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Old 26-05-2011, 18:50   #121
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Re: What Heading Am I Really On ?

Just on a reference to True Wind. Actually all the books agree, that true wind is calculated by taking the vessels "speed". That of course is water related not ground related. ( see Bowditch p 523 year 2002, sec 3608).

The fact is that "Ground wind" never in effect existed for mariners as in reality they couldnt compute it easily. So you are correct ( several of you) when you mentioned that in all your sea training only true and apparant were mentioned. But the catch is that true was always computed ( and assumed to be computed ) with the STW rather then SOG ( to use the new parlance). its Ground wind rather true that doesnt "exist" for mariners.

As for computing course to steer and tidal currents. I agree and disagree. Most cruising sailors now use rhumb line steering technigues as that suits GPS. Equally the course to steer can also get you out of whack just as easy as "crabbing". Boats tend to have a lot of speed variance, often you adjust course to sail becuase of wave motion. If you do get it wrong with course to steer it can mean a slog against the current near the destination to correct etc. I view course to steer as a good "theorectical idea" that has its problems like all such computations. But I agree in crossing places like the english channel and the Irish sea, where detailed tidal atlas are available and where also there hapens to be a good symetrical current that virtually divides the time in two, its a valuable technique.

What a lot of people do when exposed to known set and drift is make an allowance of it and run a bigger XTE, thereby not getting too far off the rhumb line ( for safety) yet not being forced to make constant heading adjustments.

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Old 26-05-2011, 19:20   #122
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Re: What Heading Am I Really On ?

In sailing, and on this forum, true wind is referenced to the water. Technical papers have a different term for the land lubbers.

As far as which tack to sail on: the one that gets you to the destination faster
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Old 26-05-2011, 20:32   #123
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Re: What Heading Am I Really On ?

Can't wait for polar navigation lesson!!

KISS Apparent wind,true wind & set where set is all the components that make up the third leg of the vector diagram.

Now I must get back to reading Burton's
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Old 27-05-2011, 02:30   #124
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True wind as far as mariners are concerned never includes the effect of set and drift.

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Old 27-05-2011, 03:03   #125
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Re: What Heading is this Thread Really On ?

Quote:
Originally Posted by wolfaroo View Post
Dockhead, I hate to throw a spanner in the works but I personally don't agree with the term 'ground wind' being used as true wind minus tide induced wind. It not only contradicts almost all writings on the subject, it may also be the cause of confusion for you in relation to lee-bowing the tide as you mentioned earlier.


It is much easier to just consider 2; TRUE and APPARENT, with the tide-induced wind being a component of APPARENT wind:


TRUE wind is the wind experienced when not underway (i.e. at anchor etc).


APPARENT wind is the wind experienced when underway, and depends on the following 6 factors:


boat heading
boat speed
tidal stream set
tidal stream rate
true wind strength
true wind direction




As for the use of the term 'ground wind' - I am more than happy for you to correct me on this, so please don't think I am trying to prove you wrong, but in all RYA, MCA & UK sailing theory I have studied TRUE wind doesn't include tide-induced wind and there is no mention of ground wind in that context?
Well, I STILL don't understand lee-bowing There is a considerable body of opinion that it is a myth altogether. See, for example: http://www.j105.org/docs/noleebow.pdf

Here's one explanation for it I found:

"Lee bowing the tide

A concept that is useful to know about, is that of lee bowing the tide.
This means that if you are beating across the Channel you arrange to tack at the turn of the tide. The intention being, to have the tide on the lee side of the vessel.

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This decreases the angle between tacks and improves the apparent wind.
If this is done it will reduce the crossing time to the minimum possible.
In practice, it can be very hard to arrange for this to happen exactly as described, but it is a factor that an experienced skipper should consider."

Sailtrain: Navigation and Chartwork, Passage Planning.

But it doesn't make the least bit of sense to me -- doing it like this will reduce the miles you sail over ground, but your boat doesn't sail over ground, does it? The miles through the water -- and hence the time -- would be the same if you tacked opposite from the way suggested here, as far as I can figure out.

So I still don't understand lee-bowing, or else it really is a myth after all.

