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Old 08-01-2013, 03:53   #196
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Re: Distinct Activities: Shackled by a Common Name?

Absolutely Carstenb, right on, Your approach is typical of the vast majority of careful navigators out there, your primary navigations tools are electronic, with backup , usually large overview types, paper charts.

Its always a funny debate for me, because I am a traditional nav fan, I even do lunars ( has anyone else tried the new Bruce Stark methods, brilliant). But I recognise through teaching what people are really using. Its noteworthy that the RYA Coastal Skipper ( Power only), considered recently to remove all traditional paper techniques from its courses, even though thankfully it did not. If you look at modern RIB navigation etc, too you see an increasingly total reliance on electronics, with a paper chart stuffed down someones boot for backup.

For me a careful navigator is just that, irrespective of the tools employed.

Where I do agree with the OP, is one of things that GPS engenders is a fascination with exact and continuous positional awareness. We have it in our phones, in our cars and in our boats, We have become used to instantly being able to precisely know where we are at all times.

On a boat, this is an unnecessary fascination, in the main we do not need immediate position updateing. One of the biggest challanges in the current RYA YM practical exam is "blind navigation", ie all electronics are off below, and the navigator must get the boat to the destination, using only paper charts, compass bearing, depth ( called from the helm) and a pencil.( the curtains are pulled below) Its brings out the sweat in many.

You are never really lost , just unsure of you accuracy.

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Old 08-01-2013, 04:28   #197
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Re: Distinct Activities: Shackled by a Common Name?

DAve,

While I would hate to have to rely on my sextant only (i'm just too rusty), I expect that I could find my position within some ascribed proximity. And get better at it as I practiced.

But it will be a long time before I jettison my paper charts - the overview they provide is just too good.

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Old 08-01-2013, 04:40   #198
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Re: Distinct Activities: Shackled by a Common Name?

Sextant work today is really now a "hobby", as a result all celestial navigation suffers from poor sextant procedure and resulting sights, mine included. Over the winter, I did as much practice as I could as where I now live I have no light splatter.Still doesnt get you prepared for small boat sights though.

The other thing that has really curbed my use is , 9/11.!! The restrictions on hand luggage means I can not easily bring the sextant along on deliveries anymore as I will not ship it in the hold. It was always a great source of pride to have it opened at X-ray security and many security people would be fascinated, Now thats so much harder, Ive even had to remove the service tools from it and several would not allow it as cabin luggage


Absolutely, today the big advantage of paper charts is their size, SO they are excellent for route planning, hazard inspection and overall situational awareness. The hour to hour stuff however is increasingly done on plotters.

Not to mention they look beautiful and its why I have 10 framed on my walls at home.

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Old 08-01-2013, 04:47   #199
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Re: Distinct Activities: Shackled by a Common Name?

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Originally Posted by goboatingnow View Post
Not to mention they look beautiful and its why I have 10 framed on my walls at home.

dave
Yes, a good paper chart is a wonderful and fascinating sight. Beside, you should always have at least one, when your impeller goes and you replace it, if for some reason you didn't get a seal in the box, you can make a perfectly good seal from an old (thick) paper chart. Works well as a seal almost anywhere (don't ask me how I know this.....)

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Old 08-01-2013, 06:17   #200
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Re: Distinct Activities: Shackled by a Common Name?

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Where I do agree with the OP, is one of things that GPS engenders is a fascination with exact and continuous positional awareness. We have it in our phones, in our cars and in our boats, We have become used to instantly being able to precisely know where we are at all times.
I would say that with some it goes beyond a fascination to an actual need - kinda non-plussed me when I first encountered it .

For me the "problem" (or at least higher risk) with GPS / Chartplotters is not that they tell folks where they are 24/7 (IMO no harm in that and generally they are pretty good) but that they allow folks to follow a course that is safe - but safe only as long as nothing goes wrong (engine fails / tack goes badly / wind shifts / helm gets distracted). I may well be a bit more cautious than some given the currents around here (and that hard to shake off!) but I do like a bit of searoom (thinking room) when navigating around hard objects - just in case. and sometimes a fair bit .....of course traditional DR tends to give you that automatically simply by being less precise (well, mine did / does ).....but even using MK1 eyeball to navigate with I am not the bravest of navigators.
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Old 08-01-2013, 06:18   #201
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Re: Distinct Activities: Shackled by a Common Name?

