of windspeed: "True" and "apparent". Whatever you call them. True windspeed is the speed over the surface of the earth and Apparent windspeed is the speed relative to the nose on your face as you go zooming along in your boat. Really, we should be saying wind velocity but we will keep it simple. Your wind gizmo has two settings True and Apparent. Both the windspeed and direction will be different as you toggle between the two settings unless you are stationary. If you are moving the computer will take into account your velocity over planet Earth to extrapolate the True windspeed. Your speed through the water has nothing to do with it as the computer cannot know the direction or speed of the current. That is it. There is no grey area here.
 
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May 17, 2004
5,032
Beneteau Oceanis 37 Havre de Grace
If you are moving the computer will take into account your velocity over planet Earth to extrapolate the True windspeed. Your speed through the water has nothing to do with it as the computer cannot know the direction or speed of the current. That is it. There is no grey area here.
Chartplotters can be set to use either GPS or speed through water as the input to calculate TWS. If using the speed through water the chart plotter can calculate the speed of the current by comparing STW and heading with SOG and COG. I understand you don’t want to call the wind speed over the water the True Wind Speed, but it absolutely is its own value and can be calculated with the right inputs.
 
Oct 19, 2017
7,733
O'Day 19 Littleton, NH
There are only two types of windspeed: "True" and "apparent". Whatever you call them.
How would you measure True Wind?
You're standing in the cockpit of your boat, you have a radio, a compass, a wind vane, an anemometer, a knotmeter, a GPS and a chart with tide tables. How do you know what "True Wind" is, if it is velocity over ground? I'm not arguing for one one definition over another. Over ground is how I've always understood it. It's just that that is less useful than wind over water, to me.

To get "True Wind" (over ground), you could anchor and measure it, but that's just where you are at that moment. You could get it from a current weather report, but that's old data and an approximation from a distant location. The only other way, out there on the water, is to measure your progress over ground and compare it to your apparent wind. That would be the most accurate. Your progress over ground includes current drift and side slip as well as forward progress. A GPS will help with that, but what's the point? Your boat sails as she does and you, her owner, understands this. You know what conditions, wind speed and direction at what point of sail produce what effects. This, however is wind over water.

With a keel buried deep in the water, your boat is half in two mediums and neither one cares about the ground except where a stationary destination exists. For sail performance, wind speed and direction over ground are meaningless. For a thought exercise and trip planning, it is a worth while consideration. Still, there is no such thing as a set-and-forget scenario in sailing.

-Will (Dragonfly)
 
Jan 11, 2014
11,323
Sabre 362 113 Fair Haven, NY
How would you measure True Wind?
You're standing in the cockpit of your boat, you have a radio, a compass, a wind vane, an anemometer, a knotmeter, a GPS and a chart with tide tables. How do you know what "True Wind" is, if it is velocity over ground? I'm not arguing for one one definition over another. Over ground is how I've always understood it. It's just that that is less useful than wind over water, to me.

To get "True Wind" (over ground), you could anchor and measure it, but that's just where you are at that moment. You could get it from a current weather report, but that's old data and an approximation from a distant location. The only other way, out there on the water, is to measure your progress over ground and compare it to your apparent wind. That would be the most accurate. Your progress over ground includes current drift and side slip as well as forward progress. A GPS will help with that, but what's the point? Your boat sails as she does and you, her owner, understands this. You know what conditions, wind speed and direction at what point of sail produce what effects. This, however is wind over water.

With a keel buried deep in the water, your boat is half in two mediums and neither one cares about the ground except where a stationary destination exists. For sail performance, wind speed and direction over ground are meaningless. For a thought exercise and trip planning, it is a worth while consideration. Still, there is no such thing as a set-and-forget scenario in sailing.

-Will (Dragonfly)
Will, you're way over thinking this. There is wind it is the same over water or over ground. It does change direction and speed as it passes over the ground or water and it is different at deck level and 50' up a mast, however, where ever you are if you are stationary with respect to the earth, it is the true wind. Apparent wind is what you feel as you move through the air. Doesn't matter if you are on a bicycle, walking, sailing, driving a car, the wind you feel as you move over the earth is the apparent wind.

There are lots of good reasons for knowing what the TWD and TWS are, especially for passage planning and deciding whether to leave the dock, but that's about it. AWS and AWD are the key to sail trim and course setting.

TWD and TWS can be calculated using GPS SOG and COG, however, due to leeway from wind, current, or sideslip, the calculations will not be accurate unless leeway is included in the calculation. To do that calculation a compass heading and speed through water is necessary. The more information that is included in the algorithm the greater the error term, because there will be error in each of the measurements.

And in all of this, does it really matter whether the TWS is 12 knots or 13? With experience an experienced sailor can estimate the wind speed with reasonable accuracy =/- a couple of knots using visual, haptic, and auditory clues. That's the Beaufort Scale.
 
May 17, 2004
5,032
Beneteau Oceanis 37 Havre de Grace
There is wind it is the same over water or over ground. It does change direction and speed as it passes over the ground or water
The movement of the air doesn’t change, but its movement is different relative to three things: (1) a moving boat; (2) the ground; (3) water that’s moving because of a current. The question of which one to measure is at the heart of the issues above with the definition of “true wind”. As Will says, the boat sails through the water, so the movement of the air relative to that water is what matters in measuring the performance of the boat. Whether that movement is “true wind” or some other term is something I’ll leave to linguists.
 
