Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 02-02-2013, 16:18   #76
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

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

Do you really think there are only a few? Given that only hourly tidal data is available ( for most of us anyway) it's really not that hard to draw up some vectors. Though agree, accuracy will start to suffer quickly. Not that I've sat down with many and discussed it at length, but if that is the case then few mariners can keep an EP going, which is basically the same thing using actual boat speed/course.
You have GPS why are you computing EPs.

Yes you have tide at a particular time and you have to interpolate to your expected time that you will experience that tide, its time consuming enough.

I'm beginning to think we have a lot f people you have not actually done any of this in real life ( by hand )



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 02-02-2013, 16:33   #77
Registered User

Join Date: Nov 2007
Location: Probably in an anchorage or a boatyard..
Boat: Ebbtide 33' steel cutter
Posts: 3,538
Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by goboatingnow View Post
You have GPS why are you computing EPs.

Yes you have tide at a particular time and you have to interpolate to your expected time that you will experience that tide, its time consuming enough.

I'm beginning to think we have a lot f people you have not actually done any of this in real life ( by hand )



Dave
EP's ? Sometimes because I can Cos sailing is fun, turn it all off and be self sufficient once in a while.

And have spent a fair amount of time round the thames estuary and channel sans gps cos it's fun to play around with that kind of stuff some days, coming within a hundred yards of a buoy after 2 or 3 hours of tide using a cts feels pretty good and gives some confidence that should the sh!t hit the fan then you can find your way home in the fog.


But the point remains, I don't think creating a multi hour cts using vectors is that difficult. If it's much use in the real world is a different matter but that's not what you said, or how it came across anyway.

The vector bit is fairly straight forward, if someone can't handle that then they can't handle an EP. It's not rocket science.

Which is not saying it's a good idea in more than a few passages.
__________________

__________________
conachair is offline   Reply With Quote
Old 02-02-2013, 17:00   #78
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

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

EP's ? Sometimes because I can Cos sailing is fun, turn it all off and be self sufficient once in a while.

And have spent a fair amount of time round the thames estuary and channel sans gps cos it's fun to play around with that kind of stuff some days, coming within a hundred yards of a buoy after 2 or 3 hours of tide using a cts feels pretty good and gives some confidence that should the sh!t hit the fan then you can find your way home in the fog.

But the point remains, I don't think creating a multi hour cts using vectors is that difficult. If it's much use in the real world is a different matter but that's not what you said, or how it came across anyway.

The vector bit is fairly straight forward, if someone can't handle that then they can't handle an EP. It's not rocket science.

Which is not saying it's a good idea in more than a few passages.
I have repeated questioned the real life valve of multi hour especially large multi hour CTS. That's been the key point of my posts in three threads for early two weeks. Using the method over 2-3 changes of tide is practical, more in my opinion is not

Working up the CTS vector is easy enough. Working up the ground tack isn't. Without the ground track you have no way of ascertaining where you should be.

What people fail to realise is that not that its difficult to determine your position, you can use EPs, sextants , GPS , voodoo whatever. Since you don't have an accurate ground track you can't easily determine if your following your precomputed plan.

So the only other way is to continually recompute the whole plan and determine a new CTS from where you are now. That's a lot of work in complex situations.

The advantage do staying reasonably close to the rhumb line is that you have a built in progress indicator.

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 02-02-2013, 17:16   #79
Registered User

Join Date: Nov 2007
Location: Probably in an anchorage or a boatyard..
Boat: Ebbtide 33' steel cutter
Posts: 3,538
Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by goboatingnow View Post
Working up the CTS vector is easy enough. Working up the ground tack isn't. Without the ground track you have no way of ascertaining where you should be.
That the bit I don't understand. There's little difference, put all the tidal vectors down first to get a course to steer then do them hourly with an hours boat speed/direction, gives you an hourly position, join the dots. Why is that so hard?
If your saying that that accuracy quickly suffers then yes, completely agree. But getting an hourly mark of where the boat should be isn't that difficult. Is it?

But back in the real world most guys i know round these parts never put pencil to paper anyway, the bulk of passages you can have a guess of how many degrees to allow for the tide and get pretty close, tweak as you go along.
__________________
conachair is offline   Reply With Quote
Old 02-02-2013, 17:29   #80
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 conachair View Post
That the bit I don't understand. There's little difference, put all the tidal vectors down first to get a course to steer then do them hourly with an hours boat speed/direction, gives you an hourly position, join the dots. Why is that so hard?
If your saying that that accuracy quickly suffers then yes, completely agree. But getting an hourly mark of where the boat should be isn't that difficult. Is it?

