

19012013, 01:57

#586

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Quote:
Originally Posted by Seaworthy Lass
It is NOT always correct, it is only correct if the tide in the last portion of the journey is EXACTLY the average in speed and direction to the tide for the previous portion of the journey. Why on earth would it be?
I most certainly am not LOL. I am not doing anything at all retrospectively. I am just taking into account the predicted tide for journey in the 5th hour, not some extrapolated amount that you (and the RYA) for some reason want to do.
I do not. You are the one mixing it up. Please go and watch the link to the second video that you posted .

Ask yourself this how do you know there is a 5th hour ! , you are getting the answer and computing the question
( like Captforce example)
Dave
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19012013, 02:04

#587

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Re: Distinct Activities: Shackled by a Common Name?
Quote:
Originally Posted by goboatingnow
Ask yourself this how do you know there is a 5th hour ! , you are getting the answer and computing the question
( like Captforce example)
Dave

Dave, I can't believe you are asking this!!!!!!!
In the hovercraft example you have a 16 mile passage and your speed is 4 miles/hour. You have a crosswind affect your journey, all from one direction. You have said in this example someone was able to predict the exact crosswind in advance (this is the same as looking up tide details before you start).
As you have any total crosswind/current affecting the journey of COURSE the journey will go into the fifth hour (and may have gone into the sixth has the wind/current been stronger).
So you need to look at the area where you think you will be in the fifth hour (close to the destination in this case) and find out what the current will be for the fifth hour. It is no more guesswork than estimating the current for the first 4 hours of the journey was.
You of course dont know how long this last bit of current is going to affect you, but the 'modified course to steer' technique I presented very elegantly (if I can say so myself) takes care of that.
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19012013, 02:19

#588

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Re: Distinct Activities: Shackled by a Common Name?
Quote:
Originally Posted by goboatingnow
Yes the RYA method applies the the Rate of known tides over the course of the actual journey. You cannot of course find all the tides in advance , because you cannot know in advance how long the journey will take. You can only estimate the tides

Of course data is available for the tides in advance. If is available for the first four hours why is it not available for the fifth?
You do of course need to estimate your position roughly, but you needed to do that for the first 4 hours anyway. Why can't you do this for the 5th?
Quote:
Originally Posted by goboatingnow
Once you have lifted whatever tidal information you determine in advance , the vector plot is a plot in TIME , not distance. ( because you only know distance by hour not in total.

Huh?
Quote:
Originally Posted by goboatingnow
Your idea is in effect back computing tides vectors based on completing the journey then re examining the journey to get more tidal vectors. Ie a recursive and hopefully converge series. In practice you can't do that as you can't determine it with any accuracy in any sort of complex case.
Dave

Read my method please. I keep adding current displacements for hour lots until the boat distance vector (= boat speed by number of hours of current you are applying) can be arced off to extend past B. It is critical to keep adding on current until this occurs, as otherwise you are hitting the rhumb line before the destination point, not at it.
Using this method if you have had any current in this last hour you draw a line from A to D, if no current in the last hour simply draw a line from A to B. This line accurately gives you your course to steer.
To work out the time taken draw a line parallel to this going through B. the length of this line is the distance the boat has travelled. The time taken is that divided by the speed of the boat. The point that this line intersects the last current displacement vector can now be labelled C.
No strange fudging needs to be done as you think it seems to. It is a very precise method if the tide values are correct (and we are always limited by that, that is irrelevant at the moment, we are discussing the accuracy of the method if the data is correct).
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19012013, 02:19

#589

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Join Date: Jun 2009
Posts: 13,649

Really Seaworthy if you worked through a real example using a real chart and chart tidal diamonds you would see what you are suggesting is nonsense.
We have skipped over the techniques to estimate tidal vectors , this is actually fairly complex
However given what we know when we extract these vectors , that's all we can do. However the RYA maths are correct yikes are wrong , the graph is a plot in TIME ( which is distance per unit time ) you are plotting speed vectors. Of the vertices of D do not coincide with the destination. You inflate the triangle ( or deflate ) ie you proportionally shrink or grow it so that the vertice touchs the destination,
the CTS remain the same , the proportions of the triangle remain the same .
If D is in front of the destination , you are travelling slower then your ground speed. Inflate the triangle at the rate sOG If its behind deflate it in proportion to SOG ( which is time from the A D line )
You keep fitting the question into the answer.
If you want to examine real world issues we can progress to that , but for a given set of tidal vectors the RYA vector addition is correct. , you are mixing time and distance. Yes you get a triangle , yes you get maths but its not the correct application of the information you have to hand
Perhaps you could review how tidal vectors are arrived at. Ie how they are deduced from charts.
The RYA method of deducing the tidal vectors suffers all the limitations of tidal information, but the subsequent maths are correct
Here's a simple example. The wind is blow from the south on the hovercraft at 2miles an hour ( that's all you know ) your destination is east , unknown , you can see it " over there " due east using your compass. , draw a CTS and tell me the rate you approach the other side. When you have determined the hourly advance ( ie the SOG ) , tell me how far away you are at say 1:30 after departure. Draw the appreciate vector triangle.
Compare that triangle with yours.
Dave
Ps I'm struggling through the snow to get graph paper !
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19012013, 02:25

