qsiv
Well-Known Member
If you do get such an effect, try increasing the damping (or period the readings are averaged over)
<hr width=100% size=1>
<hr width=100% size=1>
Fly-by-wire?
- Thread starter DeeGee
- Start date
stuartw
Well-Known Member
Hymns, Ancient & Modern
You seemed to have forgotten, that to steer 2 miles East of of Cherbourg, your rudder, possibly with the help of the windvane, will try & keep you pointing to the wind. Your initial bearing may be that, but the tidal effect will swing you away from that bearing very quickly, so you have to re-establish that. Now I would agree, that a AP just linked to a fluxgate compass will keep you on a track which is parallel to the original, thus the effects of tide will bend you round. No argument there. However as I have pointed out to Rupert W, things are not that constant, so the effects of tidal eddies, slack water will have an effect, such that your original bearing/track is wrong.
Rupert in his last answer excepts the point. If you like all the re-adjustments of sailing fair do's. But I suspect many people who pop over to say St. Vaast to re-stock their wine cellar, may well feel that the use of the AP linked to GPS is a nice option. The questionner seems to accept that my original answer was what he is looking for. I hope there is no one who does not accept an AP +GPS guided boat actually works. If they don't then they are truly one of the ancient mariners.
All that STUART will not accept, is that an AP+GPS+XTE guided boat does not work.
<hr width=100% size=1>
You seemed to have forgotten, that to steer 2 miles East of of Cherbourg, your rudder, possibly with the help of the windvane, will try & keep you pointing to the wind. Your initial bearing may be that, but the tidal effect will swing you away from that bearing very quickly, so you have to re-establish that. Now I would agree, that a AP just linked to a fluxgate compass will keep you on a track which is parallel to the original, thus the effects of tide will bend you round. No argument there. However as I have pointed out to Rupert W, things are not that constant, so the effects of tidal eddies, slack water will have an effect, such that your original bearing/track is wrong.
Rupert in his last answer excepts the point. If you like all the re-adjustments of sailing fair do's. But I suspect many people who pop over to say St. Vaast to re-stock their wine cellar, may well feel that the use of the AP linked to GPS is a nice option. The questionner seems to accept that my original answer was what he is looking for. I hope there is no one who does not accept an AP +GPS guided boat actually works. If they don't then they are truly one of the ancient mariners.
All that STUART will not accept, is that an AP+GPS+XTE guided boat does not work.
<hr width=100% size=1>
qsiv
Well-Known Member
Re: A manipulated reply.
Well - heres the maths
The classically approved route is here.
I had to rewrite part of the software to cut out the tidal optimisation. I locked in a steady wind (8 knots at 300 if I recall). The AWA and AWS figures show as different to this because the wind the boat feels is modified by the tidal vector. The difference is as expected about 40 minutes - to me thats a worthwhile percentage in cruising mode, critical in race mode.
<hr width=100% size=1>
Well - heres the maths
The classically approved route is here.
I had to rewrite part of the software to cut out the tidal optimisation. I locked in a steady wind (8 knots at 300 if I recall). The AWA and AWS figures show as different to this because the wind the boat feels is modified by the tidal vector. The difference is as expected about 40 minutes - to me thats a worthwhile percentage in cruising mode, critical in race mode.
<hr width=100% size=1>
BlueSkyNick
Well-Known Member
Freudian slip.
Your links on the post don't appear to work - maybe its just me - they are coming back with a ......"Navigation Error"!
Thanks for all your input on this, though.
<hr width=100% size=1>Never employ an illiterate dwarf - its not big and its not clever.
Your links on the post don't appear to work - maybe its just me - they are coming back with a ......"Navigation Error"!
Thanks for all your input on this, though.
