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Old 06-02-2013, 13:35   #196
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Re: Single CTS or follow the Courseline?

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Compute a second CTS?
The whole point of determining a CTS it to get you to your destination on one heading, not recompute a totally different heading when you are not even 2/3 of the way there and nothing has changed with the data.
I was merely drawing attention that in real life nobody really sails off on a multi hour CTS and just prays. You said yourself you'd generally stick to the rhumb line.

These methods have to used in real life , not on graph paper. In a boat you would arrive at D , either transist your way in , or recompute a new CTS , now knowing the real tide under teh boat.

These paper method arguments are amusing but have little real application in the real world. Other then X-Channel , were errors cancel out, and as you said yourself the difference in teh methods is minimal , nobody calculates ( sorry sails) complex tidal courses over many hours.

In fact Bowditch et al, explicity dont even mention the methods WHY, becuase the dont really work in the main, What Bowditch says is work up EPs, assign them error circles and re-compoute as you go, In reality that what everyone does.

The real errors come from the real experience, not the paper ones.
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Old 06-02-2013, 13:39   #197
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Re: Single CTS or follow the Courseline?

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Originally Posted by goboatingnow View Post
Really in most cases a helms can keep +-5 degrees often +-10 ( and the errors dont cancel)
The autohelm can (and does) do much better than that .

OK ! I am not a purist and helming manually very moment of each passage, but how many cruisers still are?
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Old 06-02-2013, 13:40   #198
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Re: Plotting the expected ground track

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Originally Posted by jackdale View Post
Two things

1) If you are using 5 minutes currents you are using a level of "prediction" that is not used by hydrographic services, I expect that you are using algorithms produced by that are not the same as those used by hydrographic services. One of the charting programs I used provided the references ports that they used to compute currents and it had no bearing those used by CHS. X-tides used algorithms provided through crowd sourcing.

2) If you are using 5 minute tides, I expect that you are using an electronic source. In which case, why not just use advanced electronic navigation and forget the vectoring process.
The UK Hydrographic Office publishes 5 minute tides at least for Europe, and maybe for the world.

Of course the tides are derived from a mathematical model, but it is an exceptionally precise model, backed up with measurements, carried out by an organization, founded in 1795, with 1000 employees. I suppose it is accurate to some tenths of a knot. It is light years away from crowd sourcing by Internet entrepreneurs.

Advanced electronic navigation will not solve this problem -- hence this discussion. Although there are programs like Neptune which will (with the help of high resolution tide data).
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Old 06-02-2013, 13:41   #199
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Re: Single CTS or follow the Courseline?

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One big advantage of the RYA method over what? I didn't quite understand. I don't see how that can be an advantage of SWL, which is also an estimate, just with much less inherent error.
Over the SWL.

You argument is only one that an economist would make.

Old Arabic expression "He who foretells the future lies, even if he tells the truth."

Both methods are riddles with parameters and assumptions that likely will not pan out. SWL keeps assuming no leeway, which I do not think is valid. I would also gather that an accurate deviation card is available - how many of those have you seen.

Both will give a good estimate. I will apply Occam's razor.
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Old 06-02-2013, 13:41   #200
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Re: Single CTS or follow the Courseline?

e
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Originally Posted by Seaworthy Lass View Post
The autohelm can (and does) do much better than that .

OK ! I am not a purist and helming manually very moment of each passage, but how many cruisers still are?
In your spare time in any sort of reasonable sea, over several journeys have a look at autopilot accuracy. In flat calm it might be good, in any seaway its sometimes very very poor, often way worse then a person ( who can see ahead and predict).

Anyway Ive never heard of sometime doing a long multi hour ( say 12 hours) CTs pasasge where they just stick to the CTS and didnt have to or felt it necessary to adjust the CTS, ( leaving aside X-Channel). Neither method is usefull. Thats what I said way way back in my orginal posts to Dockhead

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Old 06-02-2013, 13:57   #201
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RYA accuracy

Dave, let me remind you of a few things you have stated just recently regarding the accuracy of the RYA method for determining the CTS if the data is correct:

Quote:
Originally Posted by goboatingnow View Post
Using the vector addition , provides you with a CTS directly to the destination, you DO NOT arrive outside or near the destination , as far as the maths go you arrive directly at the destination without changing course.
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No again you are confused.
.....
However once you exact ( ie whatever data you decide to apply ) the tidal vectors the RYA method is mathematically correct

And if you want me to question THE WORLD greatest training organisation , that certifies sailors in 6 countries and all over the globe , you've another thing coming. The method is right mathematically.
Dave
Right up until today you have kept insisting that following the CTS determined by the RYA method will get you to the destination B (not to D) and that if the data is correct the RYA method is a mathematically precise way of getting to B.

I can find lots more quotes from you regarding this.

