Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 26-01-2013, 11:55   #421
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,750
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by TeddyDiver View Post
Sorry Lodesman, my bad . Had a couple of quotes and delated the other one and left wrong name..
I have read and participated in the thread and I still stand behind my comment 'there's no need to triangulate anything' in SWL method nor in my method.
All these methods are vector triangles. You will see if you can manage to express your method in math. You will not be able to do it without trigonometry, "triangle measuring".
__________________

__________________
Dockhead is offline   Reply With Quote
Old 26-01-2013, 12:57   #422
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,815
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Dockhead View Post
It must be said that a crucially valuable role is played by people who stubbornly insist on a wrong idea, especially if they are able to argue their position well. They force the rest of us to blow off the cobwebs and relearn all our stuff in order to prove to them the error of their thinking. Without Capt Force in the other thread, I would not know half as much as I know about CTS passages as I know now.
Most of us have, at one time or another, played both roles
Oh, yes, I have learned heaps over the last week! Particularly, sheepishly, about my own fallibility LOL .

My original faulty thoughts on the pattern of ground track under certain conditions is something I will never forget. Brings to mind some comment about asses .
__________________

__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is offline   Reply With Quote
Old 26-01-2013, 13:06   #423
Moderator
 
Paul Elliott's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 3,877
Images: 4
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Teddydiver, If you're not measuring AB (the rhumbline), then it's because the people who drafted the chart did it for you. Without the properly-scaled chart, you either need to measure it (in order to plot points A and B), or you need to do it mathematically (for which you need the length AB). However you perform the calculation / construction, you are solving a triangle.

If you are merely saying that when using a chart there is no need to draw or measure line AB, then I believe that we all agree with you.
__________________
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
www.sailvalis.com
Paul Elliott is online now   Reply With Quote
Old 26-01-2013, 13:17   #424
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Quote:
Originally Posted by Seaworthy Lass
Example 4
SWL method results:

I.....

The RYA result will either tell you your CTS (true) is 39 degrees (35 degrees off) or it will make you think the journey is not possible. Both these conclusions are incorrect!
Actually I did a quick plot based on all the RYA provisos, on a hacked up piece of paper and a CTS of 5 degrees !

Dave

Ps Seaworthy is not actually following the RTA rules.

(A) D must be within one half tide of B

(B) the RYA method is orientated for tides approx on average 25% of boat speed.


I did it roughly, I used the typical provision that I tell my YM class


The RYA method uses the rhumb line as a convent constructor. , as I repeatedly said D the intersection must be close ( within one half tide ) in all SWL cases she constructs a situation where this isn't the case and blindly applies one part of the method.

The RYA method don't require the water track vector to cut the rhumb. , its just that in real life if does as the tides are a fairly small proportion of boat speeds typically.

Anyway to suggest you should go out into a combined tide of that magnitude with that forward motion would be he height of folly.
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 26-01-2013, 13:20   #425
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,750
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
Oh, yes, I have learned heaps over the last week! Particularly, sheepishly, about my own fallibility LOL .

My original faulty thoughts on the pattern of ground track under certain conditions is something I will never forget. Brings to mind some comment about asses .
One should always be most grateful to those who disagree. If they turn out be right, then they have helped you learn something or overcome some error, and thus given you a big present. If they turn out to be wrong, then they've given you a chance to exercise your brain and have helped you perfect your knowledge. People who agree with you actually give you nothing. And sometimes have caused you harm by seducing you into confirming an erroneous view. I always thought it strange, that people so often thank others for agreeing with them. This wrong-headed, and probably results from insecurity - emotional need to be supported. Actually, all the thanks are owed to those who disagree.
__________________
Dockhead is offline   Reply With Quote
Old 26-01-2013, 13:25   #426
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Quote:
Originally Posted by Seaworthy Lass
Apologies to all.

Getting so tired here.
I was checking Paul Elliott's calculations and found they just didn't make sense. I had made an error in the text. Thanks Paul for pointing it out .

I wrote the text before tackling the 'chart' work for example 4 and for the results to be striking I had to alter the current in the second hour from 1.5 to 2 knots (that little change can mean the difference between the RYA technique giving a good result and an appalling one).

All my diagrams and ground plot were worked out with 2 knots of current, but I didn't alter the text and didn't notice the error . I put it down to exhaustion.

So here is that data again (diagrams unaltered):

Example 4 TAKE 2
RYA method results:

In the last example I was criticised for selecting an unrealistic situation. The comment was that the current was too high (8,6,2 then 2 knots) and this would never be encountered and the only reason the boat could get there was because the tide turned at the end.