<hr width=100% size=1>Never employ an illiterate dwarf - its not big and its not clever.
jfm
Well-Known Member
MoBo agreeing with yotties I\'m afraid
Stuart: As you know I'm more MoBo than yot these days so should be on your side. And I'm at the pc preparing that CD for you. But I have to say that on the basic argument (leaving aside details like local eddies etc that have been mentioned) Twister Ken is quite correct and you are wrong imho. Intriguingly what you say (keep XTE near zero) is actually what the RYA teach but even they are wrong on this and Ken et al are correct. If crossing the channel on a 12 hour trip you should NOT minimise XTE. You should let it rise and fall in a sine wave. By doing this you will cover more distance over ground (following a sine wave) but you will cover minimum distance through water. If anything therefore you should set your autopilot to hold a constant heading not a constant COG
Also, separate point, you mentioned a straight COG and dividing it up with 6 waypoints. I never do that. If a leg of my route is a straight line I dont use intermediate waypoints. Am I missing something here?
<hr width=100% size=1>
Stuart: As you know I'm more MoBo than yot these days so should be on your side. And I'm at the pc preparing that CD for you. But I have to say that on the basic argument (leaving aside details like local eddies etc that have been mentioned) Twister Ken is quite correct and you are wrong imho. Intriguingly what you say (keep XTE near zero) is actually what the RYA teach but even they are wrong on this and Ken et al are correct. If crossing the channel on a 12 hour trip you should NOT minimise XTE. You should let it rise and fall in a sine wave. By doing this you will cover more distance over ground (following a sine wave) but you will cover minimum distance through water. If anything therefore you should set your autopilot to hold a constant heading not a constant COG
Also, separate point, you mentioned a straight COG and dividing it up with 6 waypoints. I never do that. If a leg of my route is a straight line I dont use intermediate waypoints. Am I missing something here?
<hr width=100% size=1>
RupertW
Well-Known Member
Numbers of waypoints
At the risk of starting a whole new thread, I'm also intrigued by the use of a whole number of waypoints when there are no obstacles or turning points in the way.
What sort of factors help you choose your intermediate waypoints? On an Atlantic crossing last year we just set one waypoint, our destination Carribean island, and used CMG as our main (but not only) indicator.
<hr width=100% size=1>
At the risk of starting a whole new thread, I'm also intrigued by the use of a whole number of waypoints when there are no obstacles or turning points in the way.
What sort of factors help you choose your intermediate waypoints? On an Atlantic crossing last year we just set one waypoint, our destination Carribean island, and used CMG as our main (but not only) indicator.
<hr width=100% size=1>
stuartw
Well-Known Member
Re: MoBo agreeing with yotties I\'m afraid
Turncoat! I'll have you drummed off the MBC.
Joking aside, as regards multiple waypoints, for me they have no practicle value whatsoever. It is purely a comfort thing, so that I can see progress in roughly 1 hour lumps. For me viewing your end waypoint, with still say 45 miles to go, is not that comforting. Thats all.
Now to the nub of the question. I am afraid you and a few others have diverged from the original question, which was, and I paraphrase " Is it possible to go from A to B, guided by the Autopilot". I answered that it most certainly was. Now unless I am mistaken, I think most people agree it was. I at no point did ever suggest, that minimising XTE resulted in the quickest passage time. All I have ever said, it is the easiest and on occasions may well be effecient. Also, as had been mentioned, racing is another ball game altogether.
To get your idealised line as you require, you should as I understand, fix a constant heading on the AP, ie magnetic. Then you hope the tidal stream takes you one way then back the other - yes? And if you're lucky and the tidal streams do as predicted, then you may arrive on target. Excuse me for being a little sceptical, but this still seems the idealised situation. Be honest how many times have you set an initial course, with the relevent offset, and never had to change it?
By the way I am intrigued as to why the RYA teach you to use the zero XTE method. What do you and the others know, that they don't? - just a thought
<hr width=100% size=1>
Turncoat! I'll have you drummed off the MBC.
Joking aside, as regards multiple waypoints, for me they have no practicle value whatsoever. It is purely a comfort thing, so that I can see progress in roughly 1 hour lumps. For me viewing your end waypoint, with still say 45 miles to go, is not that comforting. Thats all.
Now to the nub of the question. I am afraid you and a few others have diverged from the original question, which was, and I paraphrase " Is it possible to go from A to B, guided by the Autopilot". I answered that it most certainly was. Now unless I am mistaken, I think most people agree it was. I at no point did ever suggest, that minimising XTE resulted in the quickest passage time. All I have ever said, it is the easiest and on occasions may well be effecient. Also, as had been mentioned, racing is another ball game altogether.
