What Heading Am I Really On ?

[btrayfors]

Paul, et. al..

You can respectfully (or otherwise) disagree with me all you want, but I stand by my statements.

TRUE WIND is the ACTUAL WIND REFERENCED TO A GEOGRAPHIC POINT (ON LAND OR ON THE SEA BOTTOM). It is expressed in speed (knots or MPH or KmH) and direction, in degrees clockwise from TRUE NORTH.

Any other definition is ridiculous. This is the definition used for hundreds of years by meteorologists, navigators, scientists, and everyone else before the advent of "calculated true wind instruments".

APPARENT WIND IS THE RESULT OF COMBINING THE TRUE WIND WITH ALL OTHER WINDS CAUSED BY BOAT (OR AIRPLANE) MOVEMENT...whether that movement is caused by current or waves or leeway or motors or sails.

Computers on boats attempt to calculate the TRUE WIND based on estimation of boat movement OVER GROUND...i.e., over the bottom. Once COG and SOG have been computed, and the APPARENT WIND has been established, it's a trivial computation to estimate the TRUE WIND direction and speed. The accuracy of such computation depends on the accuracy of the inputs and the algorithms used.

UNDERSTAND, HOWEVER, that TRUE WIND as computed onboard a vessel by it's navigational computer(s) is not necessarily the same as TRUE WIND. It may be off by several degrees, depending on how accurate the inputs have been.

Bottom line: THE TRUE WIND IS THE TRUE WIND. IT IS NOT SOMETHING THAT HAS BEEN CALCULATED -- OR MISCALCULATED -- ON BOARD.

Have a good and safe weekend, folks. Time for a Mt. Gay (and I'll bet Dockhead is well into the beer already, being some 5 hours ahead of me). Gotta catch up!

Bill

[Paul Elliott]

Originally Posted by wolfaroo

I made several posts and in ALL of them I stated that I either don't agree with the term 'ground wind' and that what I think of as true wind is what some others appear to think of as ground wind. And that there are only 2: true and apparent wind.

I believe that there are three wind references:

• The boat: This is Apparent Wind Angle and Speed (AWA, AWS)

• The water: True Wind Angle and Speed (TWA, TWS), calculated by taking AW and compensating for boat speed (and sometimes for leeway, either assumed or measured).

• The ground:True Wind Direction and Speed (TWS is an ambiguous term).

"Ground Wind" is a real thing, it can be directly measured from land, or calculated from the water, and there are terms for it. It is very useful in weather-routing, for example.

[HappySeagull]

...how about simple geometry.SideA,SideB,SideC.This is where this is going anyways....and then we'll get to iceboats and beyond far sooner

[Paul Elliott]

Originally Posted by btrayfors

TRUE WIND is the ACTUAL WIND REFERENCED TO A GEOGRAPHIC POINT (ON LAND OR ON THE SEA BOTTOM). It is expressed in speed (knots or MPH or KmH) and direction, in degrees clockwise from TRUE NORTH.

OK, I see that we are approaching this from opposite directions. Of course there is a True Wind, It blows across our planet at various speeds and from various directions.

But this is not what the terms TWA or TWS refer to when used on a boat. Like it or not, these have traditionally referred to a water-referenced wind, and they are calculated as I have described.

TWD is indeed ground-referenced wind, expressed exactly as you say. When used in the context of TWD, TWS also refers to the ground-referenced wind.

Computers on boats attempt to calculate the TRUE WIND based on estimation of boat movement OVER GROUND...i.e., over the bottom. Once COG and SOG have been computed, and the APPARENT WIND has been established, it's a trivial computation to estimate the TRUE WIND direction and speed. The accuracy of such computation depends on the accuracy of the inputs and the algorithms used.

TWA and TWS have been calculated since long before GPS was invented. The speed through the water and assumed leeway were used in the calculation, not SOG or COG. Until otherwise notified, I will assume that TWA and TWS refer to water-referenced measurements. If you want ground-referenced values, please invent another set of acronyms -- TWA and TWS are already taken.