But back in the real world most guys i know round these parts never put pencil to paper anyway, the bulk of passages you can have a guess of how many degrees to allow for the tide and get pretty close, tweak as you go along.
Are you thinking an hourly DR or an hourly EP?

You are only going to to be able to tweak if you get a fix. If you are using GPS, use the XTE.

BTW the major reason I use vectors in crossing large bodies of water to get the SMG. Knowing what time I can clear a pass, I need to know when I need to depart. Some of the passes into the Gulf Islands have no anchorages nearby on the Georgia Strait side, so you would be waiting six hours to get through.
__________________
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 02-02-2013, 17:54   #81
Registered User

Join Date: Nov 2007
Location: Probably in an anchorage or a boatyard..
Boat: Ebbtide 33' steel cutter
Posts: 3,538
Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by jackdale View Post
Are you thinking an hourly DR or an hourly EP?
No, thinking how difficult it is to plot a number hourly vectors on a chart to see here the boat would be each hour, which really isn't any harder than putting all the tidal vectors first. It was the statement that the process was in some way harder than just getting a cts seemed to make no sense.

If there' s a decent size of open water then why not work out a cts over a number of hours, not so difficult to figure out where you should be and keep an eye on the gps , and if you are so unlucky to loose the gps then you'll be doing EP anway. Hugging the rhumb line when you don't need to seems a bit senseless.
__________________
conachair is offline   Reply With Quote
Old 02-02-2013, 18:39   #82
Senior Cruiser

Cruisers Forum Supporter

Join Date: Nov 2005
Location: Out there doin' it
Boat: 47' Olympic Adventure
Posts: 2,635
Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by DeepFrz View Post
Read SWL's thread. She explains it all very thoroughly.
Care to point us to the post where this is explained? I read all the threads afaik, and I didn't see this answered.
__________________
Lodesman is offline   Reply With Quote
Old 02-02-2013, 19:55   #83
Senior Cruiser
 
DeepFrz's Avatar

Cruisers Forum Supporter

Join Date: Mar 2006
Location: Winnipeg
Boat: None at this time
Posts: 7,930
Re: Single CTS or follow the Courseline?

Yes, I could go back and find the posts that laid out the COG in red. But Im sure you could do it just as quickly so I will leave it to you.
__________________
DeepFrz is online now   Reply With Quote
Old 02-02-2013, 21:50   #84
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,815
Plotting the expected ground track

Hi all
I had a break yesterday, so there was lots of catching up to do on this thread. A few things stand out that I would like to comment on. The first and most important thing is how to determine the expected ground track when using a CTS method of navigating with variable cross current. This seems to be unclear to some people.

Quote:
Originally Posted by bill352 View Post
I've been following this and the other threads on the subject, and I'm going to jump in with a stupid question here (yes, they exist. I've seen them.)

If you're sailing a multi-hour CTS and you do get a reliable fix, how do you know where that is compared to your planned course? As I understand it, there is no DR plot with this method. How do you know whether you're on course or not?
Bill (and everyone else who is puzzled), the method is simple. If you are working with hourly current data, once you have computed your CTS, from the start point draw your 'distance boat is displaced by current over the first hour' line (correct length and direction for the average current amount predicted for the first hour). To the tip of this add your to your 'distance boat is displaced by boat speed (direction corrected for leeway) over an hour' line (this one is easy as it is constant until the last fraction of an hour). This marks your position at the end of the first hour. Draw a big circle around this dot or somehow take note of it. All this can and would normally be done on the chartplotter/OpenCPN etc.

To this point add your 'distance boat is displaced by current over the second hour' line (correct length and direction for the average current amount predicted for the second hour) and to the tip of this add to your 'distance boat is displaced by boat speed (direction corrected for leeway) over an hour' line. This marks your position at the end of the second hour. Draw another big circle around this dot.

Keep repeating this and marking your hourly position until you have reached your destination (the last lot of vectors will be for the fraction of the last hour if the SWL method for determining CTS is used). Join up the circled dots and you have your track : thumb:

It does not take more than a few minutes doing this.

I feel it is essential this is done, not just to check for hazards along the 'expected ground track', but so that the CTS can be recalculated if you start deviating significantly from it. You do NOT try and get back on the track.

Yes, the RYA method does need you to wing it at the end. Unfortunately you may be a lot further than half an hour away from the destination when the RYA method has you arriving on the rhumb line.