To get your idealised line as you require, you should as I understand, fix a constant heading on the AP, ie magnetic. Then you hope the tidal stream takes you one way then back the other - yes? And if you're lucky and the tidal streams do as predicted, then you may arrive on target. Excuse me for being a little sceptical, but this still seems the idealised situation. Be honest how many times have you set an initial course, with the relevent offset, and never had to change it?
By the way I am intrigued as to why the RYA teach you to use the zero XTE method. What do you and the others know, that they don't? - just a thought
<hr width=100% size=1>
NigeCh
Well-Known Member
Re: Ssssshhhh ....
Tom Cunliffe explains it on pages 57 & 61 of this month's (Nov) issue of YM.
(Going back to sleep now ...)
<hr width=100% size=1>
Tom Cunliffe explains it on pages 57 & 61 of this month's (Nov) issue of YM.
(Going back to sleep now ...)<hr width=100% size=1>
jfm
Well-Known Member
x channel race offer
OK then. I read your posts as disagreeing the sine wave theory on first principles. We're saying the same thing I think. I entirely agree that a real passage would of course take into account more factors than we can put up here on a BB, that part of the discussion was on the pure principle of this point.
Re your second last para, I dont agree that lack of 100% precision means you shouldn't bother. You are right that a calc of a constant heading to point the boat at will never be perfect. But on a x channel 12 hour passage you are better to make the best estimates you can (assuming you know the theory) and navigate the boat to that. Then monitor the GPS to see how much error you are developing, and fine tune as you go. This is much better than letting your a/pilot drive the boat at zero xte, because that is guaranteed to cause you about 10% extra distance through the water, which is a figure you can beat using the sine wave method even if your calcs are not perfect. I can promise you that if you race x-channel against Ken and me in 100% identical sailboats, everything identical except you use zero XTE and we use the sine wave, even with rough maths done with pencil paper and our heads, we will get to Le Harve before you. And I've never even met Ken!
As for the RYA, I dont believe an official body to be right just because they're an official body. The RYA are plain wrong in the way they teach this and Twister Ken et al above are correct. The fact that the constant heading/sine wave isn't used too often in practice is no excuse for actually teaching the wrong analysis of first principles imho. And if you want a list of RYA's other errors and shortcomings, we need to start a whole new thread.
On the intermediate waypoints, thanks for comment. I guess it's down to preferences. I prefer never to use intermediate points because this ensures the TTG, distance to go, ETA and velocity readings refer to a significant point, ie the next real waypoint. If you use intermediate waypoints this data becomes kind of meaningless..... imho
Stuff will be in post tomorrow, best wishes
OK then. I read your posts as disagreeing the sine wave theory on first principles. We're saying the same thing I think. I entirely agree that a real passage would of course take into account more factors than we can put up here on a BB, that part of the discussion was on the pure principle of this point.
Re your second last para, I dont agree that lack of 100% precision means you shouldn't bother. You are right that a calc of a constant heading to point the boat at will never be perfect. But on a x channel 12 hour passage you are better to make the best estimates you can (assuming you know the theory) and navigate the boat to that. Then monitor the GPS to see how much error you are developing, and fine tune as you go. This is much better than letting your a/pilot drive the boat at zero xte, because that is guaranteed to cause you about 10% extra distance through the water, which is a figure you can beat using the sine wave method even if your calcs are not perfect. I can promise you that if you race x-channel against Ken and me in 100% identical sailboats, everything identical except you use zero XTE and we use the sine wave, even with rough maths done with pencil paper and our heads, we will get to Le Harve before you. And I've never even met Ken!
As for the RYA, I dont believe an official body to be right just because they're an official body. The RYA are plain wrong in the way they teach this and Twister Ken et al above are correct. The fact that the constant heading/sine wave isn't used too often in practice is no excuse for actually teaching the wrong analysis of first principles imho. And if you want a list of RYA's other errors and shortcomings, we need to start a whole new thread.
