autohelm following chart plotter to waypoint - how does it work?

[Burnham Bob]

My ST100 will follow the NMEA output of my chart plotter to the waypoint the plotter is heading for. I'm just curious as to how it does it. Assume a 10 mile trip at 5 knots and the waypoint is due east and the tide is flowing due south. If the autohelm heads due east, it will be pushed south and the heading will have to be increased as i sail along to include a northerly element so the boat will actually sail the 10 miles in an arc with the final bit having a strong northerly element to make up for the tide - so that's a long way round - more than the two hours a 'course to steer' worked out from tidal vectors would take.

If on the other hand, the autohelm steers to the required track - ie the waypoint always remains due east then I'll sail a straight line and arrive sooner - the plotter is all the time compensating for the southerly tidal movement.

So does the autohelm simply point the boat at the waypoint regardless of the tidal drift or does it steer the required track?

This is mainly academic because my inclination would be to plan a course to steer and feed that into the autohelm as that seems the safest and most obvious way to go. If the autohelm simply keeps the boat pointed to the waypoint, then the actual course over the ground could take me into hazards along the way.

Please would someone more clever than me advise me. The manuals don't really help.

 

[sailorman]

My ST100 will follow the NMEA output of my chart plotter to the waypoint the plotter is heading for. I'm just curious as to how it does it. Assume a 10 mile trip at 5 knots and the waypoint is due east and the tide is flowing due south. If the autohelm heads due east, it will be pushed south and the heading will have to be increased as i sail along to include a northerly element so the boat will actually sail the 10 miles in an arc with the final bit having a strong northerly element to make up for the tide - so that's a long way round - more than the two hours a 'course to steer' worked out from tidal vectors would take.

If on the other hand, the autohelm steers to the required track - ie the waypoint always remains due east then I'll sail a straight line and arrive sooner - the plotter is all the time compensating for the southerly tidal movement.

So does the autohelm simply point the boat at the waypoint regardless of the tidal drift or does it steer the required track?

This is mainly academic because my inclination would be to plan a course to steer and feed that into the autohelm as that seems the safest and most obvious way to go. If the autohelm simply keeps the boat pointed to the waypoint, then the actual course over the ground could take me into hazards along the way.

Please would someone more clever than me advise me. The manuals don't really help.

the A/h will always allow for x track at the present time ,so will constantly change the heading to adjust the course so as to end up at the way point

 

[earlybird]

I don't auto-steer to a GPS waypoint that often, but when I do, the AH seems to adjust the course to keep cross track error to zero, ie steer a rhumb line.

 

[CreakyDecks]

I would guess that it will have some sort of PID control algorithm (proportional integral derivative). The integral part sums the error each time a calculation is made and then adds extra control effort. The tidal flow would manifest itself as a constantly increasing error sum. (Pure guess but that's how I'd do it!)

 

[Red Admiral]

The tiller pilot will attempt to follow the direct line over the ground. From the NMEA cross track error signal and the instruction from the GPS of course to steer it will compensate for cross tidal drift or leeway. However, if you have a change of cross tide during this passage (eg crossing the channel) the direct course over the ground might not be the quickest passage.

 

[sailorman]

The tiller pilot will attempt to follow the direct line over the ground. From the NMEA cross track error signal and the instruction from the GPS of course to steer it will compensate for cross tidal drift or leeway. However, if you have a change of cross tide during this passage (eg crossing the channel) the direct course over the ground might not be the quickest passage.

(eg crossing the channel) the direct course over the ground might not be the quickest passage.
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Will Not is the answer

 

[Simondjuk]

Allowing the auto-pilot to steer to the plotter's ground track will cost you time due a continual element of tide stemming, which in instances of a turn of tide during the leg is entirely unnecessary.

 

[David2452]

I really depens on how you set it, if you simlply "go to waypoint" then your boat will behave like it is on a piece of string connected to the waypiont i.e. always steering at the waypiont but being swept sideways off track.
If you "follow track" then it will simply follow the track over the ground, ""crabbing" as needed.
Neither is especially efficient, though the latter is safer on a plotted course where there may be obstructions or shoals either side of the proposed track.

 

[Burnham Bob]

thanks - went online and found an updated manual which explains a little bit more

 

[Stu Jackson]

Bob, if you could post a link to that manual it would be helpful.

If you have a cross tide and set the AP to go to a WP as you proposed, the course will be a big sag with, as you said, a heavy N at the end.

The AP does NOT know what the current is doing. In fact, I know of no AP that CAN do that. If so, and one exists for recreational boating, I would appreciate being enlightened.

How do I know this? I performed an experiment for pretty much just the conditions Bob mentioned in his OP: heading east with a S flowing current, exactly the same concept.

UNLESS you set a separate secondary NEW waypoint to COMPENSATE for the current, BEFORE you make "the crossing" to the waypoint you want, just like in the "old days" of basic piloting, the current will simply push the boat off course and the AP can only compensate for it.

You will NOT sail or motor a rhumb line. It'll be a loopy curve all the time.