You are talking about the thing, while I am talking about the term.

I realize that some modern instrumentation uses SOG and COG for TWA and TWS calculations. That doesn't make it right. At least, not until we all agree on the new definitions.

[btrayfors]

Originally Posted by Paul Elliott

I believe that there are three wind references:

• The boat: This is Apparent Wind Angle and Speed (AWA, AWS)

• The water: True Wind Angle and Speed (TWA, TWS), calculated by taking AW and compensating for boat speed (and sometimes for leeway, either assumed or measured).

• The ground:True Wind Direction and Speed (TWS is an ambiguous term).

"Ground Wind" is a real thing, it can be directly measured from land, or calculated from the water, and there are terms for it. It is very useful in weather-routing, for example.

Paul,

Well, well. We can agree on the basic premise, except that what you call the "Ground Wind" is in actuality the True Wind. It need not be further qualified. The True Wind is the True Wind, and True Wind Speed and True Wind Direction are measurements of the True Wind....referenced to a fixed geographic point.

And TWS isn't ambiguous. It's the speed of the true wind. Call the calculated True Wind Speed something else.

Happy Weekend.

Bill

[Paul Elliott]

Bill, we're out of sync. I will be interested in your response to my posting #154.

Happy weekend, all. In my next posting I will calculate the number of angels that can dance on the head of a pin.

[btrayfors]

Originally Posted by Paul Elliott

Bill, we're out of sync. I will be interested in your response to my posting #154.

Happy weekend, all. In my next posting I will calculate the number of angels that can dance on the head of a pin.

Paul,

Yes...out of sync. In more ways than one.

I didn't mention GPS when I was talking about COG and SOG.

Any computation of TWSc and TWDc on boat must pass thru computation of COG and SOG first, since the Apparent Wind Speed and Direction, and Boat Speed thru the water cannot be used to compute the True Wind Speed and Direction without knowing COG and SOG! viz...

Boat Speed thru the water: 5 knots
Boat Course thru the water: 045 degrees True
Apparent Wind Speed: 8 knots
Apparent Wind Direction: 120 True

Required: True Wind Speed and Direction (in knots and degrees True)

This problem can only be solved if there is no current and no leeway (set and drift), in which case the Boat Course and Speed would equal COG and SOG.

In the presence of either current or leeway, you'd need more information.

Leaving now (really)!

Bill

[Paul Elliott]

Bill,

I completely agree with respect to TWD. No argument.

Now, about TWA, do you agree that this is relative to the water? I obviously think it is, and that the TWA/TWS pair both refer to a water-referenced wind.

The TWD/TWS pair are ground-referenced.

If by "TWSc" you mean "TWS calculated", I don't think we need to differentiate. Let's try to agree what we are describing, and in particular it's reference system.

Now I'm really out of here!

[goboatingnow]

Paul

What you are describing is a virtue out of a necessity . Mariners have always attempted to determine the " true wind", but until recently that was impossible as the only speed reference that was available was compromised as it was relative to water. However it's the closest thing they had and they called it " true wind".


Ignore all this TWA etc. That's just the difference between relative and absolute angles. There's no difference between 10 degrees off the port bow on a heading f 000 and 350 degrees absolute. It's exactly the same in reality.

But this mariners" true wind " is rubbish of course in areas of little tidal effect ( oceans and the like ) true wind and real true wind are virtually identical

The " mariners true wind" is however a computational nonsense. It's not a wind you will ever experience , nor does it convey any real information, nor does the boat ever sail in it. It fact it misleads.

Now that we can calculate real " true wind " in both relative and absolute angles ( so you can have your beloved TWA) we really should ditch this computed misnomer and compute proper True wind. It's actually what's really there in the absence of the boat.