As to the rest of it, I was taught apparent wind differently from you. It's true wind plus vector for boat's heading and speed, period. Drift and set have nothing to do with it -- according to what I learned -- which of course doesn't guarantee that it's right All I can say is that your instruments definitely calculate it the way I was taught, and not like you say.

I'm also not sure about your concept of "tide induced wind". What difference does it make -- if wind is induced by tide, or by a low pressure system? Or by the earth rotating underneath your feet? Wind is the shear between the atmosphere and what is below it -- either land, if you're on land, or water, if you're on water. It's the RELATIVE motion between the two surfaces which constitutes WIND, which surface is moving and how, you don't care -- for purposes of sailing. So if you're sitting dead in the water and you've got 6 knots of wind coming over the deck from dead astern, you know you can raise your spinnaker and sail. The fact that there is a current running under you at six knots in the opposite direction, and that people on land are experiencing a dead calm, is irrelevant to you in terms of sailing. The current is relevant for NAVIGATION, of course, but that is a different discipline, which you will attack once you've worked out whether or not you can sail, and in what directions, which is a function of your TRUE WIND, which is the relative motion of the atmosphere to the surface of the earth -- that surface which you are floating in.
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Old 27-05-2011, 04:37   #126
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What Heading is this Thread Really On ?

Quote:
Originally Posted by Dockhead View Post
Well, I STILL don't understand lee-bowing ...or else it really is a myth after all.

...I'm also not sure about your concept of "tide induced wind". What difference does it make...
Well if it is a myth I am superstitious . I am also determined to help you 'believe' too (preacher?)!

Don't worry, even the legendary Tom Cunliffe makes it sound like an old wives' tale with the language he uses: "the boat will be magically set upwind, while the apparent wind will be freed by a geometrical mystery" but he then goes on to say "famous helmsmen win trophies with these methods... successful clipper captains were using these skills in the 1850's".

Mr Tim Bartlett, Fellow of the Royal Institute of Navigation, former technical editor of Yachting Magazine and Yachtmaster instructor writes:
"...by putting the tidal stream on the lee bow, the boat benefits two ways: from a 'freeing' wind shift and from an increase in the wind strength"

As a couple of people mentioned in this thread, vectors are probably the best way to think about it. See the attached pic, which also shows tide induced wind - to answer your question, it is just a component of the apparent wind so you are right it makes no difference as it is all part of the wind you feel on deck, but knowing how each component will affect the apparent wind on different headings helps you look at the wider strategical situation on a given passage and make the most efficient course.

At least in theory
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Old 27-05-2011, 05:33   #127
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Re: What Heading is this Thread Really On ?

Quote:
Originally Posted by wolfaroo View Post
Well if it is a myth I am superstitious . I am also determined to help you 'believe' too (preacher?)!

Don't worry, even the legendary Tom Cunliffe makes it sound like an old wives' tale with the language he uses: "the boat will be magically set upwind, while the apparent wind will be freed by a geometrical mystery" but he then goes on to say "famous helmsmen win trophies with these methods... successful clipper captains were using these skills in the 1850's".

Mr Tim Bartlett, Fellow of the Royal Institute of Navigation, former technical editor of Yachting Magazine and Yachtmaster instructor writes:
"...by putting the tidal stream on the lee bow, the boat benefits two ways: from a 'freeing' wind shift and from an increase in the wind strength"

As a couple of people mentioned in this thread, vectors are probably the best way to think about it. See the attached pic, which also shows tide induced wind - to answer your question, it is just a component of the apparent wind so you are right it makes no difference as it is all part of the wind you feel on deck, but knowing how each component will affect the apparent wind on different headings helps you look at the wider strategical situation on a given passage and make the most efficient course.

At least in theory
You explained that well, and for the first time (!) I start to have a glimmer of understanding.

It seems to me that this is merely introducing the vector of tide into the equations about sailing. So in that sense, it's not a myth, of course. The myth would be that you get a lift from it, but it's obviously not a myth that the tide is a vector which determines where you end up in relation to land.

It's a different way to get to the same result, although IF I'm understanding it correctly (big question, I admit) I don't like it -- I don't like it because it introduces very complicated and awkward geometry which is in my opinion not in place when you're working out how you can sail. It is mixing up sailing and navigation in one pot.