 

[timchapman]

If you were to set a course across the channel and set the autopilot on 'track'. The resultant distant the boat will travel through the water will be significantly further than the distance it will travel with a correctly calculated CTS, hence taking longer.

Learnt that the hard way!

 

[Robin]

Bob, if you could post a link to that manual it would be helpful.

If you have a cross tide and set the AP to go to a WP as you proposed, the course will be a big sag with, as you said, a heavy N at the end.

The AP does NOT know what the current is doing. In fact, I know of no AP that CAN do that. If so, and one exists for recreational boating, I would appreciate being enlightened.

How do I know this? I performed an experiment for pretty much just the conditions Bob mentioned in his OP: heading east with a S flowing current, exactly the same concept.

UNLESS you set a separate secondary NEW waypoint to COMPENSATE for the current, BEFORE you make "the crossing" to the waypoint you want, just like in the "old days" of basic piloting, the current will simply push the boat off course and the AP can only compensate for it.

You will NOT sail or motor a rhumb line. It'll be a loopy curve all the time.

If you have an autopilot linked to a GPS or GPS Plotter and set the pilot to ‘track’, it needs to have two WPTs in order to designate the track. The usual method I believe if you are simply going from the current position to a single waypoint, is that when you press the ‘track’ button the GPS/Plotter will allocate the current position as waypoint ‘0’, thus forming a track beteen it and the destination waypoint. The pilot will then follow the track by maintaining a zero XTE (cross track error), by heading adjustments to stem any deviation from the track from tide or current. Some older GPS or Plotters did not set the current position to WPT ‘0’ and just pointed the heading at the destination WPT, which in a cross tide would result in a great looping curve type ground track. I had an old Autohelm 10” plotter that did that (just pointed at the destination), but later versions of the same unit after Raymarine took over did set current position as WPT ‘0’, as did my two later Navman sets and would set up a track to follow. The old set would follow a track if it had TWO waypoints already set as a route, the problem was only if you simply wanted to set a ‘go to’ and simply asked the pilot to do so. If that makes sense!

PS added. Following a simple ground track in a tidal situation is not good, unless you really need to stay on that track, perhaps because of dangers of straying from it. It is a useful function for following a channel in cross tides, but useless in crossing the Channel when pre-plotting and calculating an overall CTS is the best way every time.

 

[jtwebb]

As I seek simplicity I plan a CTS allowing for varying tides to a point short of my destination, eg Cherbourg. My GPS will actually have a route loaded so at a suitable time out I go to track mode and let the autopilot take me in monitoring what it is doing on my Seapro system. Very lazy and a bit different to first crossings where one calculated carefully, plotted carefully and when it got foggy played with a Seafix. Still got mine!

 

[Stu Jackson]

Thanks, subtle but important difference: an AP alone won't. but linked to a GPS it can. Thanks again.

 

[Burnham Bob]

I inherited the boat and manual was missing so I downloaded a manual, but that seems to have been for the ST1000, the manual for the ST1000+ is much more informative and has the following

In Track mode, the tiller pilot maintains a track between waypoints created on a navigation system. The tiller pilot computes any course changes to keep your boat on track, automatically compensating for tidal streams and leeway.
To operate inTrack mode, the tiller pilot must receive cross track error information from either:
• a SeaTalk navigator, or
• a non-SeaTalk navigation system transmitting NMEA 0180 or 0183 data
Under most operating conditions, Track mode will hold the selected track to within ±0.05 nm (300 ft) or better. The autopilot takes account of the boat’s speed when computing course changes to ensure optimum performance over a wide range of boat speeds. If speed data is available, the autopilot will use the measured boat speed. Otherwise it will use the default cruise speed entered in Calibration Level 5 (see page 62).

So if its holding a track - isn't that the same as a course to steer? surely all that a cts is, is a heading that ends you up at the waypoint by making sure your track is the right one needed to get to the waypoint regardless of what the tide is doing to you...

anyway, thanks for the advice! the longest i'll need to do is quick hop over the channel in the forseeable future so its not as if i'll end up miles from the Azores having failed to plot a correct cts!

 

[alan_d]

So if its holding a track - isn't that the same as a course to steer? surely all that a cts is, is a heading that ends you up at the waypoint by making sure your track is the right one needed to get to the waypoint regardless of what the tide is doing to you...

No.

Minimising the cross track error (XTE) keeps you on the rhumb line, i.e.the ground track to your destination, by correcting for the tide from moment to moment. A Course To Steer is arrived at by considering the overall effects of tide throughout the whole journey and allowing for that, and except for short journeys is more efficient.

Consider the case of a destination 12 miles due East of the start point, in a boat which will do 6 knots, with a 2 knot southgoing tide for the first hour of the journey and a 2 knot northgoing tide for the second hour. Your CTS is due East, and you will reach your destination in 2 hours. If you try and minimise XTE you will be heading (but not travelling) North of East for the first hour, so making less than 6 knots towards your destination, and similarly you will be heading South of East for the second hour, once again making less than 6 knots towards your destination. Thus a vessel sticking to the rhumb line will take longer to reach her destination.