So in this example the current is as follows:
Ist hour: 3.5 knots 135 T
2nd hour: 2.0 knots 135 T (was written as 1.5 in error last time)
3rd hour: 0.5 knots 135 T

Boat speed = 1.8 knots
AB = 4.24 nm due east

You have a short journey 4.24 nm due east to get to B, which is a spot you consider safe to be before you proceed into the harbour.

This is an emergency situation and you must get there ASAP. Conditions are calm, there is no wind and the water is flat.
Unfortunately your feathering prop is jammed at the wrong pitch and your maximum boat speed is 1.8 knots (your know this from the last trip out).

Can you get to your destination to deal with this emergency?
It depends on the current for the next few hours (when you check it will actually reverse and act at 315 T, increasing in strength), so I doubt it if the RYA result is used.

What is the CTS?
That is a real dilemma. You arc off your distance after 3 hours and find it it cuts the rhumb line twice, equidistant from B.
Oh! What to do? Is it 39 degrees or is it 321? I guess if I had to pick I would choose 39.

What is the estimated time taken?
Time taken = AD divided by the boat speed (depends on which D you choose)

Anyway, you get totally wacky results!
Of course this isn't the RYA method seaworthy is not applying the rules properly ( the half tide rule )
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 26-01-2013, 13:34   #427
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,750
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by goboatingnow View Post

Anyway to suggest you should go out into a combined tide of that magnitude with that forward motion would be he height of folly.
Sorry, Dave, with all my respect to you, not if you can lay your destination with an accurately computed CTS.

All these scenarios are somewhat artificial, but they are possible, and bring out the flaws in different methods.

RYA works fine if applied with intelligence and appropriate fudges. SWL works better, and requires much less fudging, and requires of the navigator much less intelligence, a commodity in short supply among mankind
__________________
Dockhead is offline   Reply With Quote
Old 26-01-2013, 13:36   #428
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,815
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Dockhead View Post
One should always be most grateful to those who disagree. If they turn out be right, then they have helped you learn something or overcome some error, and thus given you a big present. If they turn out to be wrong, then they've given you a chance to exercise your brain and have helped you perfect your knowledge. People who agree with you actually give you nothing. And sometimes have caused you harm by seducing you into confirming an erroneous view. I always thought it strange, that people so often thank others for agreeing with them. This wrong-headed, and probably results from insecurity - emotional need to be supported. Actually, all the thanks are owed to those who disagree.
They can also cause self inflicted hair loss LOL. I now have several bald patches after some of my hair pulling sessions this last week .

Hope my grows back in very quickly, even seaworthy lasses can be guilty of a little vanity. I look odd enough going ashore here in winter with shorts and bare legs and crocs, sloshing merrily through puddles (why let kids have all the fun?) usually in something pink with my red Musto jacket that has a bright yellow hood. Having bald patches now as well really makes the locals here in Greece look twice and wonder .
__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is offline   Reply With Quote
Old 26-01-2013, 13:43   #429
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Quote:
Originally Posted by Dockhead

Sorry, Dave, with all my respect to you, not if you can lay your destination with an accurately computed CTS.

All these scenarios are somewhat artificial, but they are possible, and bring out the flaws in different methods.

RYA works fine if applied with intelligence and appropriate fudges. SWL works better, and requires much less fudging, and requires of the navigator much less intelligence, a commodity in short supply among mankind
The RYA method Dockhead is designed to model the typical situations one finds under ones keel. ( and I laid out the rules and assumptions it uses in a previous post SWl of course just ignores that and blindly cuts arcs with the SWL RYA method

I did seaworthys example using the RYA inflation method. It's gives a similar answer. You just have to deal with the situation after 2 hours where D is close tomB but not on the rhumb line. This is something the RYA simply doesn't bother teaching , why because only a madman would use a method that gives a CTS of 5 degrees. Any small error in real life would make huge issues at sea.

In class we always get asked about these " contrived" situations. I draw the 1,2,3 hour plots and we discuss what it means if D lies along way away , or if D can't actually cut The rhumb. , remember the boat can find itself anywhere along the point of the dividers depending on the CTS. Hence you can often just eyeball the solution and inflate or deflate the triangle , even if B isn't on the rhumb line.

But yes this bit simply isn't taught, the issues are discussed

Dave
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 26-01-2013, 13:46   #430
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Quote:
Originally Posted by Dockhead

Sorry, Dave, with all my respect to you, not if you can lay your destination with an accurately computed CTS.

All these scenarios are somewhat artificial, but they are possible, and bring out the flaws in different methods.