I am very glad you have now changed your mind and admit that the RYA method takes you to D (an arbitrary point on the rhumb line which may be several nm from B).
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Old 06-02-2013, 14:55   #202
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Re: RYA accuracy

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Originally Posted by Seaworthy Lass View Post

I am very glad you have now changed your mind and admit that the RYA method takes you to D (an arbitrary point on the rhumb line which may be several nm from B).
D is not arbitrary. It is a point along the CMG line that is length of the boat speed X the hours of calculation. On a multiple current scenario the furthest it will be from the destination is one hour or travel time.
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Old 06-02-2013, 16:57   #203
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Re: RYA accuracy

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Originally Posted by jackdale View Post
D is not arbitrary. It is a point along the CMG line that is length of the boat speed X the hours of calculation. On a multiple current scenario the furthest it will be from the destination is one hour or travel time.
EDIT the course / speed line which ends at D starts at the end of the set / drift line
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Old 06-02-2013, 21:48   #204
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Re: Single CTS or follow the Courseline?

The Vendee Globe tracking page uses "Speed" for boat speed and "VMG" for velocity towards the next waypoint. So because they are ending up at exactly the same place as they are starting from, even if they are sailing away from the finish line, VMG is pointed towards the next waypoint. For instance a boat may be making a Speed of 15 kn. with a VMG of 12.

I think that VMG is the most accepted modern term that eliminates confusion over SMG/SOG.

Vendée Globe 2012-2013 - Tracking

Take a look at the first 2 boats and compare their Speed and VMG with the 3rd and 4th boats.
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Old 06-02-2013, 22:50   #205
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Re: Single CTS or follow the Courseline?

DF

SOG, SMG and VMG are three different concepts.

The Vendee Globe is a good example.

VMG will never/seldom exceed SOG.

SOG is momentary. SMG is either before the fact or after the fact. VMG is used to calculate ETA based on the the current (as in time) SOG and the COG relative to a waypoint.

SOG, SMG and VMG will coincide if the vessel is making a bee line for the waypoint.

I am not sure what the VG is using for speed (GPS or knotmeter)
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Old 06-02-2013, 23:30   #206
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Re: Single CTS or follow the Courseline?

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I am not sure what the VG is using for speed (GPS or knotmeter)
I am not sure of that either but I had just assumed GPS.

They use Speed, VMG and Distance calculated for 1 hr. and 24 hr. periods so these must be reported by the boats when they poll each boat.

Basically they use Distance instead of SMG (which is after all, distance).
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Old 07-02-2013, 00:57   #207
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Re: Single CTS or follow the Courseline?

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Originally Posted by jackdale View Post
DF

SOG, SMG and VMG are three different concepts.

The Vendee Globe is a good example.

VMG will never/seldom exceed SOG.

SOG is momentary. SMG is either before the fact or after the fact. VMG is used to calculate ETA based on the the current (as in time) SOG and the COG relative to a waypoint.

SOG, SMG and VMG will coincide if the vessel is making a bee line for the waypoint.
. . .
VMG can never exceed SOG. It can equal it if and only if COG exactly equals bearing to waypoint.

I still don't know the difference between VMG and SMG -- speed and velocity are the same? Surely it's the same thing, and can be used in different ways? VMG, as someone said, was invented by electronics makers (like a lot of our nav concepts -- e.g. "ground wind"), and is ordinarily used in reference to wind -- VMG to windward (our wind instruments show that, with the ". . . to windward" bit merely implied). But I don't know why we can't use it in reference to anything: VMG to windward, VMG towards waypoint, VMG towards the North Pole . . . (which would be the same as "northing", then). "Made good" simply means recalculated with respect to a different reference. And why couldn't we use SMG in the same way, and since someone somewhere has been using SMG in reference to a waypoint -- why not? Synonym of speed of advance?


"SOG is momentary"

-- meaning, it would be complex to calculate an average SOG over any given passage, as we would have to have exact data about the ground track.


"SMG is either before the fact or after the fact. VMG is used to calculate ETA based on the the current (as in time) SOG and the COG relative to a waypoint."


This: http://www.navcen.uscg.gov/pdf/loran/handbook/APP-C.pdf

defines VMG like this:

"Velocity Along Route (VAR). Alternate name for velocity made good."

and

"Velocity Made Good (VMG).
Component of vessels ground speed in the direction of the waypoint in use. In general, VMG is less than or equal to the vessels ground speed. This will equal the ground speed, in the absence of current, whenever the vessel is on course and heading directly toward the waypoint."

I don't know why they say "in the absence of a current" -- I think the writers had a little brain f*art. Current won't make any difference to the relationship between VMG and "vessel's ground speed" (SOG).

other definitions from there:

"Speed Made Good (SMG).
Indicates the overall speed actually accomplished relative to the ground along the course line."

"Speed of Advance (SOA).
Indicates the speed intended to be made relative to the ground along the track line."

In these definitions (and I think altogether), SMG, SOA, and VMG are all exactly the same concepts. They are using SMG retrospectively (. . . "actually accomplished". . . ), SOA prospectively (. . . "intended to be made". . .), and VMG instantaneously, but it's still all exactly the same concept. So if you plan SOA of 5 knots, and your passage goes exactly to plan, you will have SMG of 5 knots, and your average VMG will have been 5 knots.
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Old 07-02-2013, 03:26   #208
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Re: Single CTS or follow the Courseline?