#590

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Join Date: Jun 2009
Posts: 13,649

Quote:
Originally Posted by Seaworthy Lass
Dave, I can't believe you are asking this!!!!!!!
In the hovercraft example you have a 16 mile passage and your speed is 4 miles/hour. You have a crosswind affect your journey, all from one direction. You have said in this example someone was able to predict the exact crosswind in advance (this is the same as looking up tide details before you start).
As you have any total crosswind/current affecting the journey of COURSE the journey will go into the fifth hour (and may have gone into the sixth has the wind/current been stronger).
So you need to look at the area where you think you will be in the fifth hour (close to the destination in this case) and find out what the current will be for the fifth hour. It is no more guesswork than estimating the current for the first 4 hours of the journey was.
You of course dont know how long this last bit of current is going to affect you, but the 'modified course to steer' technique I presented very elegantly (if I can say so myself) takes care of that.

Sorry " predict the exact crosswind " then is my bad English. ( Note i never said predict ) no one can predict the exact crosswinds or tides. , because you cannot compute in advance where you will be without knowing in advance what will affect you , which requires etc what I presented was that if the journey took four hours then these are the winds that you would experience ( or course the journey doesn't take 4 hours , so,in the absence of any other info , just use that data )
Take the example I have for the hovercraft , the wind is blowing at X mph from the south , you are traveling east, you know the hovercraft can do Y mph , you wish to travel due east. , compute CTS , compute rate of advance.
This is what really going on in chart tides. You cannot know what effects you in advance , because you cannot determine in advance where you will be.( or at what time ) , this is the RyA method ( and everyone else's. though its ignored in US text books )
Dave
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19012013, 02:32

#591

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Join Date: Jun 2009
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The RYA method is a graphical method to determine course made good , most text books are virtually all US texts books including Bowditch do not do anything other then determining course to steer over one hour. ( which is the RyA one hour method )
My triangle plotted in hourly rates appear in 3 versions of Bowditch from 1925 on , I have three , Tom cunliffes, us texts, etc etc
Dave
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19012013, 02:42

#592

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Join Date: Jun 2009
Posts: 13,649

Quote:
Originally Posted by Seaworthy Lass
Of course data is available for the tides in advance. If is available for the first four hours why is it not available for the fifth?
You do of course need to estimate your position roughly, but you needed to do that for the first 4 hours anyway. Why can't you do this for the 5th?
Huh?
Read my method please. I keep adding current displacements for hour lots until the boat distance vector (= boat speed by number of hours of current you are applying) can be arced off to extend past B. It is critical to keep adding on current until this occurs, as otherwise you are hitting the rhumb line before the destination point, not at it.
Using this method if you have had any current in this last hour you draw a line from A to D, if no current in the last hour simply draw a line from A to B. This line accurately gives you your course to steer.
To work out the time taken draw a line parallel to this going through B. the length of this line is the distance the boat has travelled. The time taken is that divided by the speed of the boat. The point that this line intersects the last current displacement vector can now be labelled C.
No strange fudging needs to be done as you think it seems to. It is a very precise method if the tide values are correct (and we are always limited by that, that is irrelevant at the moment, we are discussing the accuracy of the method if the data is correct).