As an RYA instructor commented:
Quote:
Originally Posted by goboatingnow View Post
Perform a check to ensure that D lies approx less then 30 minutes away from B , some instructors omit this bit, ie to ensure the tidal data remains valid. This can be done by simplest calculating the rate of advance , ie the average SOG to D and by inspection determine how far B is away. If its greater then 30 minutes ( or less) redraw the plot using one more or less tide ,( note this is rarely done in examples as the test questions are typically picked to ensure it isn't needed.
It bothers me terribly that the RYA Navigation Handbook (that is for general use, not just for RYA course work) mentions nothing about this half hour check. Then I am told by an instructor that on courses some instructors omit teaching it!!! The method can have huge errors if you end up further than half an hour away on the rhumb line (up to 30+ degrees in error as I have shown in some examples).

The SWL method has your computed 'expected ground track' arriving exactly at the nominated destination

PS Would anyone like me to draw this out if my explanation is unclear? Happy to do so.
__________________
"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 02-02-2013, 21:59   #85
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,815
Making corrections when following a computed CTS

Quote:
Originally Posted by bill352 View Post
I understand that the prudent navigator will start anew from here, and that is certainly my practice. But there has been talk about "correcting" a course from here, and even, IIRC, of calculating the actual set and drift by comparison with some expected position. How do you do that, if you do?

Forgive me if this has been explained before. I didn't see it.
Quote:
Originally Posted by Lodesman View Post
How many threads have been generated now over what we've been told is a flaw in the RYA 'multi-tidal set single CTS' - that it doesn't aim to destination, but up to half-hour away and you'll need to "wing it" to get to destination. If any other method of determining a 'multi-tidal set single CTS' will also require one to "wing it" to get to destination, then it seems that these threads are all sound and fury, signifying nothing. We've had a couple 'sort of' answers, but no-one has stated unequivocally what is expected during the execution of the single CTS, be it RYA, SWL or other method.

Do you: (a) plot the expected CMG for each tidal set and change course to maintain that?;
(b) take periodic fixes and recalculate a new CTS based on previously-planned and possibly incorrect tidal data;
(c) take periodic fixes and recalculate a new CTS based on tidal data adjusted by observation;
(d) stick to the planned CTS with faith that it will all even out by the end, fixing and running DRs/EPs to ensure navigational safety;
(e) stick to the planned CTS and only pansies fix; or
(f) other?
You plot your 'expected ground track' before the start of the journey and you examine your track relative to this as you would the rhumb line in the NCL method.

As soon as you deviate significantly, a new track must be computed, just as it is for the NCL method (you do NOT try and get back on the old track).

If you are racing this will need to be done frequently, particularly if the deviation is due to an unexpected variation in speed (Expedition probably does this automatically for you). Otherwise pick your duration of time to recompute this (eg hourly? - I think the most appropriate interval of time depends on the distance left to go).
__________________
"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 02-02-2013, 22:10   #86
Senior Cruiser
 
jackdale's Avatar

Cruisers Forum Supporter

Join Date: Mar 2008
Location: Calgary, AB, Canada
Posts: 5,048
Images: 1
Re: Plotting the expected ground track

Quote:
Originally Posted by Seaworthy Lass View Post

To this point add your 'distance boat is displaced by current over the second hour' line (correct length and direction for the average current amount predicted for the second hour) and to the tip of this add to your 'distance boat is displaced by boat speed (direction corrected for leeway) over an hour' line. This marks your position at the end of the second hour. Draw another big circle around this dot.
Why are you using a Fix symbol rather than an EP symbol?



Yes it does matter, especially when you switch navigators.

Quote:

The SWL method has your computed 'expected ground track' arriving exactly at the nominated destination
Assuming
  • The currents can be accurately predicted.
  • The leeway can be accurately determined
  • The winds remain constant.
  • The helm can maintain an constant course.
  • That water is flat, no swell or waves.
  • The trimmers are focused.
  • There are no TSSs along the route.
  • There is no other traffic.
  • Etc.
__________________
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 02-02-2013, 22:11   #87
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,815
Terminology

Quote:
Originally Posted by LJH View Post
In all of the problems we have been looking at the boat speed is given to us as a constant. First to avoid some terminalogy that I will use:
SOG - Speed over Ground
VTG - Velocity Track Made Good
VDR - Velocity and set of drift (assuming due to tidal current for these examples)
Vboat - Boat Speed
Positions
A - Departure point
B - Destination
C - RYA Tidal offset (this is to a full hour)
D - RYA - arc of distance traveled at Vboat and line AB
K - SWL tidal offset for duration of voyage
distances:
AB - rhumbline dist from A to B
AC - tidal offset at x hrs
CD - distance travelled though the water mass over x hours
AD - distance traveled along rhumb line over x hrs
AK - total tidal offset
KB - total distance travelled through the water mass

