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Old 28-05-2011, 07:39   #166
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I tend to sail 1 hour course to steer and not stray too many miles from the rhumb it's a compromise

I don't agree however if you just XTE crabbed that " you wouldn't get there" the difference isn't huge in practice.

Dave
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Old 28-05-2011, 08:24   #167
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Re: What Heading Am I Really On ?

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Originally Posted by btrayfors View Post
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).
TWA -- the true wind direction as expressed as an angle from the longitudinal axis of your boat -- will be different, depending on whether the wind speed and direction is referenced to water or to land. It is not just a question whether it refers to true north, or to your bow. So if you have a 6 knot N wind and you are dead in the water drifting with the tide at 6 knots to the east, and your bows are facing due north, then the wind's TWA will be 45, if you are using geographic wind, or 0, if you are using water-referenced True Wind.

Your instruments will always tell you "0" -- since they always use water-referenced True Wind in making the calculation. And indeed TWA based on water-referenced TWS is a valuable datum -- it tells you what is dead upwind and so forth. TWA based on geographical wind would be of no use for sailing.

TWD is, as everyone agrees, land-referenced. TWD tells you the big weather picture. It's wind as meteorologists understand it.


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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.
Raymarine, B&G, Furuno, Tick-Tack, Simrad, and probably others would be surprised to hear that "TWS is also geo-referenced. Always." The "always" part of this is factually incorrect, to put it mildly.
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Old 28-05-2011, 08:45   #168
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Re: What Heading Am I Really On ?

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Originally Posted by goboatingnow View Post
I tend to sail 1 hour course to steer and not stray too many miles from the rhumb it's a compromise

I don't agree however if you just XTE crabbed that " you wouldn't get there" the difference isn't huge in practice.

Dave
It depends. In most cases you are right.

But if you set your autopilot to "Track", so that it steer so that COG will be the same as bearing to your waypoint -- that is, so that the boat will be steered along the rhumb line -- and you are sailing at 6 knots, and you sail across a current of 6 knots, the pilot will steer you right into the current, so that you are standing still in relation to the ground.

It will do that because that will provide minimum deviation from the rhumb line and maximum VMG to your waypoint. Any other direction will take you away from the rhumb line and give you a temporary negative VMG to your waypoint. So the pilot will be faithfully doing what it was asked to do, and you will stay on the rhumb line.

But that would be manifestly the wrong way to sail. You will stay on the rhumb line but you would never get there. You cannot be sure of steering in the optimum manner by steering to the highest VMG to waypoint. That will only work if you have the same current over the whole passage -- same speed and direction -- and your boat speed is faster than the current.

If, for example, you're crossing a river, say, with a narrow 6 knot stream in the middle and still water on both sides of the stream, calculating a course to steer will give you an near-optimum crossing which is obviously better than setting your autopilot to "Track". If you set your autopilot to "Track", you will sail on a course directly towards your destination, and when you get to the stream, the autopilot will crab you across, fighting to stay on the rhumbline. You will thus spend much more time in the 6 knot stream than if you simply headed your boat perpendicular to the stream. If your boat speed is equal to the speed of the stream, the pilot will head you directly into the stream, and you will lose all motion in regard to the ground. You would stay therefore forever, or until you ran out of fuel.

If, however, you flipped off the autopilot and steered perpendicular to the stream once you got into it, you would get across the stream optimum time, but you would be swept downstream at the rate of 6 miles per hour in the stream. So emerging from the stream, you would have a long sail back up to your destination -- obviously not an optimum crossing.

If you calculate a course to steer, you will steer at an angle upstream which will get you into the stream at the same distance above the rhumbline as the distance below the rhumbline where you will emerge from the stream. Thus you will obviously sail a much shorter distance, than by setting your pilot on "Track", even if you flip off your pilot when you hit the stream.

But in this case -- to introduce a further complication -- course to steer will be "near optimum" but not "optimum" -- why? Because at a certain point you really need to cross the stream at 90 degrees to minimize time in it even at the cost of changing headings over the passage. So at some point a better crossing will be obtained by steering a single heading "x" on both sides of the stream in order to minimize distance to and from the stream, but in the stream, a different heading "y" which is perpendicular to the stream.

All of this could be shown mathematically, if someone is less lazy than I am.
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Old 28-05-2011, 09:13   #169
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Re: What Heading Am I Really On ?

What other reference is there if not " geo" ?