You cannot reiteratively compute time based on the incorrect application of rate vectors. Tides are time based, as you journey gets longer because of the effect of preceding tides , you have to then go back and increase the preceding tidal vectors thereby changing your solution
What you are suggesting is that irrespective of tide the boat only spends one hour in its influence , irrespective of the speed of the boat. Draw a simple one hour triangle at a given boat speed, draw another for the same tide at an other speed. They are not multiples of one another
Similarly. When you compute the journey time due to the first say 4 vectors. , you cannot state that the remaining time is subject to some other vector that you now go looking for. You spend the time proportionately being affected by all the vectors. ( since they are specified in time ( ie say mph )
Dave
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19012013, 02:45

#593

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Join Date: Jun 2009
Posts: 13,649

Quote:
Originally Posted by Seaworthy Lass
Of course data is available for the tides in advance. If is available for the first four hours why is it not available for the fifth?
You do of course need to estimate your position roughly, but you needed to do that for the first 4 hours anyway. Why can't you do this for the 5th?
Huh?
Read my method please. I keep adding current displacements for hour lots until the boat distance vector (= boat speed by number of hours of current you are applying) can be arced off to extend past B. It is critical to keep adding on current until this occurs, as otherwise you are hitting the rhumb line before the destination point, not at it.
Using this method if you have had any current in this last hour you draw a line from A to D, if no current in the last hour simply draw a line from A to B. This line accurately gives you your course to steer.
To work out the time taken draw a line parallel to this going through B. the length of this line is the distance the boat has travelled. The time taken is that divided by the speed of the boat. The point that this line intersects the last current displacement vector can now be labelled C.
No strange fudging needs to be done as you think it seems to. It is a very precise method if the tide values are correct (and we are always limited by that, that is irrelevant at the moment, we are discussing the accuracy of the method if the data is correct).

So what you are saying is that if you have slack tide at the end , then a single CTS is not right. !
Your method requires you to estimate your position ? , how can you do that in advance in a complex situation , you mean I have to determine ground track ?
You are clearly computing the question to suit the answer. , you are in effect determining tides based the journey time, which cannot be computed on till you know all the tides , which then affect the journey time etc etc.
Again to illustrate graph a hovercraft destination due east , speed 5mph, wind from the south 2kn, graph the triangles as you progress east , show me the triangle at say 1:30, 1:45, 2 hours
When you've done that I will tell you a distance to the destination. Use any one of the triangles to tell me the passage time.
This is in essence a one hour tidal plot.
If anyone draws it tell will me the method is correct for the information we can determine in advance
We can then put that aside and look at the inherent errors in determining the tidal vectors in advance.
Your way off hear.
Dave
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19012013, 02:45

#594

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Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,937

Re: Distinct Activities: Shackled by a Common Name?
Quote:
Originally Posted by goboatingnow
The RYA method is a graphical method to determine course made good , most text books are virtually all US texts books including Bowditch do not do anything other then determining course to steer over one hour. ( which is the RyA one hour method )
My triangle plotted in hourly rates appear in 3 versions of Bowditch from 1925 on , I have three , Tom cunliffes, us texts, etc etc
Dave

Bowditch needs to to be revised. They do this regularly. Do you think every scrap of information in there is the same as the original version Bowditch wrote?
My method is simply more accurate.
In the hovercraft example if the genius who you said predicted wind so accurately in advance for four hour did so for five instead and announced that it would be zero, your method of course to steer would have you going off 7 degrees from the correct angle to steer and you would never arrive at your destination. My method has you steering straight for B and I would arrive there.
If you present any challenge I am happy to draw it up on a chart for you. I will then compare the answer for the two techniques. As long as you don't make the current in the final hour the average of the current in the previous hours, I will have an accurate course to steer to get you to B. The RYA method will not.
Gauntlet thrown down.
I am about to head ashore so will be offline until this evening. You have several hours to prepare an example and give me the figures for boat speed, the distance and direction from A to B and plenty of tide data. I will look at it tonight.
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19012013, 02:48

#595

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Re: Distinct Activities: Shackled by a Common Name?
Quote:
Originally Posted by goboatingnow
So what you are saying is that if you have slack tide at the end , then a single CTS is not right. !
Dave

I am saying no such thing. I am saying if you have slack tide at the end you join the end of the final current displacement vector directly to B, instead of D, where D is a point BEYOND B (the first time you could have put D behind B).
This line gives you the course to steer and you arrive at B.
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19012013, 02:54

#596

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Boat: Aluminium cutter rigged sloop
Posts: 12,937

Re: Distinct Activities: Shackled by a Common Name?
Quote:
Originally Posted by goboatingnow
Sorry " predict the exact crosswind " then is my bad English. ( Note i never said predict ) no one can predict the exact crosswinds or tides. , because you cannot compute in advance where you will be without knowing in advance what will affect you , which requires etc what I presented was that if the journey took four hours then these are the winds that you would experience ( or course the journey doesn't take 4 hours , so,in the absence of any other info , just use that data )
Dave