The example from one site which I gave above from a site attempting to explain -- the example of a Channel crossing -- it is definitely false, in terms of miles sailed -- they will be the same and there is no advantage in the given example from "lee-bowing". It is a fallacy to think that because you sail fewer miles along the bottom, that you will get there faster -- you sail the exact same number of miles through the water. It is also a fallacy to think that you will tack through a narrower angle -- that is an illusion -- only with reference to the bottom is that so, and you don't give a fig about that. Your tacking angle through the water will be the same. There might be an advantage in that the apparent wind will be stronger if you tack the way the author suggests. But that could be a disadvantage if the wind is strong enough with the opposite tacking strategy and you have to reef doing it as suggested.

Personally, I would keep the sailing and navigation separate. In the example of the Channel crossing above, I would do the standard tidal vector calculation -- an exercise in navigation -- to find out in WHAT DIRECTION I need to make progress in order to end up at my destination. THEN I would think about how best to sail in that direction, given the wind I expect to experience over the passage. Here your earlier comment is right: I do want to know what is the geographic wind, because I do need to understand how the wind will change when the tide changes. Trying to imagine some bizzarre assymetrical tacks with reference to the ocean bottom seems to be a bass-ackwards way to do it, IMHO.
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Old 27-05-2011, 07:35   #128
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Re: What Heading Am I Really On ?

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.

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Old 27-05-2011, 07:57   #129
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Re: What Heading Am I Really On ?

Quote:
Personally, I would keep the sailing and navigation separate. In the example of the Channel crossing above, I would do the standard tidal vector calculation -- an exercise in navigation -- to find out in WHAT DIRECTION I need to make progress in order to end up at my destination. THEN I would think about how best to sail in that direction, given the wind I expect to experience over the passage. Here your earlier comment is right: I do want to know what is the geographic wind, because I do need to understand how the wind will change when the tide changes. Trying to imagine some bizzarre assymetrical tacks with reference to the ocean bottom seems to be a bass-ackwards way to do it, IMHO.

Most people and I include experienced sailors, dont make such calculations to any great extent. Where there is a string tidal set and drift over the course of the voyage, then its usually factored in my using pesonal experience, ie sail up a bit to counter the expected tide. Most sailors like to stay within a mile or two of the rhumb, rather then fully using course to steer and allow themselves to be offset by several miles over the course of the voyage.

This is because things go wrong, vessels dont maintain the same speed, wind shifts occur and all these complicate course to steer. people just compensate to a degreee, so that if they have to change plans they arnt too far from the rhumb line.

As to using "ground wind", in reality you never use it other then say a forecast says " F5 NW", thats ground wind, the "true wind" might be slightly different due to tidal issues, but in reality there is so much inaccuracy , that there is rarely any practical difference.

On board, I use truw wind computation, ie wind computed against my speed through the water, to mainly understand whats really out there, and what I might experience on a course change. This wind is not ground reference, but that doesnt matter. You could argue that "Ground wind" is what you will really see there, ie, if you have a current on teh bow, your "true wind computation" would actually be not what you experience if you did a 180. ( The true wind would actually less then the ground wind, but the apparent wind is affected by ground wind.).

Its really semantics, true wind helps to compare tacks etc, but in reality most cruisers interpret true wind as " ground wind", in that they relate it to the forecast.

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Old 27-05-2011, 08:04   #130
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Re: What Heading Am I Really On ?

Quote:
As to the rest of it, I was taught apparent wind differently from you. It's true wind plus vector for boat's heading and speed, period. Drift and set have nothing to do with it -- according to what I learned -- which of course doesn't guarantee that it's right All I can say is that your instruments definitely calculate it the way I was taught, and not like you say.
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".
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Old 27-05-2011, 08:21   #131
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Re: What Heading Am I Really On ?

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.

These two conditions don't often occur in any part of the world that I have yet sailed. My experience does not include "The Channel", and I do understand that because of the strong tidal currents and heavy traffic it has been well studied and charted, but I don't believe that the tidal atlas considers the effects of wind driven currents, atmospheric conditions and all of the other minor things that add up to the actual flow on any given day/time. And in most other parts of the world, there is no tidal current data available at all.