RYA works fine if applied with intelligence and appropriate fudges. SWL works better, and requires much less fudging, and requires of the navigator much less intelligence, a commodity in short supply among mankind
Seriously Dockhead , I gave you a CTS of 5 said you would experience 5 knots of tide in two hours , and your boat could do 1.8 kn , you'd leave the harbour. Like hell you would.

Leeway alone could make it that you never reached your destination. Add +- 5 in helmsman heading , and you could be out there all day.

Again SWLs method assumed abrupt transistions of time at time boundaries. The reality is of course far different.


The destination is mathematically possible. , but that's not relevant at all really.
Please.


Dave
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 26-01-2013, 13:52   #431
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,750
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by goboatingnow View Post
Seriously Dockhead , I gave you a CTS of 5 said you would experience 5 knots of tide in two hours , and your boat could do 1.8 kn , you'd leave the harbour. Like hell you would.

Leeway alone could make it that you never reached your destination. Add +- 5 in helmsman heading , and you could be out there all day.


The destination is mathematically possible. , but that's not relevant at all really.
Please.


Dave
A proper passage plan, including proper CTS calculation, allows for leeway and wind shifts. Whether you can lay your destination or not is not a question which can be answered with "oh, please"! It requires more work than a momentary emotional outburst. The problem is really a damned good question, worth working up with some thought and effort.
__________________
Dockhead is offline   Reply With Quote
Old 26-01-2013, 13:56   #432
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,750
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by goboatingnow View Post
The RYA method Dockhead is designed to model the typical situations one finds under ones keel. ( and I laid out the rules and assumptions it uses in a previous post SWl of course just ignores that and blindly cuts arcs with the SWL RYA method

I did seaworthys example using the RYA inflation method. It's gives a similar answer. You just have to deal with the situation after 2 hours where D is close tomB but not on the rhumb line. This is something the RYA simply doesn't bother teaching , why because only a madman would use a method that gives a CTS of 5 degrees. Any small error in real life would make huge issues at sea.

In class we always get asked about these " contrived" situations. I draw the 1,2,3 hour plots and we discuss what it means if D lies along way away , or if D can't actually cut The rhumb. , remember the boat can find itself anywhere along the point of the dividers depending on the CTS. Hence you can often just eyeball the solution and inflate or deflate the triangle , even if B isn't on the rhumb line.

But yes this bit simply isn't taught, the issues are discussed

Dave
I agree with most of this, and I already said, that RYA method is perfectly reasonable if applied with taste and intelligence and appropriate fudges. But none of this takes anything away from the fact that SWL method is better.
__________________
Dockhead is offline   Reply With Quote
Old 26-01-2013, 13:57   #433
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Remember. The RYA method takes a boat water track vector and draws an arc. It marks point D which lies closest to the the destination B, it inflates of deflates the whole triangle to make D reach B

The method does not rely on D being on the rhumb line. It's just that with typical figures it does. But the method works just as well even if D is not on the rhumb.

The main thing I want to get across is that you don't get the silly SWL RYA CTS, she claims. You only get that where you blindly apply the vector construction with no idea what you are actually doing.

Dave
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 26-01-2013, 14:00   #434
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,750
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
. . . I look odd enough going ashore here in winter with shorts and bare legs and crocs, sloshing merrily through puddles (why let kids have all the fun?) usually in something pink with my red Musto jacket that has a bright yellow . . ..
This is becoming pornographic, and as a moderator, I must warn you to stop posting this smut. . . . before I start looking up plane tickets to Greece in Travelocity. . . .
__________________
Dockhead is offline   Reply With Quote
Old 26-01-2013, 14:04   #435
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Quote:
Originally Posted by Dockhead

I agree with most of this, and I already said, that RYA method is perfectly reasonable if applied with taste and intelligence and appropriate fudges. But none of this takes anything away from the fact that SWL method is better.
The essence of SWLs method is she applies the proportion to the last tide ( irrespective of her arguments ), the RYA applies the average of they set of closest tides that apply ( whether D is on the rhumb is irrelevant. )

Therefore it boils back to a discussion about the accuracy of tidal time boundaries. I would argue that both SWL and RYAs error circles lie WITHIN the overall passage error circle ( ie an EP work up ). Hence in that regard either method is suitable. aND neither gives more precision .

precision a means relating the model to real life , not comparing each model to each other. In physics you compare theory to practice not theory to theory.

Say I have a theory on Lead acid modelling. , you have one , do we compare models no we not not. We compare to the actual situation, then we can deduce which is the more accurate or precise

sWLs whole claim to fame is using the concept of precise time boundaries to arrive at a more precise result. That only works if you beleive the assumptions is valid.
The arguments about the rhumb line are a red herring.

Dave
__________________

__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 15:11.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.