( or course there are accuracy issues with SOG and COG, but that another days work)

Dave

[Paul Elliott]

Originally Posted by goboatingnow

The " mariners true wind" is however a computational nonsense. It's not a wind you will ever experience , nor does it convey any real information, nor does the boat ever sail in it. It fact it misleads.

It misleads only if you think it is a ground-referenced wind. I claim that TWA and TWS are by definition water-referenced.

I agree that you do not directly experience water-referenced True Wind, and it is of little practical value (other than helping people understand what happens to the Apparent Wind as they speed up, slow down, turn, etc). In my ideal world we would have boat-referenced Apparent Wind, and ground-referenced True Wind, but you or I wishing that TWA/TWS were ground-referenced doesn't make it so.

Again, most of us understand the concepts quite well. We get confused and argumentative when we discuss them using differing definitions. I've got all the calculations for wind in my NavMonPc program, and I can derive any of the wind values from whatever data is available. My program even lets you select ground or water as the reference for true wind calculations, since I realize that some people prefer the ground-referenced convention. I firmly believe that water-based is the proper reference for TWA/TWS, based on historical usage.

So tell me -- if I'm wrong, when did the definition of TWA/TWS become ground-referenced?

[btrayfors]

Paul,

You're mixing apples and oranges.

TWA (True Wind ANGLE), of course, is boat-referenced, just as is any "angle" on the bow. This, however, is NOT True Wind Direction (which is geo-referenced....always).

TWS (True Wind SPEED) is also geo-referenced. Always. Not water or boat referenced. It is what it is.

Just because your program treats the definitions differently doesn't make it right or common usage.

Bill

[Paul Elliott]

Bill, we agree on all but TWS. I believe it to be, when paired with TWA as is usually the case on a boat, the water-referenced windspeed. When paired with TWD it is a ground-referenced parameter. (But for what it's worth, see the Ockam example below for a water-referenced TWD!)

I realize that the usage is at the least evolving, but to show I'm not entirely alone on this point, here's a quote from Ockam (True Wind):

Relativity
There are two frames of reference for wind, each with its own advantages.

• Ground relative. This is the 'normal' way of thinking when on land. "Wind's from the nor'east at 60". If you strapped an instrument on a car and drove around, the wind direction would always report as 45° (ignoring magnetic variation of course).
  • All meterology is earth-based and true north oriented, so earth-true is the number you have to use when route planning.
  • Using earth-based boatspeed & heading prevents the calculation of current.
  • When converting to water-true, say for next-leg apparent, the local current and magnetic variation must be considered.
• Water relative. On the water, the true wind that the boat feels is a combination of the earth-true and current. If the current is significant, water-true wind will be different from earth-true.
  • The boat responds to water-true because the keel is immersed in the water. Polar plots always use water-relative reference.
  • True wind angles are the same on both tacks (except for wind shear). If wind direction were calculated using COG/SOG, the true wind angles would be different on the opposite tacks (more on the difference here).
  • When being used for performance, because of this symmetry, water-true is the number to use.
  • Current is calculated by comparing water-based boatspeed & heading+leeway with COG/SOG.

The Ockam system displays water-true unless COG/SOG is replacing boatspeed/heading.

Here is the "VWT" NMEA sentence definition. Note that this is for "True Wind Speed and Angle", referenced to the water:

VWT - True Wind Speed and Angle
True wind angle in relation to the vessel's heading and true wind speed referenced to the water. True wind is the vector sum of the Relative (Apparent) wind vector and the vessel's velocity vector relative to the water along the heading line of the vessel. It represents the wind at the vessel if it were stationary relative to the water and heading in the same direction.

I am sure that we could find examples that conform to the usage you are advocating. At this point I will stop trying to force my opinion on everyone, but let me suggest that we all define our terms unambiguously, at least in the rare cases where it might make a practical difference.