The Wind doesn't care if it's blowing over land or water... True wind is measured from a fixed point on the earth.

Wind relevant to a vessel moving is apparent wind.

True wind Direction and Speed experienced on a vessel moving through the water or a car on land....is an attempt to calculate the TWD and the TWS, taking into consideration all the affects of COG, SOG, Heading, Current etc.

In the example above the speed over ground..of 6 knots E.would have to be taken into consideration in order to attempt to determine the True Wind Speed of 6 knots north, If you are going to attempt to use it for any weather prediction.
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Old 28-05-2011, 10:35   #170
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Re: What Heading Am I Really On ?

Absolutely!

In the example Dockhead gave, the 6Kt wind felt from 45 degrees on the bow is NOT the True Wind. It is the Apparent Wind....the wind felt on the vessel...caused by a combination of the True Wind and wind caused the movement of the boat with the current.

Sorry, Dockhead...no cigar! I think you drank too much beer last nite :-)

Bill
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Old 28-05-2011, 11:56   #171
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Re: What Heading Am I Really On ?

My head is spinning. All this hot air is giving me the vapours!

Calling this thread hot air is just a joke. Some parts of this discussion are very informative, just need to figure out which parts.

True wind, ground wind, apparent wind!

I'm taking the USPS course next week. I'll get this started there and see what they say, the course will never end.

I am not by any means an experienced pilot. I have made long crossings but not as navigator. Any time I have been out on the water as pilot I have not had to deal with current. It has mostly been on a bay and the distances were not that great, mostly point and shoot navigation. My short forays into the ocean have also been like that. I can tell I need a couple of good navigation books to round out my navigational education. Anybody have a good recommendation? Something like "Sail boat Navigation For Dummies"?
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Old 28-05-2011, 12:43   #172
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Re: What Heading Am I Really On ?

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Originally Posted by Don1500 View Post
I can tell I need a couple of good navigation books to round out my navigational education. Anybody have a good recommendation? Something like "Sail boat Navigation For Dummies"?
Hi Don,

I can recommend some great books (have PM'd you) but none of them mention 'ground wind'

Cheers
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Old 28-05-2011, 12:53   #173
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Re: What Heading Am I Really On ?

Dockhead-
"lee-bowing" is a racing maneuvre/strategy, it has been misused and misappropriated by a few folks as a navigational concept. EIther they're misunderstanding the earlier use of the word (which goes back decades in racing) or they've borrowed it badly. See:
32nd America's Cup Official Website

or any 30 year old book on racing tactics.

It also seems like there's a lot of confusion (which is normal in navigation <G>) about "course through the water" which doesn't exist. There's a heading in the water, but a course is always a course over ground (COG) because your "course" always goes from one terrestial location to another, and that's always over ground, no matter how much water lies over those two points. Points defined as lat/lon locations, which are always terrestial ones.

All the rest if a matter of vector math, and if you get a Weems plotter or the old E6B pilot's calculator, with instructions, you can "spin" all the vectors up and avoid lots of gnarly math.

Cruisers can afford to get it wrong--racers need to get it right. Any racing navigation book.course should set it in short order.
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Old 28-05-2011, 13:01   #174
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Re: What Heading Am I Really On ?

Don,

The US Coast Guard Auxiliary offers Basic and Advanced Coastal Navigation courses that arae excellent. The texts are available as PowerPoint presentations here...USCGAUX Training Dept AUXOP Training Downloads
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Old 28-05-2011, 13:07   #175
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Re: What Heading Am I Really On ?

Quote:
Originally Posted by btrayfors View Post
Absolutely!

In the example Dockhead gave, the 6Kt wind felt from 45 degrees on the bow is NOT the True Wind. It is the Apparent Wind....the wind felt on the vessel...caused by a combination of the True Wind and wind caused the movement of the boat with the current.

Sorry, Dockhead...no cigar! I think you drank too much beer last nite :-)

Bill
Sorry, in this case True (water-referenced) wind and Apparent are equal, because you have zero boat speed. If you were to motor ahead, the Apparent wind would be further aft and stronger. But your instruments would tell you True wind stays the same - at 45 degrees off the bow. Ground wind is at zero degrees.

