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Old 24-03-2015, 15:44   #406
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Originally Posted by Seaworthy Lass View Post
As far as I'm aware, all weather forecasts give ground wind.
Yes they do, they also say between this speed and that speed and may be 50%(?) higher etc etc. Couple that with the area covered by typical forecasts and its all just a little bit approximate.

Even the station reports can be misleading... In Bass Strait frinstance Cape Otway and Wilsons Prom are invariably off the scale while at the same time Crib Point ( being in amongst the pine trees at HMAS Cerebus) is having balmy breezes...
FB 7 SC 8 FR 0 PW 8 -- Bay Winds Page

edited to add....

http://www.baywx.com.au/coast.html

All a little on the far side of academic really...
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Old 24-03-2015, 16:30   #407
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Originally Posted by goboatingnow View Post
Dockhead, please point to me a US reference manual that details multi hour CTS in a non trivial situation

Answer these issues

(a) Who in reality sails an unknown ground track

(b) in most cases , you dont have accurate position dependant hourly tidal data , multi hour CTS cannot be commuted with any precision at all, and what actually happens is a kind of hourly " eyeball" CTS ( which is fine)


in a complex non symmetrical non reversing multi hour tide, none uses a single vector computation to sail off into the sunset. in reality its just not safe
I think we're going in circles, a bit.

I already answered (a), and in some detail. Your ground track is not "unknown". You have a good idea to begin with, and if there is any question of hazards, you work it up on the chart. It is entirely safe if you do it soundly, and keep a decent watch along the way. It is really not totally different from the same problem when you're sailing the rhumbline.

As to (b) -- I also answered this. Successful CTS sailing does not indeed depend on high precision. Going off on generally the right direction is far better than sailing in intentionally the wrong direction. It is easy to correct it as you go along. The better the data you have, the more precise, and therefore efficient. If the data is worse -- then efficiency will suffer, and you'll have to correct, but you're still far better off than the guy intentionally crabbing off stupidly down the rhumbline.


As to "hourly eyeball CTS" -- I have a feeling that you have a mistaken idea (correct me if I'm wrong) that a series of hourly CTS's equal a whole-passage CTS -- they do not. Your hourly CTS is trivial, and approximates a rhumbline passage. It will vary from a rhumbline passage exactly to the extent of the variation in current during that hour. So the potential for efficiency, if it exists at all, is vastly less than if you do a CTS for the whole passage. The whole point is to be as far away from the rhumbline, as you need to be, in order to sail a straight line or close to straight line through the water.


In order to really understand all this, you really have to be able to visualize a water-referenced world. Let's take the opposite situation -- dead still water without the slightest current. So here a straight line over ground equals a straight line in water. Let's say you're planning a passage and there are some obstacles in the way -- how do you do it? Well, you draw a straight line on the chart and stay as close to that straight line as you can while avoiding the obstacles, correct? And efficiency doesn't suffer much as long as you stay somewhere vaguely close to the straight line. We all do this almost every day, even on land. Even navigating through a city on foot, you use the same technique.

Well, exactly the same is true of a CTS passage -- the constant heading is your straight line through water. So you can't calculate it exactly -- ok. Well, you have to at least try. Even a vague guess will get you going in the generally right direction. You can correct it as you go along. Just like you can't sail a perfect straight line in the previous example -- but a decent approximation is still perfectly fine.

So the whole point is to get a CTS to your ultimate destination, and then stay as close to that heading as you can over the passage, considering corrections, and considering any obstacles. If you set waypoints along the rhumbline -- that has no effect at all.


Another myth which has been propagated here is that CTS nav is really useful only in extremely rare cases like the English Channel. This is not true -- reversing tides make the efficiency of CTS nav vividly evident, but almost every sailor encounters situations where the straight line through the water does not coincide with the straight line over ground. Another one equally vivid to La Manch is crossing a strong stream -- like the Gulf Stream. And this is maybe easier to understand than the Channel. If you follow the rhumbline across the Gulf Stream, you will be in the absolutely wrong place when you reach the stream. You will be swept down and you will be fighting your way back up -- with the result that you will have a long, long slog back up, maybe doubling your passage time in a slowish sailboat.

