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Old 20-02-2013, 13:29   #796
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by jackdale View Post
Waiting is always an option.
Jackdale, if this computation is a recalculation for the last part of a long journey while underway, waiting is NOT an option.
Postion A can be in open water.

Quote:
Originally Posted by jackdale View Post
The example is too extreme and unrealistic
I do not believe that an average current of 3 knots for one hour, followed by one of 1.4 knots for one hour is extreme or unrealistic. the boat speed may be lower than average, but all sorts of situations crop up on the water, eg equipment failure along the way and fuel problems (eg diesel bug blocking the last spare filters along the way).

In all the examples you have presented, little time would have been saved by following a single CTS (crabbing would have been almost as quick and certainly safer given that students are not taught to plot their expected ground track beforehand).

The whole value of having a single CTS is either for long journeys (where you can simply join the tip of the last current vector directly to the destination without using any method) or for situations where the average current is significant compared to boat speed (and these may well be emergency situations where a reliable technique for determining a CTS would be extremely useful).

What is the point of teaching a complex CTS computation for variable cross current if your method only covers you for situations where crabbing would have been preferable?
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Old 20-02-2013, 13:31   #797
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by HappySeagull View Post
no, it's there, but the mark isn't showing .Check the proportions of current vectors- both are equal.
scales are not same though.these appear as one picture but there are two. linked from up-thread.
Ah, hidden marks and different scales! You need to make things a little easier for weary lassies .
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Old 20-02-2013, 13:35   #798
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

here's a better example of the different idea to find last hour proportion
The problem is the same as I repostedpictures above.


per drift= 264 @.8miles,266@1.3 mi, 254 @2.0 mi
A->B =s 337@11 mi
BoatSpeed=4 knots

CTS found is 356@10.7 miles
solid reds are proportion last hour current found
and therefore CTS found
red dotted's are vectors to prove track and "arrival"

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this is my parallelogram phase!
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Old 20-02-2013, 13:43   #799
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by Seaworthy Lass View Post
Ah, hidden marks and different scales! You need to make things a little easier for weary lassies .
sorry,SWL. sometimes when I label the points, they overwrite each other and are unreadable, so I turn the name off.

ADDED the names of drifts might be wrong per the original example too. aargh. the originals are on a scratchpad in the weed somewhere....
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Old 20-02-2013, 13:52   #800
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by HappySeagull View Post
here's one I made up this morning. There are more, I guess, but it takes awhile to figure out how to draw them. This solves the last problem, in case it's not plain. A bit of a toughie to line up in the O routing sketcher but on paper, I think should be practical.

Not hanging my gonads on it so fire away.

Attachment 55298
Happy, your artistry is remarkable, but I need a little help interpreting this example. I see an arrival point in the centre of the diagram (or is that the departure point?). I can't see any vectors lined up in the same direction indicating boat displacement following a single CTS. Are the green lines the current?
I know true art should not be dissected, but please lend a wing here.

PS Thankfully your gonads were not at stake. Castrated seagulls (not that I am into violence, but some other CF members do seem to have a tendency towards this) are no longer very happy ones
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Old 20-02-2013, 14:04   #801
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by HappySeagull View Post
here's one I made up this morning. There are more, I guess, but it takes awhile to figure out how to draw them. This solves the last problem, in case it's not plain. A bit of a toughie to line up in the O routing sketcher but on paper, I think should be practical.

Not hanging my gonads on it so fire away.

Attachment 55298
Happy, your artistry is remarkable, but I need a little help interpreting this example. I see an arrival point in the centre of the diagram (or is that the departure point?). I can't see any vectors lined up in the same direction indicating boat displacement following a single CTS. Are the green lines the current?
I know true art should not be dissected, but please lend a wing here.

PS Thankfully your gonads were not at stake. Castrated seagulls (not that I am into violence, but other CF members do seem to have a tendency towards this) are no longer very happy ones
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Old 20-02-2013, 14:12   #802
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

oh, that's a bear, SWL . ..Flag is destination.
...The idea is really from the Hapless model...Resolve each drift with boat speed per hour(green dots)...sort of rhumbline crabbing. It overshoots destination of course.

finding how much it overshoots is "simply" a matter of drawing the last current vector from the destination flag and using that dimension to discover the last portion of the last current vector needed.