You have three different winds - Raymarine, B&G, and Furuno call them (1) Apparent, (2) True, and (3) Ground, respectively. If you are a meteorologist by training or are otherwise strongly attracted to the concept of true wind being referenced to ground, you might prefer to call them (1) Apparent, (2) True - Water Referenced, and (3) Really Truly True. You might even ignore (2). But (2) is what your instruments give you as True
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Old 28-05-2011, 15:58   #176
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Re: What Heading Am I Really On ?

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Sorry, in this case True (water-referenced) wind and Apparent are equal, because you have zero boat speed. If you were to motor ahead, the Apparent wind would be further aft and stronger.
Really? Seems to me that if you "motor ahead" the apparent wind moves forward, not aft.
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Old 28-05-2011, 17:12   #177
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Re: What Heading Am I Really On ?

I will agree that all of this True Wind discussion revolves around terminology.

Bowditch defines " True Wind" as " Wind relative to a fixed point on the earth"
( relative to a compass rose ) It's been around longer than Raymarine..

I also understand that electronics manufacturers have decided to call what we old-timers have always called " True Wind" ---> ground wind.

What they call True wind...is "relative" to the boat. Depending on how you set up your instruments, they may calculate true wind from SOG...or STTW...sailors may choose STTW, power boaters may be more interested wind relative to SOG.

With enough electronic inputs, you can even get your instruments to calculate the True ...True wind...or ground wind ..if you want. ;-)

I don't think one needs to be a meteoroligist to know/care how winds move around a low.
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Old 28-05-2011, 18:06   #178
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Re: What Heading Am I Really On ?

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Sorry, in this case True (water-referenced) wind and Apparent are equal, because you have zero boat speed.
Zero STW but 6kts SOG


Quote:
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You have three different winds - Raymarine, B&G, and Furuno call them (1) Apparent, (2) True, and (3) Ground, respectively.
NO! The NavNet3D allows for calculating True Wind using either STW or SOG, but they still call it True Wind. There is no such thing as Ground Wind in the NavNet3D.

NO! The Raymarine ST70 nav gear calls it ground wind.

I prefer knowing true wind calculated using SOG as the waypoint I'm heading for is referenced to the ground and I'm trying to figure out if I'm capable of a point of sail to get there.
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Old 28-05-2011, 18:23   #179
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Re: What Heading Am I Really On ?

As you say, Bowditch defines "True Wind" as ground-referenced. In Bowditch's discussion of Apparent Wind, current is not mentioned, and the Apparent wind factors are course and speed further reinforcing the ground-reference concept. However, in the Apparent Wind example Bowditch used "heading" rather than "course", which, in the real world will result in incorrect results (with either ground or water reference). You really have to include both heading and course/leeway for a correct answer. I have to say that Bowditch is a bit problematical on this point.

Here's another datapoint: Sailboat "polars" are used to show the predicted or measured speed through the water, as a function of True Wind Speed and True Wind Angle. These factors are all water-referenced. I GUARANTEE THIS. Wind instruments on racing sailboats provide AWA/AWS, and using speed through the water calculate water-referenced TWA/TWS. These water-referenced TWA and TWS values are plugged into the polars and used to generate a Target Boat Speed.

Modern instruments may measure (ground-referenced) SOG/COG, and convert the Apparent wind measurements to ground-referenced TWD/TWA/TWS, but they are usually (and I submit should be) converted to water-referenced values when being used for sailing performance analysis.

Many sailors don't have polars for their boat, and don't really care if they are getting the best performance possible. Most of the time I'm in that camp. For those folks the finer points of wind measurement aren't very important. A masthead windex, some bits of yarn on the shrouds, a wet finger in the breeze, these are perfectly adequate. Most of our instruments are calibrated so poorly that these finer points are lost among the errors. But TWA/TWS/TWD are terms of art, and they have specific meanings. Sometimes they have more than one meaning.

Bottom line: It might be a good idea to check your own instruments, and see what they are telling you. You might be surprised.
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Old 29-05-2011, 03:27   #180
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Re: What Heading Am I Really On ?

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It also seems like there's a lot of confusion (which is normal in navigation <G>) about "course through the water" which doesn't exist. There's a heading in the water, but a course is always a course over ground (COG) because your "course" always goes from one terrestial location to another, and that's always over ground, no matter how much water lies over those two points. Points defined as lat/lon locations, which are always terrestial ones.
The course through the water is normally known as the water track in RYA theory and is a perfectly appropriate thing to consider as it effectively defines your time to the next terrestrial location when divided by boat speed.
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