On the intermediate waypoints, thanks for comment. I guess it's down to preferences. I prefer never to use intermediate points because this ensures the TTG, distance to go, ETA and velocity readings refer to a significant point, ie the next real waypoint. If you use intermediate waypoints this data becomes kind of meaningless..... imho
Stuff will be in post tomorrow, best wishes
peterb
Well-Known Member
Multiple waypoints
I will sometimes set up a route using multiple waypoints for a trip like the Solent-Cherbourg one, but more because I enjoy navigating than anything else.
I start by plotting a course using total tide over the run. Then, given this course through the water, I plot the snake-like progress that should be made over the ground (start at slack tide, banana shaped; start at peak stream, S-shaped). Put in waypoints at every hour, and connect them as a route. If everything goes right, then the boat will follow this route (with automatic or manual changes of leg at each waypoint on the hour) keeping a steady heading. If, however, things start to go wrong (perhaps through wind changes or the tide not behaving according to prediction) then I'll get warning by the heading changing or the leg changes not coming up at hour intervals. Then I can either be lazy, in which case the autopilot will continue to follow my waypoints and I'll get there but not at optimum speed, or I can enjoy myself by replotting and modifying the waypoints. In practice, unless there are major changes en route, being lazy rarely loses more than 2-3 minutes.
<hr width=100% size=1>
I will sometimes set up a route using multiple waypoints for a trip like the Solent-Cherbourg one, but more because I enjoy navigating than anything else.
I start by plotting a course using total tide over the run. Then, given this course through the water, I plot the snake-like progress that should be made over the ground (start at slack tide, banana shaped; start at peak stream, S-shaped). Put in waypoints at every hour, and connect them as a route. If everything goes right, then the boat will follow this route (with automatic or manual changes of leg at each waypoint on the hour) keeping a steady heading. If, however, things start to go wrong (perhaps through wind changes or the tide not behaving according to prediction) then I'll get warning by the heading changing or the leg changes not coming up at hour intervals. Then I can either be lazy, in which case the autopilot will continue to follow my waypoints and I'll get there but not at optimum speed, or I can enjoy myself by replotting and modifying the waypoints. In practice, unless there are major changes en route, being lazy rarely loses more than 2-3 minutes.
<hr width=100% size=1>
RupertW
Well-Known Member
Re: MoBo agreeing with yotties I\'m afraid
I hope everyone agrees with you that the zero XTE approach gets you there - it must do. Zero-XTE is also safer if you don't have a good feel for where the S-curve will take you as you wind your way over the ground towards your goal. It just takes a little longer.
Offsetting for the tides has never worked perfectly for me and I've always had to adjust a few hours before arrival, but I've often adjusted by very little.
I'm a bit surprised that the RYA teach zero-XTE as the recommended way. I finally did the RYA courses 18 months ago and they taught the way that I expected - they suggested offsetting for the tides. They did suggest zero-XTE as another option where boat speeds were much greater than tidal speeds, where obstacles lay on either side of the course, OR if you didn't feel up to navigation calculations at sea.
<hr width=100% size=1>
I hope everyone agrees with you that the zero XTE approach gets you there - it must do. Zero-XTE is also safer if you don't have a good feel for where the S-curve will take you as you wind your way over the ground towards your goal. It just takes a little longer.
Offsetting for the tides has never worked perfectly for me and I've always had to adjust a few hours before arrival, but I've often adjusted by very little.
I'm a bit surprised that the RYA teach zero-XTE as the recommended way. I finally did the RYA courses 18 months ago and they taught the way that I expected - they suggested offsetting for the tides. They did suggest zero-XTE as another option where boat speeds were much greater than tidal speeds, where obstacles lay on either side of the course, OR if you didn't feel up to navigation calculations at sea.
<hr width=100% size=1>
jfm
Well-Known Member
Re: MoBo agreeing with yotties I\'m afraid
Rupert - I might then have blamed the RYA unfairly, in which case I apologise. It might vary by instructore. My instructor was adamant that you should do separate vector calcs for say every hour and adjust the heading as the passage progresses, in order to follow a straight line groundtrack. When I told him that meant a longer journey through the water he disagreed and said his way was official RYA! Maybe he was wrong. (Come to think of it he didn't understand apparent wind either).