It's MUCH plainer in the later example (above), because I marked the proportion needed in solid red. and carried it down to find the place to start CTS twards destination.

In effect, I chopped off the portion of triangle I needed to arrive at destination via Hapless method....all the triangles combined can therefore be resolved as ONE CTS

If wanted it'd be easy to crab along the Rhumb per Hapless and adjust the last vector only.
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Old 20-02-2013, 14:39   #803
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
Jackdale, if this computation is a recalculation for the last part of a long journey while underway, waiting is NOT an option.
Postion A can be in open water.
Waiting is still an option, just as one does not enter an unfamiliar harbour in the dark.

If I had a boat capable of only 2 knots in those conditions, I would probably have problems entering an anchorage or a harbour.

I might be tempted to issue a Pan Pan

Quote:



I do not believe that an average current of 3 knots for one hour, followed by one of 1.4 knots for one hour is extreme or unrealistic. the boat speed may be lower than average, but all sorts of situations crop up on the water, eg equipment failure along the way and fuel problems (eg diesel bug blocking the last spare filters along the way).
Those currents could be accurate.

Again I am not going to attempt anchoring or docking in those situations. I have had to dock 40 and 50 foot boats under sail - at docks about which I had intimate knowledge.

Quote:
In all the examples you have presented, little time would have been saved by following a single CTS (crabbing would have been almost as quick and certainly safer given that students are not taught to plot their expected ground track beforehand).
When one established a course to steer, that is the heading one gives to the helm. You would be crabbing.

I am confused about what you are arguing.

Quote:

What is the point of teaching a complex CTS computation for variable cross current if your method only covers you for situations where crabbing would have been preferable?
I am not sure what you are asking?
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Old 20-02-2013, 15:08   #804
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Lodesman View Post
I'll give you that your method is relatively easy, but it is certainly not more accurate. You're comparing two different triangles to estimate the partial hour. .
My method involving determining the proportion of the last bit of current applicable by looking at SB/SB+BL is more accurate than determining D1B/D1B+BD2 as the latter will vary depending in the angle of the last bit of current relative to the angle of the rhumb line. My proportion will not change as it does not refer to the rhumb line at all.

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As I said previously, your method has merit where someone regularly needs to calculate a multi-tidal single CTS, but it has little utility beyond that. The basic tidal triangle can be used for all your navigation needs.
Yes, the only purpose of the technique is for calculating a CTS with variable cross current. It gives a dependable method for doing this.
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Old 20-02-2013, 15:30   #805
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by jackdale View Post
Waiting is still an option, just as one does not enter an unfamiliar harbour in the dark.

If I had a boat capable of only 2 knots in those conditions, I would probably have problems entering an anchorage or a harbour.

I might be tempted to issue a Pan Pan
Waiting would see you pushed significantly down current (not the best option).
You are arriving in this case with just 1.4 knots of current, not an extreme or dangerous amount. There is no significant current in the shallow waters of the protected anchorage you are heading to and planning to anchor in).
You are not just capable of 2 knots, you may be keeping fuel in reserve for a safe entry, but 2 knots was all you deem safe for the hours beforehand, so you could enter the harbour at a higher speed.
This situation is not some weird extreme one, but the RYA cannot handle the computation at all (it gives a result more than 20 degrees in error).

Quote:
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When one established a course to steer, that is the heading one gives to the helm. You would be crabbing.

I am confused about what you are arguing.
By "crabbing" I mean crabbing along the rhumb line. The RYA method works well for all the situations where simply following the rhumb line would have been a reasonable alternative.

I am just querying the value of a technique to determine a single CTS if it only works consistently well in situations where following the rhumb line would have been nearly as quick, a lot simpler and probably a lot safer. Where is the RYA technique consistently useful?