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.........................If you are crossing tidal waters, you need the shortest distance over water. That is because your boat doesn't sail in the bottom; it sails on water. The shortest distance over water is a constant compass bearing.
I was puzzled by this statement in the same manner as "goboatingnow" stated, "only if the tide is symmetrical". It would be very rare for the current drift to remain at the same velocity over a passage. Most often the drift would increase to a maximum at the center of the crossing and then dimenish at the end of the crossing. Maintaining a constant compass course would then result in a sigmoid curve that takes you off course into the set of the current first and then offcouse away from the set of the current for the last half of the passage. You would actually only be on the rhumb line at the midway point only! Of course, the constant update of position by GPS removes this error and you remain on course by changing your compass heading throughout the trip.
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Old 08-01-2013, 12:38   #202
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Re: Distinct Activities: Shackled by a Common Name?

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huh, only if the tide is symmetrical.

I for one, only calculate CTS if I have good tidal stream data, in most places you do not. Equally I set a limit on how far my XTE should get as I have found if I get the CTS wrong, I can get out of position, often ending downtide of the destination. Id have to say that most people today are merely using XTE.

in many areas that I am familiar with , I merely run CTS in my head, ie I allow for a degree of tide, over a period, often just compensating for it in my head, then use XTE as a quide to my success. While I have taught the RYA YM night classes to many people, Im not a great fan of the "compute all the vectors " approach. The on water events often throw everything out and force repeated recalculations,
Why would the tide need to be symmetrical? And why only if you have good tidal stream data?

In any case, calculating or even guessing set and drift and adding it up to get CTS is the only way to get across a moving body of water efficiently.

Let me put it another way -- if you were an omniscient navigator, who had tidal stream data accurate to 1 degrees and 0.0001 knots with resolution of one minute, and enough computing power to deal with it all, you would still make your transit by steering a constant compass bearing calculated using just that data. That is the shortest path to your destination. If you have to change course, you are increasing your distance through the water. The ideal path is a straight line through the water. A big S curve over ground, with XTE exceeding 10 miles at times in the case of a typical Channel crossing at springs.

We are not omniscient, and our tidal atlases have resolution of only one hour, and random arrows drawn on the chart, so a vagueish approximation of the streams. But that's good enough -- already gives us a much better CTS than we get from just holding a finger in the air. And we correct it as we go along. What mostly plays hell with these calculations is passage speed. A knot faster than you assumed and you are exposed to the current you thought for that hour for that much less time -- 12% to 15% less depending on that speed.
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Old 08-01-2013, 14:13   #203
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Re: Distinct Activities: Shackled by a Common Name?

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There are many people that sail by rote or others that get out of their depth and made mistakes through fear or lack of experience. I've seen people in extremis just forget to think or suggest crazy ideas. None of this has to do with GPS or chart plotters. Anyone that has sailed anywhere will know there are always issues with GPS positions on electronic charts. Harbours have different layouts, marinas are not present , buoy age has changed etc. then you add in anomalies. , for example all the chart plotters show me entering San Sebastián de la Gomera going over the breakwater and when I'm in the marina , I'm I the lobby of the parador hotel !

In practice I never rely on exact position fixing anyway ( too much holdover from by bad sextant sight days !!) I'd never put the boat in a tight situation just because a chart plotter said it was all right.

Dave
Yes I am "old fashion" and follow the KISS principle, but I also have electronics. My only pev about electronics (or any gizmo) is peoples over reliance on them....that and people who seem to think that excessively expensive/high tech methods (that is more the fault marketing though) are the only way to successfully get from point A to point B.

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But the reality is that 99%(?) of the time that is sufficient.
That means that 1% of the time the fancy gizmos would be insufficient, which is actually pretty bad odds when your life could be at stake.
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Old 08-01-2013, 15:21   #204
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Re: Distinct Activities: Shackled by a Common Name?

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That means that 1% of the time the fancy gizmos would be insufficient, which is actually pretty bad odds when your life could be at stake.
I put a question mark next to the 99% figure (which Stat was of course plucked from thin air ).

The real figure could be higher. could be lower - but I stand by my basic position that most of the times folks can get away with using electronic gizmos with no greater understanding of WTF the dotted line means or where it comes from. In practice also likely augmented by looking out the window. Well, at least now and again.

Hell, even in the pre Gizmo days plenty could get away with little more than a compass course and no greater understanding - just not so many. and not so much!

If boats involved rocket science most of us would be dead........
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Old 08-01-2013, 17:10   #205
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Re: Distinct Activities: Shackled by a Common Name?

97.54% of all statistics are made up on the spot.
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Old 08-01-2013, 18:36   #206
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Originally Posted by Dockhead

Why would the tide need to be symmetrical? And why only if you have good tidal stream data?

