Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 06-02-2013, 10:55   #166
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Single CTS or follow the Courseline?

Quote:
3) It seems we agree that a constant heading, CTS strategy is only appropriate when the substantial advantages of 'crabbing the rhumb' are more than outweighed.
Im glad to see despite all the fog here that this jewel is recognised.

I further would say that many protagonists have simply said " plot you intended ground track" , In multi hour CTS this isnt easy, nor can it be done on the vast majority of chartplotters. It requires an understanding of vector addition that very many sailors do not have and hence are likely to make mistakes. Equally in some cases , as has been very aptly demonstrated , the intuitive track is in fact not what actually happens at all.

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 06-02-2013, 10:56   #167
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,819
Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by Seaworthy Lass View Post
Sorry, no, I do not think this is correct.
SOG is not the same as SMG.
SMG is relative to the destination, SOG is not.
Quote:
Originally Posted by jackdale View Post
Good I am saying they are not the same as well.
Quote:
Originally Posted by Lodesman View Post
I didn't realize there was such confusion over the term SMG. Sorry, I'm a traditionalist - to me it's the same as SOG. Starpath Online Classroom Glossary
OK, Jackdale which is it LOL?
You can't sit on both sides of the fence!

If it truly is:
"A term used in the past, but without a unique definition according to modern conventions. It could mean speed over ground or velocity made good",
then perhaps the term should be dropped from current usage (no pun intended ).
__________________

__________________
"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 06-02-2013, 11:03   #168
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,819
Plotting the expected ground track

Quote:
Originally Posted by goboatingnow View Post
I further would say that many protagonists have simply said " plot you intended ground track" , In multi hour CTS this isnt easy, nor can it be done on the vast majority of chartplotters. It requires an understanding of vector addition that very many sailors do not have and hence are likely to make mistakes. Equally in some cases , as has been very aptly demonstrated , the intuitive track is in fact not what actually happens at all.
dave
I think you underestimate the average sailor. If they are able to compute a CTS for variable cross currents, then I am sure most would be easily capable of going one step further and plotting the expected ground track.

It is only one small more step in the process and it is a very easy way to check if the computed CTS is correct, as using the RYA method you end up exactly at D on the rhumb line and using the SWL method you end up exactly at B, the destination .
__________________
"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 06-02-2013, 11:03   #169
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Single CTS or follow the Courseline?

SMG does not really have a hard definition , it has been used as VMG or as SOG by many books etc

IN general it has been used to represent speed towards a destination, computed retrospectively. whereas SOG is a instantaneous value. Hence in that way its analogous to CMG.

The real poorly defined one is VMG of course.

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 06-02-2013, 11:20   #170
Moderator
 
Paul Elliott's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 3,882
Images: 4
Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by goboatingnow View Post
[...] whereas SOG is a instantaneous value.
SOG can be a filtered (averaged) value as well. It is the distance traveled over the ground in *any* direction: from where you were (some amount of time ago, could be seconds or minutes, or I suppose hours), to where you are now. Destination or waypoint have nothing to do with SOG.
__________________
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
www.sailvalis.com
Paul Elliott is online now   Reply With Quote
Old 06-02-2013, 11:24   #171
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by Paul Elliott View Post
SOG can be a filtered (averaged) value as well. It is the distance traveled over the ground in *any* direction: from where you were (some amount of time ago, could be seconds or minutes, or I suppose hours), to where you are now. Destination or waypoint have nothing to do with SOG.

The fact that its filtered makes no difference, SOG is an instantaneous reading, in that its the current speed over the ground , rather like a cars speedo. In that way STW and SOG are the same.

SMG is similar to CMG is that to some extent its a retrospective computation, rathe like SOA is a prospective computation

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 06-02-2013, 11:30   #172
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Plotting the expected ground track

Quote:
Originally Posted by Seaworthy Lass View Post
I think you underestimate the average sailor. If they are able to compute a CTS for variable cross currents, then I am sure most would be easily capable of going one step further and plotting the expected ground track.

It is only one small more step in the process and it is a very easy way to check if the computed CTS is correct, as using the RYA method you end up exactly at D on the rhumb line and using the SWL method you end up exactly at B, the destination .

The vast majority of sailors however SL, dont compute long complex multi hour CTS, in fact the method isnt even illustrated in many navigation texts, including Bowditch. Outside of the RYA ( whatever you make think of it) its not mentioned at all in the vast majority of navigation texts, ( I have currently 14 of these various texts). Even the RYA books often illustrate it very badly and most people only vaguely get the idea in the class.