[Dockhead]

Originally Posted by goboatingnow

lee-bowing is simple, The boat is being pushed by the current, in a partcular direction, such that the apparent wind goes slightly more "aft". This equates to freeing the boat, allowing you to point slightly more upwind, then were the current not there. The effect is small and only applies when you are hard on the wind.

Dave

The apparent wind goes aft from a land-based perspective, but not from a a water-based perspective. You can look at it -- with the exact same result -- that the current is causing a favorable change in your COG, compared to your course through the water. Isn't that a more sound way to look at it? Why turn what is fundamentally an issue of navigation into an issue of sailing?

The way I see it, you can look at it two different ways:

"The current is pushing me over to starboard, I'm on a starboard tack, so the wind seems to be freeing since the current makes it appear to back and I'm getting a lift"

OR:

"I'm sailing just the way I expect considering the wind direction and speed from the point of view of the water. The current is changing my COG closer to my destination, so I won't have to tack as soon as I expected.

Which is more logical, more sound, and more elegant? I submit the second.

[Dockhead]

Originally Posted by goboatingnow

Dockhead. Thats not the way its taught. Apparent wind is NOT calulated, its measured. Its the actual breeze in your face that the boat feels, and the wind instruments reads. Its combines all possible vector effects, tide, leeway, speed, the heavy guy on the port side etc. Hence it combines set and drift and everything else. Underway its the one known wind value.

True wind is a computation, or a measurement at zero water speed. which is actually impossible to read since the boat is always moving even when stopped. If you anchor , your true wind calculation become "ground wind".

Certainly, this is correct in every detail. If I implied anything different, let's say it was a brain fart

[Dockhead]

Originally Posted by Jim Cate

What an interesting theoretical discussion, guys. Well done.

But, as far as I can see, in the real world, these nits have been over picked. In order for the "steer a constant course" method to work, one must have consistent wind speed and direction and to have tidal and current info that is completely correct as well.

Actually, it does not need to be completely correct. It only needs o be slightly correct. If your predictions about the currents are even slightly more correct than they are not correct, it will be better than crabbing along the rhumb line. This is a mathematical fact.

Sailing Needles to Cherbourg across the English Channel, you've got 60 miles of water with strong tides mostly perpendicular to your course (but not entirely). On the French side, the tides really rip and at springs you can see 6 knots of current (!) at some points. Six knots of current, of course, is 6 miles of XTE over one hour of sailing.

The tidal atlases are not that precise, and the biggest uncertainty is your speed. If you end up in a lull spending two hours crossing one nine mile piece of water, instead of one hour, you will end up miles further downtide than you expected.

BUT -- the "course to steer" method works astonishingly well. I make an allowance for leeway and steer one degree (one mile at 60 miles) to whichever side will be uptide upon arrival, as a margin of error. I check how we're doing every hour -- which is a snap, because all you have to do is read the XTE on the plotter and compare it to the sum of the tidal vectors at that hour. In the middle of the Channel I make the calculation all over again from scratch and correct the course. But I have never had to correct by more than one or two degrees, and usually there is no correction at all. We generally arrive close to the coast a mile or two uptide of our destination. The last hour you just turn downtide and steer COG.

It helps I guess that the wind is good in this part of the world. I've made eight crossings so far and only had to motor one of them. 60 miles usually takes us about seven hours (a pace of 8.5 knots). Three were at night. Sailing on a pace of 8.5 knots (once it took just over 6 hours or 9.5 knots -- a rollicking beam reach in 23 knots of wind) means we do not get in two full tides, as you would on a 5 knot pace, so the tides do NOT cancel each other out, in our case. We are always steering a fair distance east or west of our destination.

That's 60 miles through the water. The distance over ground is very much further, as you sail a great big "S" through the water as shown on the plotter. If you tried to sail this particular body of water by sailing COG towards your destination to stay on the rhumb line, you might never get there. So this particular exercise -- admittedly, an extreme case -- burns into your brain the rightness of navigating this way. But -- in less extreme cases, it does become less and less important how you do it.