The quintessential CTS case is actually not the Channel, but the Gulf Stream, because EVERYONE can visualize this. A CTS Channel crossing seems like PFM to most sailors, but the Gulf Stream is easy -- obviously, you have to head off way more southerly than the rhumbline, in order to meet the Stream at a point where the current will take you towards the Bahamas. Even the dimmest sailor grasps this instinctively. All CTS nav is, is how to find at least the approximate heading, which will put you in the right place as you go across -- just like in the Channel. Because the shortest way across the Stream -- just like in the Channel, just like everywhere -- is that way which has you on the same heading all the way across. Even sailors who have no idea how to do the calculations, will be miles and miles ahead, just by eyeballing it -- just head South. Miles ahead of the awful rhumbline. But if you CAN do the calculation, you'll save up to half the passage time.

There you are - a multi-hour CTS, no reversing stream. Is it "dangerous", not to know your ground track? Hell, no. You can figure your ground track fairly easily, to the extent you need to (which you don't, much, since like most of these passages, there AREN'T any obstacles). Is it a "party trick"? Only if you think saving literally half the passage time is a "party trick".

We sail in water, not over land. Every passage is a CTS passage -- a constant heading is always the direct route. If the water is not moving, or if it is moving in a way which does not vary over the time of your passage, or over the areas you pass through, then the rhumb line will approximate the ideal CTS way across. And in those specific cases, you can use the rhumb line as an approximation of the right way across, because it is easier to navigate to with our electronics.

But the One True Path is always the constant heading way, every day, every where. This is not taught much, and I fear not understood much, for a simple reason. It has a material effect only on those of us who sail so slowly, that the currents reach an appreciable proportion of our boat speed. Ships making 16 knots or more don't care -- the rhumbline always works more or less, as a rough approximation of the One True Path through the water, when you're making 16 knots. Motorboaters are oblivious to these issues for the same reason. But for those of us who are doomed to travel at 9 knots, or 8, or 6 -- in water which moves at 2, or 4 or 5 knots -- the rhumbline is the Stupid Crooked Path.
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Old 24-03-2015, 18:01   #408
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

An excellent try, but will you get through I wonder? Big ship drivers and big engined motor yachters will never get this message I suspect. Those that need to probably have always known, but don't dare stick heads above the parapet for fear of being gunned down! Ooops, sorry wrong thread.
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Old 24-03-2015, 18:06   #409
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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An excellent try, but will you get through I wonder? Big ship drivers and big engined motor yachters will never get this message I suspect. Those that need to probably have always known, but don't dare stick heads above the parapet for fear of being gunned down! Ooops, sorry wrong thread.
Robin and I go way back --

When I was nervously getting ready for my very first Channel crossing about six years ago -- a terrifying prospect when you've never done it -- Robin, who lived in the UK at the time (Poole, right?) was the one who gave me the most useful advice, patiently answering stupid question after stupid question of mine, and in fact explaining to me for the first time a lot of these concepts I am now spouting off about. He even ran my passage for me on his Neptune program. Thanks, Robin!
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Old 24-03-2015, 18:08   #410
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Originally Posted by Dockhead View Post
But the One True Path is always the constant heading way, every day, every where.
OK, I'll ask the question.... what is the One True Path from Auckland to Valporaiso?
An enquiring mind wants to know....
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Old 24-03-2015, 18:11   #411
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

East.
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Old 24-03-2015, 18:17   #412
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Originally Posted by El Pinguino View Post
OK, I'll ask the question.... what is the One True Path from Auckland to Valporaiso?
An enquiring mind wants to know....
A philosophical question!

There is a One True Path from there to there, just like from anywhere to anywhere. Whether it is knowable or not -- another question, but it exists whether or not we can know it.

Whether we can know it or not depends on whether we have any data about the currents, or not. Even a little information will get us closer. Maybe even just a hunch. We who sail in slow boats, cannot navigate well, without knowing something about how the water moves.
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Old 24-03-2015, 18:23   #413
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Originally Posted by Wotname View Post
East.
Is that 'East' or... 'Due East'?
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Old 24-03-2015, 18:29   #414
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Originally Posted by Dockhead View Post
A philosophical question!