The fact is that in my experience most people dont even get the ideas of moving the current vectors together to add then up. Expecting them to them re-apply these vectors against the the then computed CTS to extract ground track isnt going to happen. Witness the Troup/Dockhead non-intutitive sinusoidal case and most real vectors are more weird then that

However Im glad that the Troup/SWL axis at least agree on the safety and applicability of teh crabbing method unless its disadvantages are greatly outweighed. Plotting and sailing a complex course to "theoretically" save 30 minutes isnt really worth it , given the issues involved.

ps Im suprised at this
Quote:
It is only one small more step in the process and it is a very easy way to check if the computed CTS is correct, as using the RYA method you end up exactly at D on the rhumb line and using the SWL method you end up exactly at B, the destination
The RYA method gets you to B, ( by inflation or deflating the averages), you may argue as to the accuracy of that method , thats different. I would argue there are huge other variables at work interferring with arriving at B using any precomputed method. But thats a different argument

Neither method makes computing a complex multi hour CTS ground track easy for non mathematicians, nor do modern chartplotters in general offer any aid in that regards.
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 06-02-2013, 11:59   #173
Senior Cruiser
 
jackdale's Avatar

Cruisers Forum Supporter

Join Date: Mar 2008
Location: Calgary, AB, Canada
Posts: 5,048
Images: 1
Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by Seaworthy Lass View Post
OK, Jackdale which is it LOL?
You can't sit on both sides of the fence!

If it truly is:
"A term used in the past, but without a unique definition according to modern conventions. It could mean speed over ground or velocity made good",
then perhaps the term should be dropped from current usage (no pun intended ).
The starpath definitions are related to electronic navigation
Quote:
This section includes the discussion of several related terms, all speeds derived electronically in navigation equipment: SOG, SOA, SMG, and VMG.
I thought this whole exercise was based on traditional paper charting. It is to derive a compass course to provide a helm to get from a departure to a destination. And to provide the navigator with a SMG which can used to establish an ETA.

For me SOG is found in electronics as is VMG (although the latter can be estimated on a chart).
__________________
ISPA Yachtmaster Ocean Instructor Evaluator
Sail Canada Advanced Cruising Instructor
IYT Yachtmaster Coastal Instructor
ASA 201, 203,204, 205, 206, 214
As I sail, I praise God, and care not. (Luke Foxe)
jackdale is offline   Reply With Quote
Old 06-02-2013, 12:17   #174
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,819
Plotting the expected ground track

Quote:
Originally Posted by goboatingnow View Post
The RYA method gets you to B, ( by inflation or deflating the averages), you may argue as to the accuracy of that method , thats different. I would argue there are huge other variables at work interferring with arriving at B using any precomputed method. But thats a different argument

Neither method makes computing a complex multi hour CTS ground track easy for non mathematicians, nor do modern chartplotters in general offer any aid in that regards.
Dave
Dave, I have been repeating for a couple of weeks now that unlike the SWL method which gets you very close, the CTS using the RYA method does NOT get you to B, it gets you to an arbitrary point on the rhumb line that the RYA call D. You keep insisting the RYA method will have you arriving at B. This is how an important issue, so lets settle it with a simple example and plot the expected ground tracks using the CTS computed by each method.

Destination (B) is 6.5 nm due east of A
Boat speed is 5 knots (log reading)
Average current for the first hour is 3 knots to the south
Average current for the second hour is 1 knot to the south

What is the expected CTS using both methods?
What is the expected time taken using both methods?
What is the expected ground track using both methods?

I will compute this and reply shortly.
Could you please do the same for the RYA method and show me how you can possibly get to B using one CTS?
__________________
"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 06-02-2013, 12:37   #175
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Plotting the expected ground track

Quote:
Originally Posted by Seaworthy Lass View Post
Dave, I have been repeating for a couple of weeks now that unlike the SWL method which gets you very close, the CTS using the RYA method does NOT get you to B, it gets you to an arbitrary point on the rhumb line that the RYA call D. You keep insisting the RYA method will have you arriving at B. This is how an important issue, so lets settle it with a simple example and plot the expected ground tracks using the CTS computed by each method.