If the RYA method can't handle having any data thrown at it, why bother teaching it and examining it and using it?
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Old 20-02-2013, 15:53   #806
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
Yes, the only purpose of the technique is for calculating a CTS with variable cross current. It gives a dependable method for doing this.
In "real world" examples your method provides no significant difference, but involves more steps. I see absolutely no advantage.

Victoria to Port Angeles

My Solution
Inaccurate RYA Teaching : CTS - Quest For a New Method

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Inaccurate RYA Teaching : CTS - Quest For a New Method

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Inaccurate RYA Teaching : CTS - Quest For a New Method

I may see advantage if you can post a real world example that shows a sigificant difference. The criteria are:
  • A real passage
  • Real current data based in current tables, current diamonds, electronic data, etc..
  • A boat that will make decent speeds ,e.g., 5 knots is my usual planning speed.

I have downloaded all of the NOAA charts for CG District 1 (NE US). It has lots of possibilities and loads of current data. For example. New Haven, CT to Port Jefferson on Long Island looks interesting.
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Old 20-02-2013, 16:07   #807
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post



By "crabbing" I mean crabbing along the rhumb line. The RYA method works well for all the situations where simply following the rhumb line would have been a reasonable alternative.
In the RYA method, you are crabbing down the rhumb line.

In this example, you vessel is on a heading of 044T to attain a CMG of 023 T. With a CTS of 26 M (with 18 E variation) I would tell the helm to steer 025 C (assuming no deviation).

Your COG/CMG would be 023 T. That is, you are following the rhumb line. Your vessel's centreline is not parallel to the rhumb line, it is parallel to the CTS.

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Old 21-02-2013, 08:32   #808
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

per Jackdales picture above,
say,
Start to Destination (A->B) 23T 11.25 miles
Start time 0900

current from Opencpn table Feb13,2013
0900 1.63 kn 290deg
1000 1.85 kn 290 deg
1100 1.75 kn 290 deg
1200 1.35 kn 290 deg
current direction is all the same but velocities are not. Real life is quite detailed.

from this table, previous slack @ 0619. Max @ 1012 1.86kn (ebb). Next slack 1418.)
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Old 21-02-2013, 08:41   #809
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Old 21-02-2013, 08:46   #810
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
By "crabbing" I mean crabbing along the rhumb line. The RYA method works well for all the situations where simply following the rhumb line would have been a reasonable alternative.

I am just querying the value of a technique to determine a single CTS if it only works consistently well in situations where following the rhumb line would have been nearly as quick, a lot simpler and probably a lot safer. Where is the RYA technique consistently useful?

If the RYA method can't handle having any data thrown at it, why bother teaching it and examining it and using it?
This is an entirely misleading statement, All SWL is doing is apportioning the last tide, SO the only advantage is where such tides are dramtically different that you get into a "dispute " about accuracy.

Furthermore the RYA method can of course be extended to handle any of the circumstances. Then the argument simply boils down to a discussion about the application of such tide data and the assumptions underlying them

I notice SWL wont debate me on this as she has steadfastly ignored the debate on the assumptions underlying her use of tide data.

The RYA method in real life is more then adaquate. You can use it to calculate a 12 hour cross channel trip and it will work out fine

Hence this in particular statement is just pure dis-information

Quote:
I am just querying the value of a technique to determine a single CTS if it only works consistently well in situations where following the rhumb line would have been nearly as quick, a lot simpler and probably a lot safer. Where is the RYA technique consistently useful?
As Ive just said a X-channel crossing is definitely not crabbing along the rhumb line, and teh RYA method works fine.

I find it strange that an obviously intelligent person like SWL, has turned an amusing debate about mathematical computations into a shrill , mis-informed and mis-guided attempt to denigrate a method proven to be useful by thousands, has been demonstrated here and elsewhere to be accurate enough for the task. SWL has no interest in what actually really works, just mis-information about the RYA ( and by extension Bowditch and others) and constantly repeating a tired now old mantra.

Why keep interjecting SWL, what are you seeking to do, Take it up with the RYA, even better have a go on YBW.com , on its forums, loads of RYA instructors there to take pot shots at.


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