And as to the wind (which greatly affects one's boatspeed and perhaps course), it seems to vary a lot when I'm sailing. One can use various means to predict what it will do over the course of a voyage, but IME none of these are accurate enough to make the "constant heading" method work over a whole voyage, or I suspect, even a channel crossing.

So, while the theoretical advantage of that method of navigation may be real, in practice frequent corrections must be made to reflect actual conditions, and these in turn make rhumb line sailing look pretty practical.

On ocean passages I reckon one is doing well just to avoid areas of known adverse currents, and even this is more difficult than it appears. Those nice broad arrows shown on the pilot charts are fictitious -- in actuality, the currents are made up of lots of eddies and swirls that are only averaged out on the charts. The advent of GPS has for the first time made observation of these small scale phenomena possible. We have experienced large variations in speed and set of currents in mid ocean, events that would not have been measurable by traditional navigational methods, and which would destroy the usefulness of the constant heading course.

At any rate, what Ann and I have done for our passages is to lay out a course that avoids (on paper) the worst adverse currents, avoids seamounts when possible, avoids reefs and such impediments by reasonable margins and eventually arrives at our destination. Then we set out in the best weather conditions we can find and then sail in whatever conditions Neptune throws at us, attempting to stay near enough to our proposed track to avoid the hard bits. It isn't as elegant as the preplanned constant heading, but it has gotten us where we wanted to go in reasonable time frames for a long time now, and I suspect that most oceanic voyagers do something similar.

Cheers,
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Old 27-05-2011, 08:54   #132
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Re: What Heading Am I Really On ?

heres an interesting puzzle as I beleive there are several answers


Youre sailing with the wind , youre apparent wind meter says the wind is 10 knots, and the log says the speed is 5 knots. The gps says the SOG is 8 knots ( all alligned with direction of travel).

compute the ground wind, true wind and then what is the wind the observer feels on the land immidiately aft of you. ( ignore all other factors).


Then the current reverses, directly 180. The SOG obviously falls to 5 knots, what would you expect the apparent wind meter to show, and what would the true and ground wind computations be.

Lets see if we can agree.

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Old 27-05-2011, 09:30   #133
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Re: What Heading Am I Really On ?

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Originally Posted by goboatingnow View Post
Youre sailing with the wind , youre apparent wind meter says the wind is 10 knots, and the log says the speed is 5 knots. The gps says the SOG is 8 knots ( all alligned with direction of travel).

compute the ground wind, true wind and then what is the wind the observer feels on the land immidiately aft of you. ( ignore all other factors).

Then the current reverses, directly 180. The SOG obviously falls to 5 knots, what would you expect the apparent wind meter to show, and what would the true and ground wind computations be.
Lets see if we can agree.
Dave
If I undertand your situation correctly then:

Current = 3 knots (8 knots SOG - 5 knots STW)
Apparent = 10 knots
True wind = 18 knots
(from 10 + 5 STW + 3 current)

Observer on land feel 18 knots

AFTER 180 deg current change with SOG of 5 knots (as stated by you - I would have expected SOG to be less) then:

Boat speed STW must now have somehow increased to 8 knots
Apparent wind = 13 knots
True wind 18 knots

Observer on land obviously notices no change

Or did I miss something???
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Old 27-05-2011, 10:00   #134
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What's the ground wind calculation ? Also redo your calculation as true is computed using only boat speed through the water.

And sorry yes SoG falls to 2 knots.
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Old 27-05-2011, 10:13   #135
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Re: What Heading Am I Really On ?

Are you sure this is a good idea?.
But I agree with wolfaroo
Sea/true=15
Shore =18
current switches..
Sea/true=21
The second example gets muddy quickly...unknowns involved.
Sailboat speed increases in fresher breeze... how much?...but when log/boat speed increases ApparentWind drops, but Log/Boat speed will still be more than 5...I'd say 6?

SOG is Log/Boat speed - 3 contrarycurrent..so giving the log/sailboat about 6knots,I get roughly 3 SOG(It is getting darned choppy though.)
A motor boat's a different case (except as windage)so it's easier.5-3=2 + a touch of windage.
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