There is a One True Path from there to there, just like from anywhere to anywhere. Whether it is knowable or not -- another question, but it exists whether or not we can know it.

Whether we can know it or not depends on whether we have any data about the currents, or not. Even a little information will get us closer. Maybe even just a hunch. We who sail in slow boats, cannot navigate well, without knowing something about how the water moves.
Yes , we have information , we have the West Wind Drift at about half a knot more or less.....
Knowing how the water is moving and the shape of the earth suggests to me that a GC with daily course adjustments would be the way to go... can't see a Shining Path anywhere..
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Old 24-03-2015, 18:47   #415
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Originally Posted by El Pinguino View Post
Yes , we have information , we have the West Wind Drift at about half a knot more or less.....
Knowing how the water is moving and the shape of the earth suggests to me that a GC with daily course adjustments would be the way to go... can't see a Shining Path anywhere..
If the water moves at the same rate and direction the whole way, and all the more at such a slight rate, then the One True Path will pass close by the rhumbline. So you wouldn't need to strain yourself to work out a CTS -- you can just follow the rhumbline, which is not the ideal path, but close enough for government work, so to speak.

If there is water moving at a much different rate (or direction) anywhere along the way, then you would be well served by trying to work out at least an approximate CTS to give you a more or less constant heading all the way.
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Old 24-03-2015, 19:39   #416
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Originally Posted by El Pinguino View Post
OK, I'll ask the question.... what is the One True Path from Auckland to Valporaiso?
An enquiring mind wants to know....
The first thing you want to remember with these things is that it is cross-currents that are going to mess you around the most. They are the ones that can cause differences in CTS.

Going from NZ to Chili, you are largely going along with the current, so its influence is mainly in how long it is going to take you to get there.
There are other factors in route planning. The shortest ground route (great circle) could take you further south than you care weather-wise for example. Once you have made such strategic decisions, you can look at the ocean currents to be expected. Along the coast of Chili you will find the Humboldt Current flowing north quite noticeably and you might want to account for it in your planning.
Work out how long it should take you to cross it and how much drift it is going to create for you when you get there, so you can "aim" further south with your passage and never have to buck that current to make Valparaiso at the end.
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Old 24-03-2015, 19:53   #417
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Originally Posted by El Pinguino View Post
Is that 'East' or... 'Due East'?
I stand corrected.
Make that Due East(ish).
Wake me when you close the Fairway Buoy (and that is pronounced " boy" -as I'm sure you know)
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Old 24-03-2015, 21:10   #418
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Robin and I go way back --

When I was nervously getting ready for my very first Channel crossing about six years ago -- a terrifying prospect when you've never done it -- Robin, who lived in the UK at the time (Poole, right?) was the one who gave me the most useful advice, patiently answering stupid question after stupid question of mine, and in fact explaining to me for the first time a lot of these concepts I am now spouting off about. He even ran my passage for me on his Neptune program. Thanks, Robin!
De nada, that is what friends especially sailing ones are for. I miss sailing and navigating over there from Poole too, beats map reading up and down the Ditch for sure. I miss having waters over a few feet deep too. and channel buoyage that is positioned the right way round.
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Old 24-03-2015, 21:13   #419
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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The first thing you want to remember with these things is that it is cross-currents that are going to mess you around the most. They are the ones that can cause differences in CTS.

Going from NZ to Chili, you are largely going along with the current, so its influence is mainly in how long it is going to take you to get there.
There are other factors in route planning. The shortest ground route (great circle) could take you further south than you care weather-wise for example. Once you have made such strategic decisions, you can look at the ocean currents to be expected. Along the coast of Chili you will find the Humboldt Current flowing north quite noticeably and you might want to account for it in your planning.
Work out how long it should take you to cross it and how much drift it is going to create for you when you get there, so you can "aim" further south with your passage and never have to buck that current to make Valparaiso at the end.
Thank you for the advice ( doffs cap ).

However I was actually seeking enlightenment regarding Dockhead's 'Shining Path' but I guess I will have to ride the Humbolt up to Peru to gain enlightenment in that regard.

The Humbolt isn't really an issue on that passage ... what you do have to watch out for is the South East Pacific High which can glass you out well south of 40*S and a long way offshore as you approach Chile.
The rhumb line is a definite non starter unless you want calms and easterlies and a 60 day passage.
The GC isn't real flash either ... best is a composite GC and a crossing in about 45*/48*S.