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Right up until today you have kept insisting that following the CTS determined by the RYA method will get you to the destination B (not to D) and that if the data is correct the RYA method is a mathematically precise way of getting to B.

I can find lots more quotes from you regarding this.

I am very glad you have now changed your mind and admit that the RYA method takes you to D (an arbitrary point on the rhumb line which may be several nm from B).

Sheesh, In the RYA method D is at B if you continue on the CTS for either less time ( D being behind B) or more time D being in front of B. IN all cases the method says follow the CTS derived and sail for the appropriate time.

I have not changed my view, the RYA CTS methods "claims" by following the CTS so computed you will arrive at the destination.

All you are arguing with me about is the "Error" and its a theoretical error at that and As I have repeatedly said from the very first posts two threads ago, multi hour theoretical SINGLE CTS calculations have virtually no real life use and are not used that way with the exception of some poster child applications. All sailors in real life recompute as they go , based on the actual conditions experienced. Hence the theory models converge anyway,

You seem obsessed with "catching me out" rather and going on about theoretical models, rather then dealing with the actual way these courses are really sailed.

You said it yourself, You would not depart too much from the rhumb line, in that you agree exactly with what I said at the outset of this debate, before all the mathematcians tried to tell a person with 60,000 sea miles how to cross a couple of tides!

( I am reminded of the bumblee bee paradox)
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Old 07-02-2013, 03:36   #209
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Re: Single CTS or follow the Courseline?

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I still don't know the difference between VMG and SMG -- speed and velocity are the same? Surely it's the same thing, and can be used in different ways? VMG, as someone said, was invented by electronics makers (like a lot of our nav concepts -- e.g. "ground wind"), and is ordinarily used in reference to wind -- VMG to windward (our wind instruments show that, with the ". . . to windward" bit merely implied). But I don't know why we can't use it in reference to anything: VMG to windward, VMG towards waypoint, VMG towards the North Pole . . . (which would be the same as "northing", then). "Made good" simply means recalculated with respect to a different reference. And why couldn't we use SMG in the same way, and since someone somewhere has been using SMG in reference to a waypoint -- why not? Synonym of speed of advance?

Dockhead , as I see it

SMG ( like all made good), is the average speed over the ground that may theorectically be experienced , or has actually been back calculated by using the distance travelled over time. IN practice today we use SOG or more correctly average SOG.

SOA, the speed that we are closing the waypoint, its the scalar part of VMG


SOG today is in effect the GPS groundspeed.


VMG, is a vector , so it is the direction and SOA to a designated waypoint. imagine an arrow indicator pointing at the next waypoint and showing a speed underneath that many gauges do this , Confusingly many display VMG of the wind angle rather then the destination , ( which happened because proper VMG needs to know the waypoint) SO many racers talk about VMG as a function of sailing angle ( i.e. the source of the wind is the VMG waypoint)

So while SMG represents what you actually achieved, back calculated, VMG is in effect an instantaneous indication of SOA( to a virtual waypoint ) and not SMG
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Old 07-02-2013, 04:26   #210
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Re: Single CTS or follow the Courseline?

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Sheesh, In the RYA method D is at B if you continue on the CTS for either less time ( D being behind B) or more time D being in front of B. IN all cases the method says follow the CTS derived and sail for the appropriate time.

I have not changed my view, the RYA CTS methods "claims" by following the CTS so computed you will arrive at the destination.
Have a look at my example above. If you follow the CTS computed using the RYA method (53 degrees true in this case) you will NOT ever arrive at B. You arrive at D, 2.5 nm short of your destination. And if you continue at 53 degrees you will NEVER reach B.

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All you are arguing with me about is the "Error" and its a theoretical error at that and As I have repeatedly said from the very first posts two threads ago, multi hour theoretical SINGLE CTS calculations have virtually no real life use and are not used that way with the exception of some poster child applications. All sailors in real life recompute as they go , based on the actual conditions experienced.
If they have "no real life use" why are they included in the RYA Navigation Handbook? And why are the calculations taught on RYA courses and examples worked through in examinations and why are students told they will fail if they don't use a sharp enough pencil as that will affect their accuracy? .

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You said it yourself, You would not depart too much from the rhumb line, in that you agree exactly with what I said at the outset of this debate, before all the mathematcians tried to tell a person with 60,000 sea miles how to cross a couple of tides!
I said that if the average cross current was negligible compared to boat speed (in Jackdale's English Channel example the average current was only 0.2 knots for a predicted boat speed of 6 knots) then I would personally prefer to crab and follow the rhumb line. If the boat speed was expected to be lower or the current greater, I most certainly would compute a CTS.

Regardless of the perfection of the data, we need a reasonably accurate method for determining this, not one that that may leave us several nm short of the destination as the RYA method does, without any regard for the current on arrival at that point.

PS I am not a mathematician and I have more than a few sea miles behind me as well .
And I am labouring the point as you are an RYA instructor!
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