Please don't be pedantic. Your exact words were:
"You know, because some nerd has worked out the winds speeds and direction across the 16 mile lake. He tells you that the winds are all blow North to south as follows as you know from experience that it has this effect on your hovercraft
first hour 3 miles/hour ( ie it will blow your hovercraft 3 miles south in an hour if it was just sitting there)
2nd hour 2 miles/hour
3rd hour 4 miles/hour
4th hour 1 mile/hour"
So why can't the nerd in this example tell me what the wind is for the 5th hour?
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19012013, 02:56

#597

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Re: Distinct Activities: Shackled by a Common Name?
Folks, I have five minutes to go and make myself look presentable before we head ashore. I need every moment of that five minutes .
Will be back online tonight.
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19012013, 03:11

#598

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Join Date: Nov 2007
Location: Probably in an anchorage or a boatyard..
Boat: Ebbtide 33' steel cutter
Posts: 3,858

Re: Distinct Activities: Shackled by a Common Name?
Moving forward with Desmos graph app in chrome..
Still a way to go with sliders for easy manipulation of the data but moving in the right direction
From Capt example but with a single tide max flow of 4Kn, as a sine wave.
https://www.desmos.com/calculator/6quwldikyl
The curve from 0  8 is (hopefully) total distance the tide has moved (north in this example) calced by Y=(cos(x)+1)*4. Suspect this might be wrong though, the "cos(x)" is straight out of google but can't see why I had to add 1 to get it all positive. Looks about right though.
The next curve from 0  4 back to 0 should be the tidal rate. y=sin(x) x 4
Straight line should be the boat heading through the water.
And the final line is the tidal distance and boat track through water added together, ie boat track over the ground.
If you are using chrome the the graph app is a simple add on 
https://chrome.google.com/webstore/d...ojekgdko?hl=en
EDIT. I've just divided the Y axis by 5 to make the whole thing look a bit better. Dunno how to zoom just one axis yet.



19012013, 03:13

#599

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Join Date: Jun 2009
Posts: 13,649

Quote:
Originally Posted by Seaworthy Lass
Bowditch needs to to be revised. They do this regularly. Do you think every scrap of information in there is the same as the original version Bowditch wrote?
My method is simply more accurate.
In the hovercraft example if the genius who you said predicted wind so accurately in advance for four hour did so for five instead and announced that it would be zero, your method of course to steer would have you going off 7 degrees from the correct angle to steer and you would never arrive at your destination. My method has you steering straight for B and I would arrive there.
If you present any challenge I am happy to draw it up on a chart for you. I will then compare the answer for the two techniques. As long as you don't make the current in the final hour the average of the current in the previous hours, I will have an accurate course to steer to get you to B. The RYA method will not.
Gauntlet thrown down.
I am about to head ashore so will be offline until this evening. You have several hours to prepare an example and give me the figures for boat speed, the distance and direction from A to B and plenty of tide data. I will look at it tonight.

Seaworthy. When I said the winds were estimated to be x,x y , that means for the rhumb line journey that's what you would experience if you where " flying across" not affected by those winds.
I'm not giving you the winds ( or its affect) base on what actually happens since I cannot know in advance what you journey time will be.
I will do some graphs on a proper chart to,illustrate the method ,
The RYA method progressively inflates all known vectors ( determined at the start ) equally as time progresses if you don't accept that then we are comparing apples and oranges.
Remember in the vast majority of cases you cannot determine tides in positions , tidal data is presented as an average over quite huge areas of sea ( often 100s of squares miles )
All you can you is proportionally increase all tides you can determine in advance due to the extended time spent in them.
Loads of graphs to follow if I can get to the shop in this snow !!
Enjoy ashore I love Greece
Dave
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19012013, 03:18

#600

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Join Date: Mar 2009
Location: The boat: Cowes (Winter), Above 60N (Summer); me: somewhere in the air!
Boat: CutterRigged Moody 54
Posts: 21,291

Re: Distinct Activities: Shackled by a Common Name?
Quote:
Originally Posted by Andrew Troup
Dockhead
Maybe you thought I used the CAD package to solve the triangles. I didn't, Excel did all that, with a bit of coaxing, and gave me the sideways displacement from the rhumbline at each constant heading slice.
So the CAD package just fitted a curve through the xy coordinates I extracted from the spreadsheet.
I might be wrong, but I don't see any sideways displacement from the rhumbline in your spreadsheet, so I'm not sure what you would like me to plot ?

Oh, so you just used it to graph the data . Excel can do that too.
Yes, now that you have prodded me, I see that I could easily get the sideways displacement out of Excel, too. I guess one could plot the whole thing in Excel. Just need to get it to spit out the coordinates at the end of each slice. Again, your distanceslicing makes this vastly simpler . All you need to calculate is one axis
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