So... once again Dockhead's Shining Path has been found impractical in real life. There is a lot more to a successful passage than the shortest distance through the water...... consider Valpo to Auckland if you will.
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Old 25-03-2015, 05:00   #420
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Thank you for the advice ( doffs cap ).

However I was actually seeking enlightenment regarding Dockhead's 'Shining Path' but I guess I will have to ride the Humbolt up to Peru to gain enlightenment in that regard.

The Humbolt isn't really an issue on that passage ... what you do have to watch out for is the South East Pacific High which can glass you out well south of 40*S and a long way offshore as you approach Chile.
The rhumb line is a definite non starter unless you want calms and easterlies and a 60 day passage.
The GC isn't real flash either ... best is a composite GC and a crossing in about 45*/48*S.

So... once again Dockhead's Shining Path has been found impractical in real life. There is a lot more to a successful passage than the shortest distance through the water...... consider Valpo to Auckland if you will.
Not indeed -- there are just other dimensions, some of which may be much more important, in some cases. To say that this example shows that CTS nav is impractical, is exactly like saying that plotting a course to a waypoint is an impractical nav technique, just because in this example there's an obstacle on the course line. Both here and there, the nav technique shows you the ideal path according to your frame of reference (the course line, or rhumb line, for ground-referenced nav), which is a guide you stay as close to as practical, while departing from it to the extent necessary to deal with other considerations like avoiding obstacles, weather, or whatever.


The One True Path is the direct route through water -- one needs to start with that, rather than a direct route over land, which is fallacious and inapplicable. Obviously, if you need to dodge weather, avoid obstacles, or get around a continent, you need to modify that plan. Or if you can pick up a fair current somewhere -- then in many cases, it's reasonable to forget all other kind of navigation and just hie thee to the fair stream. But unless you are completely abandoning normal navigation, you should start with the CTS way, and try to stay as close to it as practical, considering your other requirements (dodging stuff).

Once a passage is long enough to bring up great circle questions, then that adds another dimension. CTS nav and great circle nav are perfectly compatible with each other -- just substitute a heading varying at a precise rate for a constant heading. That is just a more precise "constant heading" in geographic terms; the unvarying constant heading is again just a proxy which can be used for passages too short to make a difference. Just like the rhumbline can be a proxy for a CTS in case the water is not moving enough to make a difference!

And this relationship may be illustrative:

The One True Path is actually a great circle varying heading -- even for a passage of one mile!

However, the difference between the great circle varying heading and constant heading for passages of under 100 miles is measured in fractions of a degree, and so have no practical consequence -- so we use a constant heading as a proxy.

If the water is not moving, or is moving at a constant rate, we don't even need the simplified constant heading CTS -- we can just use the dumb rhumb line, which is even easier to calculate. It's already the second order proxy for the Real Thing.


Now I'm stating all of this in a half-serious categorical way -- people who teach chart work will protest "that's not navigation!". Because that's not the way it's done on ships. Navigation for ships is done in a purely ground-referenced way, and the science of navigation has developed that way. But it is only because ships travel too fast to feel much difference between water-referenced, and ground-referenced navigation, that we have this situation -- and that does not apply to us.


So if we want to be practical about it -- I think it should be stated like this: Obviously we need to know how to navigate, and those skills are not that widespread among us. We even have people making up their own definitions of heading :scary:. We should know how to do it, the way it's done on ships, as a starting point. We should fight the tendency for people to let the electronics do everything and not even understand the principles.

But at the same time, we should be aware that ground-referenced navigation will only give a second order proxy for the direct away across a given piece of water -- many times close enough. But sometimes -- really not. You might not even get there -- if you try to use ground-referenced nav, in a slow enough boat, in a lively enough body of water -- like crossing the Channel at springs. That's how far ground-referenced nav can diverge from the right way to your waypoint. But ground-referenced nav always diverges from the right way -- the only question is whether it diverges enough to matter. So I suggest that understanding water-referenced nav techniques like CTS nav is really important, for anyone who sails a vessel slow enough, for currents to make a significant proportion of our boat speed.
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