<hr width=100% size=1>
Rupert - I might then have blamed the RYA unfairly, in which case I apologise. It might vary by instructore. My instructor was adamant that you should do separate vector calcs for say every hour and adjust the heading as the passage progresses, in order to follow a straight line groundtrack. When I told him that meant a longer journey through the water he disagreed and said his way was official RYA! Maybe he was wrong. (Come to think of it he didn't understand apparent wind either).
<hr width=100% size=1>
qsiv
Well-Known Member
Re: Freudian slip.
Arrgh! We've had a powere cut, and my web server has rebooted with a different IP - I will try and sort them out, but time is short as I have to fly to UK/Lymington to bring the boat back home, so I may run out of time....
<hr width=100% size=1>
Arrgh! We've had a powere cut, and my web server has rebooted with a different IP - I will try and sort them out, but time is short as I have to fly to UK/Lymington to bring the boat back home, so I may run out of time....
<hr width=100% size=1>
Rich_F
Well-Known Member
Re: MoBo agreeing with yotties I\'m afraid
Hang on, we're all forgetting that the zero XTE approach will make us break the Colregs! If we're maintaining zero XTE while crossing a TSS in a strong cross tide, the autopilot will not maintain a heading directly across the shipping lanes, but instead point the boat at an angle.
Whereas, if we maintain a constant heading (taking tide into account), and also ensure that we're on starboard tack, then we can be safe in the knowledge that there's nothing to worry about, and go below for a well-earned nap after all those tidal calcs. /forums/images/icons/wink.gif
Ducking for cover,
Rich
<hr width=100% size=1>
Hang on, we're all forgetting that the zero XTE approach will make us break the Colregs! If we're maintaining zero XTE while crossing a TSS in a strong cross tide, the autopilot will not maintain a heading directly across the shipping lanes, but instead point the boat at an angle.
Whereas, if we maintain a constant heading (taking tide into account), and also ensure that we're on starboard tack, then we can be safe in the knowledge that there's nothing to worry about, and go below for a well-earned nap after all those tidal calcs. /forums/images/icons/wink.gif
Ducking for cover,
Rich
<hr width=100% size=1>
stuartw
Well-Known Member
Rubbish!
I am sorry, that is typical single waypoint thinking. You set an additional waypoint to get across in the safest way possible. And lets face it, how many boats go across shipping lanes exactly at right angles. Even the bloody ferries don't, or do they a divine dispensation?
<hr width=100% size=1>
I am sorry, that is typical single waypoint thinking. You set an additional waypoint to get across in the safest way possible. And lets face it, how many boats go across shipping lanes exactly at right angles. Even the bloody ferries don't, or do they a divine dispensation?
<hr width=100% size=1>
stuartw
Well-Known Member
Thank you
At last, someone who is prepared to let the AP assist. It may not be pretty, but it's safe.
<hr width=100% size=1>
At last, someone who is prepared to let the AP assist. It may not be pretty, but it's safe.
<hr width=100% size=1>
Rich_F
Well-Known Member
Re: Rubbish - NOT!
Yes, single waypoint thinking, but we're tallking about using the auto-helm to get us across using XTE, which implies using a single waypoint. By the time you've put in three or five waypoints, you might as well have calculated the tide and done the job properly.
And if you want to get across the lane using waypoints, you'll have to take the tidal flow into account. Which will probably be no easier to work out than doing a 12-hour tidal calculation!
I agree that the Colregs point is a bit nit-picky. The main points are that using XTE on a crossing like this results in a) A slower passage (40 mins?), b) Additional changes to point of sail, possibly forcing you to beat, and c) A saving of about 2 minutes when planning the passage.
I know which I'd choose, but each to his/her own.
Rich
<hr width=100% size=1>
Yes, single waypoint thinking, but we're tallking about using the auto-helm to get us across using XTE, which implies using a single waypoint. By the time you've put in three or five waypoints, you might as well have calculated the tide and done the job properly.