Using the RYA method
VTG (AD) = Vboat x (AD/CD)
Using the SWL method
VTG (AB) = Vboat x (AB/KB)
Note that given more than one curent your drift will vary with the current and your boat track will not follow the rhumbline. Your speed over ground will not be the same as the VTG. If you want, you can calculate one using the Law of Cosines using Vboat, VDR and the angle between the CTS and the tidal set for each tidal hour. It would not survive first contact with the elements!
Great summary LJH. You may want to add Vboat = BSP, as Bewitched keeps referring to that.

The only thing we are missing here is a better term for the 'course line' or the 'intended track'.

The RYA define both of these as the straight line between A and B (whether or not you will NCL)
. I feel this is very misleading as the term implies this is the ideal line/track you want to follow at all times. It most certainly is not if you have any variable cross current for the CTS method. Also for the NCL it is only the line/track to follow if you have not once deviated significantly from it during a leg. That is unlikely also.

It would stop all confusion if the straight line between A and B was simply termed the 'rhumb line'.

'Expected ground track' (EGT) may be a better term than the 'course line' or the 'intended track'. It is a curve for the CTS method if you have variable cross current (determine this beforehand), and a straight line for the NCL method and the CTS method if the current is constant for that leg. In both methods it may vary as you progress along the leg.

Some of the reasons you may deliberately deviate from the 'expected ground track' determined at the start of the race/ journey (for both the NCL and CTS methods) and need to recompute a new one are:
- when scooting around to chase better breeze when it is visible ahead of you on the surface of the water or its affect is seen on boats ahead of you
- when dodging other boats
- when gybing to improve speed going dead downwind and when tacking
- when seeking to avoid adverse current or take advantage of it by deviating to shallower/ deeper water

In all of these circumstances (for both methods) as soon as you have deviated, your aim is NOT to get back on the old 'expected ground track' (unless you have only deviated momentarily on the NCL method), but to find and follow a new one. This is an important consideration.

The CTS method does not navigate using one course to steer, just as the NCL method does not navigate one course line!
__________________
"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 02-02-2013, 22:18   #88
Senior Cruiser
 
jackdale's Avatar

Cruisers Forum Supporter

Join Date: Mar 2008
Location: Calgary, AB, Canada
Posts: 5,048
Images: 1
Re: Terminology

Quote:
Originally Posted by Seaworthy Lass View Post
[B]The only thing we are missing here is a better term for the 'course line' or the 'intended track'.

It does have a name: CMG Course Made Good. I will, when doing "predictions", use the phrase Course to Make Good.
http://www.sailing.ca/images/uploads...201%202009.pdf
__________________
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 02-02-2013, 22:20   #89
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,815
Re: Plotting the expected ground track

Quote:
Originally Posted by jackdale View Post
Why are you using a Fix symbol rather than an EP symbol?
Yes it does matter, especially when you switch navigators.
Sorry, my error. Mark it conventionally.

Quote:
Originally Posted by jackdale View Post
Assuming
  • The currents can be accurately predicted.
  • The leeway can be accurately determined
  • The winds remain constant.
  • The helm can maintain an constant course.
  • That water is flat, no swell or waves.
  • The trimmers are focused.
  • There are no TSSs along the route.
  • There is no other traffic.
  • Etc.
Yes, the data input and the method used and the ability to maintain the heading will all determine the accuracy of the result.
The validity of the data and the ability to maintain the heading will equally affect the RYA and SWL methods for determining the CTS.

The SWL method will, however, at least get you within a degree of two of the destination if the data is correct.
The RYA method can be 10 or 20 or 30+ degrees in error.

Even given the limitations of the data, I know which method I would prefer to be using .
__________________
"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 02-02-2013, 22:26   #90
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?

The only problem with LJH defintions is that some are idiosyncratic.

SOG is a GPS term refering to a momentary speed based on a the average of some fixes. The term used in vectoring is SMG.

Set/drift refers to current.

S is boat speed through the water.

You should check these with either the CYA link above or with recognized manuals like Bowditch. It is essential that watch navigators use standard terms and symbols.

There a a few recent additions to symbols such as electronic fixes where there no consensus among various agencies.
__________________

__________________
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
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 13:26.


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.