Destination (B) is 6.5 nm due east of A
Boat speed is 5 knots (log reading)
Average current for the first hour is 3 knots to the south
Average current for the second hour is 1 knot to the south

What is the expected CTS using both methods?
What is the expected time taken using both methods?
What is the expected ground track using both methods?

I will compute this and reply shortly.
Could you please do the same for the RYA method and show me how you can possibly get to B using one CTS?

Hang on Seaworthy, before we start taking examples. I beleive you know the RYA good enough now, in the RYA method if D is before B and within the 30 minute rule, the RYA method merely says the CTS is the same., ie it ignores the effect of the first 30 minutes of the last tide.

Equally if D is behind B, and within 30 minutes, teh RYA again simply uses the full effect of the last tide , even though it it only being applied for 30 minutes. ( all this for hourly tide intervals)


Im not arguing this. Nor am i arguing that your method more correctly proportions the last tide.

BUT all I said was that your method is more accurate if you simply use fixed time quantums as a given. ie
(a) that the tide abruptly changes as stated for the next hour
(b) that the tide remains exactly constant for that stated hour.

In real life of course none of these conditions are true, leaving aside lee-way and other induced errors the tide extraction data is subject to

(a) manual interpolation to arrive at the Set and Drift
(b) An assumption that in fact you will physically be in that location at that time ( which is strange given the fact that you dont actually know where your gound track is at the start). we have of course totally ignored that tide is also position based and we use the rhumb line to exact the data in the first place hence an error
(c)Tide does not change much ( in general ) even using tide data from hour to hour, hence the errors in the inflation method are generally less in real life

SO , merely picking examples as you do , does not illustrate the underlying error in arriving at the variables in teh first place.

Hence i would contend that neither method is particularly accurate and that the error circle of either method is in fact smaller and contained within the error of the whole process.

Paradoxically, its you take more and more precise tide timing then in effect both methods converge , ie running the SWL and RYA methods on 5 minute tide data would result in very little difference.

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 06-02-2013, 12:49   #176
Senior Cruiser

Cruisers Forum Supporter

Join Date: Nov 2005
Location: Out there doin' it
Boat: 47' Olympic Adventure
Posts: 2,637
Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by Seaworthy Lass View Post
Lodesman, I just had a look at your link. The last paragraph reads:

"SMG is speed made good. A term used in the past, but without a unique definition according to modern conventions. It could mean speed over ground or velocity made good."

In previous threads Jackdale gave a method for calculating the SMG when using the RYA method for determining the CTS. In that instance he was referring to SMG as VMG, not SOG, so I have followed this convention.

Perhaps the term should be dropped if it can mean either SOG or VMG "according to modern convention". Highly confusing if you ask me!!!!
You should have added the rest of the last paragraph:

Quote:
Before modern electronics with various derived outputs and their subsequent naming conventions, this term was used to mean the combined result of your knotmeter speed and that of the current. Its counter part was course made good, but both these have been replaced with speed and course over ground in most applications.

To me VMG is a vector - it denotes direction as well as speed. That is SMG and CMG taken together give you VMG.

As I said, I'm a traditionalist, but to further cloud the issue, I used SMG in the "overall" sense - that is the overall speed made good over the two-hour passage. I think I can see what Dockhead was talking about - if you look at my past example, the NCL boat follows the rhumbline between A and B, that is the CMG/COG and the speed (distance/time) is SMG/SOG. The CTS boat actually follows an ESE path from A to C in the first hour, then an ENE path from C to B in the second hour. If you look at each chunk AC and CB, I guess it would be more accurate these days to describe these by COG and SOG. But in describing the relationship between the time taken to cover the effective distance between A abd B would still be SMG. In this case, obviously AC+CB is longer than AB, so SOG is greater than SMG. Still, if you shrink AB, then AC and CB will also shrink by a corresponding amount.
__________________
Lodesman is offline   Reply With Quote
Old 06-02-2013, 12:51   #177
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: 19,751
Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by Seaworthy Lass View Post
Lodesman, I just had a look at your link. The last paragraph reads:

"SMG is speed made good. A term used in the past, but without a unique definition according to modern conventions. It could mean speed over ground or velocity made good."

In previous threads Jackdale gave a method for calculating the SMG when using the RYA method for determining the CTS. In that instance he was referring to SMG as VMG, not SOG, so I have followed this convention.

Perhaps the term should be dropped if it can mean either SOG or VMG "according to modern convention". Highly confusing if you ask me!!!!
Another definition of Speed Made Good is this:

"speed made good (navigation) The actual average speed in knots which was maintained in proceeding along the intended track to the ultimate destination or an intermediate point."

speed made good definition of speed made good in the Free Online Encyclopedia.