In any case, calculating or even guessing set and drift and adding it up to get CTS is the only way to get across a moving body of water efficiently.

Let me put it another way -- if you were an omniscient navigator, who had tidal stream data accurate to 1 degrees and 0.0001 knots with resolution of one minute, and enough computing power to deal with it all, you would still make your transit by steering a constant compass bearing calculated using just that data. That is the shortest path to your destination. If you have to change course, you are increasing your distance through the water. The ideal path is a straight line through the water. A big S curve over ground, with XTE exceeding 10 miles at times in the case of a typical Channel crossing at springs.

We are not omniscient, and our tidal atlases have resolution of only one hour, and random arrows drawn on the chart, so a vagueish approximation of the streams. But that's good enough -- already gives us a much better CTS than we get from just holding a finger in the air. And we correct it as we go along. What mostly plays hell with these calculations is passage speed. A knot faster than you assumed and you are exposed to the current you thought for that hour for that much less time -- 12% to 15% less depending on that speed.
Agh I see you were talking about theoretically perfect course to steer calculations. dr cooper I presume.

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Old 08-01-2013, 22:11   #207
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Re: Distinct Activities: Shackled by a Common Name?

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Agh I see you were talking about theoretically perfect course to steer calculations. dr cooper I presume.

Dave
Yes.

What I mean is that the shortest way to sail any passage across tidal or otherwise moving water is to point the bow in the same compass direction the entire way, notwithstanding how the water pushes you all over the place with the tide. The problem is knowing what compass direction to point the bow in, more about anon.

Important to note: when I write pushes you all over the place, I meanin relation to the ground. Chart plotters actually make it harder to visualize and understand this, because they show you where you are in relation to the ground, giving us a ground fetish which can be very harmful to good navigation. You are actually sailing a straight line through the water, notwithstanding the crazy "S" curve you see on the plotter.

On such a passage, every change of heading lengthens the distance sailed -- distance through water, which is the only way your boat can sail.

In order to sail the shortest distance, you would have to have an ideal CTS. In order to compute an ideal CTS, you would have to have perfect knowledge about the motion of the water over your course, and of course your own speed.

You do not have this perfect knowledge, so you have to make the best possible approximation.

It is absolutely amazing how well it works considering the low quality of the data. I guess if you do your approximations with feeling, the errors tend to cancel each other out -- I would be interested to know the theory behind it (my brother is a prof of math and physics -- need to ask him). I am rarely more than a mile off at my destination, after a mid-Channel course correction of never more than a degree or two, as long as my estimate of speed is reasonably close, although the XTE gets up to more than 10 miles along the way.
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Old 09-01-2013, 00:14   #208
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Re: Distinct Activities: Shackled by a Common Name?

Dockhead is precisely right. I has spoken.
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Old 09-01-2013, 01:57   #209
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Re: Distinct Activities: Shackled by a Common Name?

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You do not have this perfect knowledge, so you have to make the best possible approximation.

It is absolutely amazing how well it works considering the low quality of the data. I guess if you do your approximations with feeling, the errors tend to cancel each other out -- I would be interested to know the theory behind it (my brother is a prof of math and physics -- need to ask him). I am rarely more than a mile off at my destination, after a mid-Channel course correction of never more than a degree or two, as long as my estimate of speed is reasonably close, although the XTE gets up to more than 10 miles along the way.
This is where GPS and chart plotters are so useful. You've done your "proper" nav, and the chart plotter is ideal for monitoring your progress. You'll also get a much better idea of the actual tidal conditions if you have a good heading sensor and accurate log.
Crossing the channel you'll not be wanting to spend too much time below with pencil and rule.
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Old 09-01-2013, 02:45   #210
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Re: Distinct Activities: Shackled by a Common Name?

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Yes.

What I mean is that the shortest way to sail any passage across tidal or otherwise moving water is to point the bow in the same compass direction the entire way, notwithstanding how the water pushes you all over the place with the tide. The problem is knowing what compass direction to point the bow in, more about anon. ............................
It's difficult for me to accept that I would stand in such complete opposition to someone that seems to present such intelligent discourse. I sense that we must have some basic agreement, but I can't find it. Basic geography would present the shortest passage between two points crossing over the sea as a straight line and also maintaining a straight line over the ocean floor. It would be the mistake of maintaining a constant compass heading and keeping the bow pointed in the same direction regardless of variations in current set that would result in the inefficient sigmoid curve.
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