And if you want to get across the lane using waypoints, you'll have to take the tidal flow into account. Which will probably be no easier to work out than doing a 12-hour tidal calculation!
I agree that the Colregs point is a bit nit-picky. The main points are that using XTE on a crossing like this results in a) A slower passage (40 mins?), b) Additional changes to point of sail, possibly forcing you to beat, and c) A saving of about 2 minutes when planning the passage.
I know which I'd choose, but each to his/her own.
Rich
<hr width=100% size=1>
bedouin
Well-Known Member
Re: MoBo agreeing with yotties I\'m afraid
I did my RYA training in the days before they admitted that GPS existed!
However I think one of the reasons RYA may teach zero XTE is that it is safe. That way the boat follows a straight line ground track (i.e. the one you should have drawn on the chart) so should not hit anything unexpected
Any method that relies on BTW only is inherently dangerous in that you know where you are going, but not where you are.
So if all you want to do is set the route and press the button then I agree with stuartw - the zero XTE method is the only safe way to do it.
However as I sail a small sailing boat the extra hour or two that would add to the crossing is too long, so I prefer the classic "fixed compass course and reassess 75% of the way across". At that point a surprising degree of error can be accommodated without adding significantly to the journey.
I do sometimes use the zero XTE method for getting into harbour with a strong cross current.
<hr width=100% size=1>
I did my RYA training in the days before they admitted that GPS existed!
However I think one of the reasons RYA may teach zero XTE is that it is safe. That way the boat follows a straight line ground track (i.e. the one you should have drawn on the chart) so should not hit anything unexpected
Any method that relies on BTW only is inherently dangerous in that you know where you are going, but not where you are.
So if all you want to do is set the route and press the button then I agree with stuartw - the zero XTE method is the only safe way to do it.
However as I sail a small sailing boat the extra hour or two that would add to the crossing is too long, so I prefer the classic "fixed compass course and reassess 75% of the way across". At that point a surprising degree of error can be accommodated without adding significantly to the journey.
I do sometimes use the zero XTE method for getting into harbour with a strong cross current.
<hr width=100% size=1>
stuartw
Well-Known Member
There is light at the end of the tunnel!
Do I take it then, there are schools of thought that suggest that the zero -XTE method is the more precise and safer option? Interesting.
I certainly am prepared to accept in ideal conditions, the sinewave approach has got to work. But, as I keep pointing out, and so do others, that in non-ideal conditions, where re-calculation of bearing is being done say ever 30 mins, then the XTE option has got to be better, unless of course you're a masochist.
<hr width=100% size=1>
Do I take it then, there are schools of thought that suggest that the zero -XTE method is the more precise and safer option? Interesting.
I certainly am prepared to accept in ideal conditions, the sinewave approach has got to work. But, as I keep pointing out, and so do others, that in non-ideal conditions, where re-calculation of bearing is being done say ever 30 mins, then the XTE option has got to be better, unless of course you're a masochist.
<hr width=100% size=1>
RupertW
Well-Known Member
Re: There is light at the end of the tunnel!
Hang on a second. Zero-XTE is more precise, obviously, but at the expense of extra time, so if you don't need to thread a needle of real obstacles, then that precision adds nothing.
More importantly, though, Zero-XTE is safer in the same way that it would be safer to call an electrician to change a light-bulb than to risk doing it yourself. It depends on your understanding and confidence. I think you choose your own risk-level.
For me, going to sea in a single-engined boat without sails would be too risky, but then one thing my electrical engineering background taught me was to always have a backup to cope with any single failure.
<hr width=100% size=1>
Hang on a second. Zero-XTE is more precise, obviously, but at the expense of extra time, so if you don't need to thread a needle of real obstacles, then that precision adds nothing.
More importantly, though, Zero-XTE is safer in the same way that it would be safer to call an electrician to change a light-bulb than to risk doing it yourself. It depends on your understanding and confidence. I think you choose your own risk-level.
For me, going to sea in a single-engined boat without sails would be too risky, but then one thing my electrical engineering background taught me was to always have a backup to cope with any single failure.
<hr width=100% size=1>