I think it's not worthwhile to get all bogged down in definitions.

"Made good" simply means recalculated with regard to what for you is "good" -- towards your waypoint, upwind, whatever. "Speed" versus "velocity" is trivial.

So I think "speed made good" or "VMG towards the waypoint" or "speed of advance" can all be reasonably used to describe the same thing, and will equal the SOG in case you are sailing along the rhumb line. If you're sailing far off the rhumb line, then the difference between speed of advance and SOG becomes vivid.
__________________
Dockhead is offline   Reply With Quote
Old 06-02-2013, 12:53   #178
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Single CTS or follow the Courseline?

Quote:
To me VMG is a vector - it denotes direction as well as speed. That is SMG and CMG taken together give you VMG.
Thats a key point that is missed, the debate however being what is VMG vector pointing at.
__________________
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 06-02-2013, 12:55   #179
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by Dockhead View Post
Another definition of Speed Made Good is this:

"speed made good (navigation) The actual average speed in knots which was maintained in proceeding along the intended track to the ultimate destination or an intermediate point."

speed made good definition of speed made good in the Free Online Encyclopedia.




I think it's not worthwhile to get all bogged down in definitions.

"Made good" simply means recalculated with regard to what for you is "good" -- towards your waypoint, upwind, whatever. "Speed" versus "velocity" is trivial.

So I think "speed made good" or "VMG towards the waypoint" or "speed of advance" can all be reasonably used to describe the same thing, and will equal the SOG in case you are sailing along the rhumb line. If you're sailing far off the rhumb line, then the difference between speed of advance and SOG becomes vivid.
In my experience, Ive always presented SMG and CMG as historical calculations , ie having known what you experienced you compute these figures.

SOG and STW, HDG and (to some extent) COG are real time variable computed on the fly . I beleive that is how most people and electronics interpret them.

IN reality SMG was an attempt in preGPS times to effectively find an average SOG.
__________________
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 06-02-2013, 13:05   #180
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: 19,751
Re: Plotting the expected ground track

Quote:
Originally Posted by goboatingnow View Post
Hang on Seaworthy, before we start taking examples. I beleive you know the RYA good enough now, in the RYA method if D is before B and within the 30 minute rule, the RYA method merely says the CTS is the same., ie it ignores the effect of the first 30 minutes of the last tide.

Equally if D is behind B, and within 30 minutes, teh RYA again simply uses the full effect of the last tide , even though it it only being applied for 30 minutes. ( all this for hourly tide intervals)


Im not arguing this. Nor am i arguing that your method more correctly proportions the last tide.

BUT all I said was that your method is more accurate if you simply use fixed time quantums as a given. ie
(a) that the tide abruptly changes as stated for the next hour
(b) that the tide remains exactly constant for that stated hour.

In real life of course none of these conditions are true, leaving aside lee-way and other induced errors the tide extraction data is subject to

(a) manual interpolation to arrive at the Set and Drift
(b) An assumption that in fact you will physically be in that location at that time ( which is strange given the fact that you dont actually know where your gound track is at the start). we have of course totally ignored that tide is also position based and we use the rhumb line to exact the data in the first place hence an error
(c)Tide does not change much ( in general ) even using tide data from hour to hour, hence the errors in the inflation method are generally less in real life

SO , merely picking examples as you do , does not illustrate the underlying error in arriving at the variables in teh first place.

Hence i would contend that neither method is particularly accurate and that the error circle of either method is in fact smaller and contained within the error of the whole process.

Paradoxically, its you take more and more precise tide timing then in effect both methods converge , ie running the SWL and RYA methods on 5 minute tide data would result in very little difference.

Dave
We're kind of going around the houses here, so sorry for repeating what has been said before.

The difference between RYA and SWL is very simple -- they deal with the last partial hour in different ways. RYA subsitutes the average of the passage, where SWL substitutes the average of the last hour. Neither, of course, is extremely precise, but SWL is obviously better, ranging from somewhat better to vastly better, depending on whether or not you run into the limitations of the SWL method.

Now that I have 5 minute tide data I can show it with examples -- just need a bit of quiet time to do it. Maybe tomorrow.
__________________

__________________
Dockhead is offline   Reply With Quote
Reply

Tags
single

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 13:58.


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.