May 17, 2004
5,032
Beneteau Oceanis 37 Havre de Grace
That is the definition of apparent wind. ;)
No, apparent wind is the wind relative to the boat. (And yes, I agree that matters a lot for sail trim and reefing). I’m talking about wind relative to the water. That’s what I’d use to measure performance with things like Polar charts. And that may be different from wind relative to ground, if there is current.
 
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May 25, 2012
4,333
john alden caravelle 42 sturgeon bay, wis
the apparent wind gives you the horse power to move the boat, regardless of any current. your vmgs are affected by current, ok
true wind speed can be read by the eye. no wind is steady, the wind pulses, waves, tumbles, funnels, spreads out, .....
a computer tells you what is happening now, which i think is to late for the racing helmsman or the cruising helmsman in my never humble opinion. you want to be able to anticipate, not respond to the situation.
remember the sails steer the boat, the rudder is just for trim :)
 
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Nov 26, 2012
1,653
Hunter 34 Berkeley
How would you measure True Wind?
You're standing in the cockpit of your boat, you have a radio, a compass, a wind vane, an anemometer, a knotmeter, a GPS and a chart with tide tables. How do you know what "True Wind" is, if it is velocity over ground? I'm not arguing for one one definition over another. Over ground is how I've always understood it. It's just that that is less useful than wind over water, to me.

-Will (Dragonfly)
Simple. the boat has an anemometer that measures the apparent windspeed and direction and it has a GPS that measures the boat's speed and direction over ground. Through the miracle of linear algebra your computer is able to subtract the boat's speed and direction to give you the windspeed as if you were standing still. If you have a really fancy pants computer you can add in the boat's speed through the water measured at the paddle wheel and heading (GPS) and it will tell you the current's speed and direction as well. Again, this is just math.
 
Mar 26, 2011
3,401
Corsair F-24 MK I Deale, MD
The elephant in the room, of course, is whether the meter is accurate at all (is it recording the actual speed the air is moving past the instrument?). I've climbed masts with a real anemometer (one that has not been in the elements for years and sat on by birds) to calibrate masthead instruments, and 10-20 percent error is common place. The reading is also affected by the wind coming off the sails and their effect on the wind field. Deep off the wind it is often laughably far off (30-60%).

The point is that you are better off working towards a personal best than believing the figure is dead nuts accurate. It probably is not. But that dos NOT reduce it's value.
 
Jan 11, 2014
11,323
Sabre 362 113 Fair Haven, NY
Thanks, Mark. That and, just as importantly, the definitions of the terms.
So, it's just math and English!:):):)
i.e., in Scott's OP, his link to Panbo, Figure 2 DEFINES ground and true wind in the vector diagram.
But @Stu Jackson if we just accepted that article on face value what we have done all day while sitting at home socially distancing? Huh? ;)
 

PaulK

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Dec 1, 2009
1,223
Sabre 402 Southport, CT
No, because the wind is constantly changing in direction and speed.
So couldn't you stop more often if knowing the true wind is that important and it is constantly changing? You're not racing, so you can come to a complete stop, take your readings, and then continue, whenever you like. Since apparent wind is what we sail in, why does knowing the true wind - which if you're moving is going to be dependent upon calculations from instruments that may or may not be too precise - make any difference?
 
Oct 26, 2008
6,045
Catalina 320 Barnegat, NJ
The movement of the air doesn’t change, but its movement is different relative to three things: (1) a moving boat; (2) the ground; (3) water that’s moving because of a current. The question of which one to measure is at the heart of the issues above with the definition of “true wind”. As Will says, the boat sails through the water, so the movement of the air relative to that water is what matters in measuring the performance of the boat. Whether that movement is “true wind” or some other term is something I’ll leave to linguists.
Basically, I opened this post because I was intrigued by the Panbo diagram that ties the 3 relative wind components together with vectors. My premise was that if I assume that the paddlewheel isn't accurate, how can you arrive at the True Wind component? I figured that the instruments should be able to calculate the true wind component with apparent wind, ground wind and current set and drift, instead of inputing the paddlewheel speed. I am interested in this because by adding a heading sensor to the network, the current set and drift is accessible. But I'm not sure how the chartplotter knows set and drift (it is displayed when the heading sensor is active). Does the chartplotter know set and drift from data input or does it measure it while you are moving across the water? I'm guessing that if it is measured while moving across the water, it must rely upon the accuracy of the paddlewheel speed somehow. So now I am going full circle and back to relying on an accurate paddlewheel. :what: @thinwater you really know how to upset the apple cart! Now I'm supposed to distrust my wind sensor? :banghead:;)

Since I'm not on my boat as I am musing over these mental exercises, I will assure you @jon hansen , when I'm sailing, I see the glint of sunlight across the water surface, I feel the wind over my balding head, my forearms are sensitive to the pressure on the helm and I am sailing in all its erotic glory! I don't need an instrument, I notice the telltales and I can usually sense the pressure differences in the wind shifts before they happen. ;) I just so happen to like my gadgets, too. And I like thinking about what they are supposed to be telling me and how! :what::what: