GPS/autopilot handling of cross-tide

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JimC

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I have interfaced my GPS/plotter to my autopilot. I tried the setup whilst crossing Morecambe Bay from the Barrow side. I know from experience that to find the Lune No 1 buoy one has to steer about 140 degrees to allow for the incoming tide, though the buoy actually bears 106 degrees. I put a GOTO onto the buoy and engaged NAV on the autopilot. The LED lit confirming that it was receiving NMEA but, to my disappointment, all it did was put the boat onto a 106 degree heading straight for the buoy. This would have put me ashore somewhere near Heysham so I had to disengage it. I should have thought the system would have monitored the XTE and adjusted the heading to maintain the correct COG.

The GPS/plotter is a Standard-Horizon and the autopilot a Simrad, interfaced via their NMEA connections. Any suggestions ?
 
Yes - I agree with this. We would normally be making fairly constant changes when sailing in strong cross-tides but the autopilot/gps doesn't seem to correct so quickly or constantly.
I used to aim at Blackpool Tower, sailing roughly along the 3degree line if I remember correctly - which was the course I'd calculated would land me somewhere near Danger Patch on an incoming tide and then just switch the autopilot on. It worked fine but then I'd done all the calculating!
 
There are various formulae that GPS's and A/Ps use for handling cross track error. A simple one is to steer the boat toward a point on the original rumb line measured between the position on the rumb line perpendicular from the current position and the destination waypoint. This means that the amount of correction depends on the amount of deviation and also the distance left to travel. There are other, more sophisticated methods but they all rely on detecting that the boat has deviated from the required course. This means that, as you found in your case, they are never as good as the human navigator who predicts the effects of the tide and sets a course accordingly.
 
Agreed. Some autohelms seem to need to develop a cross-track error (typically around 0.02 mile or so) before they put you on a rhumb line (which can be especially infuriating if sailing close-winded). A correctly set-up system should have put you on the correct course within a few minutes and kept you there within 40 metres or less. Of course that's not to say it would necessarily put you on the quickest route on longer passages with cross-tides varying over time. Incidentally, bear in mind that the autopilot is dependant on the GPS for X-track error, and the GPS has not the vaguest notion of which way the boat is pointing.
 
In some setups I have found the 'Follow track' function doesn't work on a simple 'Go To', it will only work between two waypoints. Current position is usually seen as Wpt '0' and ignored, so you need first to set a 'route' between two Wpts that gives you the required ground track. What then happens is that the pilot initially steers the boat on the direct course for the next Wpt, in this case your destination and I think you said about 106 degsM, it takes this information from the fluxgate compass in the pilot. Then, if the tide is causing he boat to go off track, it will tweak the course to return to the track by using the cross track error from the GPS/Plotter, however it doesn't make 90 deg turns to instantly correct the side movement (imagine the consequences!) but applies a gradual adjustment over a few minutes.

We have a Raymarine 7000 pilot and GPS data either from a Raymarine GPS/Plotter or a Navman plotter and previously on another boat had a different Raymarine pilot fed from a variety of GPSs. In all cases we have been able to use the 'Track' function to closely follow a required groundtrack, but always between 2 Wpts in a route not as a simple 'Go To'.

We have several complex 'routes' set where cross tides are a significant feature and fog often a factor and have pre-tested these using 'track' on many occasions in good visibility, with an occasional slight tweak of a Wpt position resulting. An example would be the route down and through Chenal Du Four between Ushant and the mainland of France. We have followed these routes using 'track' enough times to be fairly confident of using them if we had to in fog, bearing in mind we also have radar and chart plotter running at the same time.
 
I use a Garmin GPS72 + Raymarine AP2000+. If I've set up the GPS route and don't follow it immediately it's possible to build up quite a cross track error - few 100s of meters is large in GPS terms. If I then set the autopolit to track, it will try to turn to intercept the original track and reduce the XTE to zero which can involve quite a course change. Therefore it's better to use a simple "goto" from the current position. This then resets XTE to zero and the tracks correctly once it learns what the drift is.

Track works OK where the passage is short. i.e. if the tide doesn't change much during the passage. You'll always head into the tide which will slow you down. If for instance the passage is 12 hours long the tide may come close to cancelling over the period. If you have several hours of tide to contend with then it will be better to either:

1. Work it out the old fashioned way and just set a course to steer. Then do a quick "goto" as you get close to the destination.

or

2. Work it out the old fashioned way and then work out where you should be at regular intervals - say every hour. Set those up as intermediate way points.

Just setting up to follow the rhumb line will cause the boat to head into the tide all the time and slow you down. If you have smallish slow boat like me then it can make quite a difference.

Of course do remember the guys who set up a track to Cowes a few years ago going straight across the Solent. They set up the autopilot while they had lunch. Lunch time was extended rather a long time while admiring the Brambles Bank.
 
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I use a Garmin GPS72 + Raymarine AP2000+. If I've set up the GPS route and don't follow it immediately it's possible to build up quite a cross track error - few 100s of meters is large in GPS terms. If I then set the autopolit to track, it will try to turn to intercept the original track and reduce the XTE to zero which can involve quite a course change. Therefore it's better to use a simple "goto" from the current position. This then resets XTE to zero and the tracks correctly once it learns what the drift is.

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The Raymarine ST4000 I had on my old boat and the current ST7000 pilots both would not make a sudden turn without an 'acknowledgement' from the crew following beeped warnings of a required large alteration of course, they do the same with a required course change as a Wpt in a route is reached. I agree the GoTo is a simpler if the GoTo if the pilot will track it but as I said in the previous post, some will apparently not recognise the start position as a point on a ground track that they can follow, maybe it was a software thing updated on later versions.

'Track' is a useful aid on occasions but I certainly wouldn't use it for normal passages. As you rightly say it is the slow way and never really possible on a raggie under sail because of the wind angles contantly changing though I suspect mobos use it more. As I said, we use it occasionally to follow routes where it is essential not to stray to far off the groundtrack, like the route through Chenal du Four I mentioned where the tides are strong and can shift from one side to the other side as the track changes as Wpts are passed.

I have used 'track' to cross large bays, like Lyme Bay which is about 42mls I think. There is a 'set' into the bay between Portland Bill and Start Point and if on a free wind or motoring it is easier to put it on 'track', even though over the years I do know the approximate compensation to the straight course to apply.

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Of course do remember the guys who set up a track to Cowes a few years ago going straight across the Solent. They set up the autopilot while they had lunch. Lunch time was extended rather a long time while admiring the Brambles Bank.

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Sounds like a Scumsail special! That however is a reason I'm not a fan of 'GoTos' as it is easy to forget what is in the way. If you plot a route, either from the chart and entered into the GPS or on a chart plotter you can see that some twit left a bit of sand in the way that you need to avoid. It is a good reason too for making up and recording (either electronically or in a notebook as a sequence of WPTs) regularly used routes that can be instantly recalled, there is less risk of a GoTo mishap.

I don't use GoTos even at the end of a crosstide passage say Poole-Cherbourg. It is simple enough to tweak the pilot course if needed to match CMG to BTW over the last mile or two. However, if we have been on a calm trip with a free wind or motoring I would expect not to need to tweak the course at all until, otherwise I want to know why I got the predicted course wrong! Of course it could have been SWMBO's turn for the plan or we used the computer one, if we have done it all 3 ways they rarely agree! /forums/images/graemlins/smile.gif
 
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Of course it could have been SWMBO's turn for the plan or we used the computer one, if we have done it all 3 ways they rarely agree!

[/ QUOTE ]Which computer one do you use that didn't agree with your (or SWMBO's) manual plan? Not CCX I hope! /forums/images/graemlins/smile.gif
 
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Which computer one do you use that didn't agree with your (or SWMBO's) manual plan? Not CCX I hope!

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/forums/images/graemlins/smile.gif No we use Neptune both the original and the C-Map version.

Over many years of restocking the booze locker I made out a pre-printed form for rapid calculation of a CTS having manually collected and interpolated the tide streams, from the Stanfords Tide Atlas rather than chart diamonds. For crossings at close to 90 degs like Poole-Cherbourg it would be a simple plus/minus calculation to give the expected offset and a simple '8mls = 8 degs' difference from the straight course for the 60 mls, useful short cut! In addition I also have an old Casio computer that is programmed with all sorts from Astro to CTS programs gleaned when PBO published them in Basic centuries ago. The tide program I put in this will handle non-90 deg crossings and instances not easily interpolated from the '1 deg at 60 mls is 1 mile' rule!

The errors I suspect creep in because a) Inshore tides rarely match the diamonds or the atlases, especially as the tide turns b) the tide never 'turns' from one way to the other directly, it comes round in a circle, rather than simply drops/stops/turns/runs again c) Atmospheric pressure, seems to influence either the rate of flow or the time of reversal or both. for example a high pressure in the eastern Channel seems to slow the flood rate and increase the ebb rate or delay the changeover from ebb to flood and bring forward the one from flood to ebb. This is my excuse for when we are set too far west! Joking apart it is something I believe happens. Other factors are of course are leeway or no leeway, surface drift, speed variations and that is without compass, autopilot or helmsman error!

All good fun to pass a boring day at work or a long motor with no wind..

Your program looks very useful I'm sure many on here will find it very helpful.

Robin
 
We left the autopilot to it for about 10 minutes, during which time we were deviating further and further from the required COG. i.e. the XTE was steadily increasing. Next time I'll try it as a route between two pre-programmed waypoints rather than a simple goto.
 
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We left the autopilot to it for about 10 minutes, during which time we were deviating further and further from the required COG. i.e. the XTE was steadily increasing.

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That to me confirms that all it was doing was reacting to the changing BTW and following the BTW heading, not compensating for the tide at all by reference to the changing and increasing XTE. So it seems that your setup like ours will work only with a route between two WPTs not a GoTo where WPT '0' is the start position. Let me know if I got it right!

Robin
 
Robin, I think what you're saying is absolutely right in as much as if you just use a GOTO then the XTE starts at 0 and then may increase as the boat moves away from the rumb line - whereas if you are already following a leg of a route then you probably already have a significant XTE so the A/P can react straight away.

JimC, if you can, check that the GPS is supplying the RMB sentence in its NMEA output. Autopilots will track with just the XTE sentence but may take longer to get 'sorted out' as they don't have any BTW information to compare to the current heading (they just assume the boat is pointing at the wp initially!).
 
Thanks Danny - I'll investigate that - though when I don't know as next weekend is neaps and I'll be lucky to get out of the Lune at all - also weather up here is crap.

Jim
 
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