How does the autopilot work?

[lustyd]

OK so I assumed I knew how the autopilot worked, as I'm sure do we all. Then I set one up and now I'm reading about great circles. Now I'm in one of those loops we all get into occasionally...

So I select a destination, for instance the Carribean, and I'm in the UK, and the plotter knows the great circle. How does it do the steering? I thought I knew that the plotter told the autopilot where the waypoint was and the AP steered to it. Then I realised probably not, and maybe the plotter just sends a course to steer, but then I realised probably not since I have to acknowledge the waypoint on the AP, not the course and with a GC that would need to be updated. So....does it just send out a heading constantly, or does it send an initial course and the boat would end up on a rhumb line?

All theoretical of course, given all the other considerations on a yacht like weather I can't imagine I'd ever use a great circle for long enough for it to make a difference, and certainly not with an electronic AP!

 

[AntarcticPilot]

That's a very interesting question, and the answer probably hinges on how the chart plotter works; in particular, what the internal coordinate system used by the chart plotter is.

Practically, the chart-plotter outputs cross-track error, which the autopilot uses to get back onto the track laid down by the chart plotter. The question is, what is the track that the chart-plotter is using? As most navigational charts use the Mercator projection, I'd guess that it is likely that the chart-plotter operates in Mercator coordinates, and therefore the track is the Rhumb line. Unless the internal coordinate system is spherical coordinates and the software is clever enough to do great circle calculations, I'd be a bit surprised if the track was a Great Circle.

As you say, of theoretical interest only - if anyone really wants to follow a Great Circle route, they'll put in waypoints computed at points along the Great Circle, but few sailing boats want to follow a Great Circle anyway - better to follow Trade-wind routes!

 

[newtothis]

OK so I assumed I knew how the autopilot worked, as I'm sure do we all. Then I set one up and now I'm reading about great circles. Now I'm in one of those loops we all get into occasionally...

If you're steering loops and circles, it's definitely a software issue.

 

[johnalison]

It would be easier to use traverse tables. You'd be just as unlikely to arrive at your destination.

 

[lustyd]

Practically, the chart-plotter outputs cross-track error, which the autopilot uses to get back onto the track laid down by the chart plotter.

So in theory would it just keep increasing the XTE to get the AP to steer the circle? In which case I guess the AP doesn't care what the course is, the XTE just works like the port/starboard buttons

 

[dunedin]

I think your batteries will run out long before you discover whether doing a great circle route or not

Actually, I am still struggling to see any valid reason to integrate my plotters to the autopilot. Whilst I am generally a gadget person, and lots of on board integration, I have that option disabled.
Unless aiming to send boat from one marina to another unattended whilst I drive the car there, I always want to set the autopilot on course, and ALWAYS want to decide when to change to next waypoint (just to give visual check that safe to alter course). Takes less than 5 seconds to alter course manually, and much safer.

 

[AntarcticPilot]

I think your batteries will run out long before you discover whether doing a great circle route or not

Actually, I am still struggling to see any valid reason to integrate my plotters to the autopilot. Whilst I am generally a gadget person, and lots of on board integration, I have that option disabled.
Unless aiming to send boat from one marina to another unattended whilst I drive the car there, I always want to set the autopilot on course, and ALWAYS want to decide when to change to next waypoint (just to give visual check that safe to alter course). Takes less than 5 seconds to alter course manually, and much safer.

On a long passage, I found the ability to follow a route set up on the chart-plotter invaluable. My chart-plotter requires a confirmation before changing course; it never changes course without human intervention, so your objection (in the case of my Raymarine C80) doesn't hold; I always have to confirm a change of course. It meant I could readily plan my passages on the chart-plotter, and then follow them as convenient when under way. Of course, things like interactions with other vessels meant that I often over-rode the pre-planned route, but that was simple to do and meant that I could easily get back on track afterwards.

 

[TernVI]

That's a very interesting question, and the answer probably hinges on how the chart plotter works; in particular, what the internal coordinate system used by the chart plotter is.

Practically, the chart-plotter outputs cross-track error, which the autopilot uses to get back onto the track laid down by the chart plotter. The question is, what is the track that the chart-plotter is using? As most navigational charts use the Mercator projection, I'd guess that it is likely that the chart-plotter operates in Mercator coordinates, and therefore the track is the Rhumb line. Unless the internal coordinate system is spherical coordinates and the software is clever enough to do great circle calculations, I'd be a bit surprised if the track was a Great Circle.

As you say, of theoretical interest only - if anyone really wants to follow a Great Circle route, they'll put in waypoints computed at points along the Great Circle, but few sailing boats want to follow a Great Circle anyway - better to follow Trade-wind routes!

Are you sure about that?

Firstly, some AP systems will simply adjust the course to match the bearing to the waypoint. They won't take the boat back to the original track.
Not saying that applies to all.

Secondly, even some quite ancient GPSs can do great circles.
Try putting in a waypoint on the same lattitude but with a much different longitude.

These things may be options buried under a few menus?

 

[DJE]

Actually, I am still struggling to see any valid reason to integrate my plotters to the autopilot. Whilst I am generally a gadget person, and lots of on board integration, I have that option disabled.
Unless aiming to send boat from one marina to another unattended whilst I drive the car there, I always want to set the autopilot on course,

I find it useful sometimes in strong cross tides to let the autopilot keep the boat on the intended track while I deal with something else.

 

[TernVI]

I find it useful sometimes in strong cross tides to let the autopilot keep the boat on the intended track while I deal with something else.

I'm more likely to be setting it to steer a course that takes me up tide a bit, so there's something in hand if the wind changes.
There are several ways of getting from A to B with a GPS etc, no one of them is always better than any other.
It can be good to know what options are available.

 

[pvb]

On a long passage, I found the ability to follow a route set up on the chart-plotter invaluable. My chart-plotter requires a confirmation before changing course; it never changes course without human intervention, so your objection (in the case of my Raymarine C80) doesn't hold; I always have to confirm a change of course. It meant I could readily plan my passages on the chart-plotter, and then follow them as convenient when under way. Of course, things like interactions with other vessels meant that I often over-rode the pre-planned route, but that was simple to do and meant that I could easily get back on track afterwards.

Agreed. I had a Raymarine C-series on my last boat, and I found the bleeping alarm at every waypoint to be a bit annoying. My current boat has a Garmin plotter and autopilot, and it will follow a route without any intervention from me.

 

[[163233]]

Does anyone actually set them up with such complex routes anyway? Even on longer trips I just get the boat on a course and review it every few hours. In essence just holding a bearing. I don't think I've ever contemplated setting up any kind of complex track for the autopilot to follow even if I've put one in the plotter for my own benefit.

Not that I've done anything where a great circle route would make any difference of course.

 

[pvb]

Does anyone actually set them up with such complex routes anyway? Even on longer trips I just get the boat on a course and review it every few hours. In essence just holding a bearing. I don't think I've ever contemplated setting up any kind of complex track for the autopilot to follow even if I've put one in the plotter for my own benefit.

Not that I've done anything where a great circle route would make any difference of course.

Even on coastal trips, setting up a route can be useful.

 

[TernVI]

....

Not that I've done anything where a great circle route would make any difference of course.

The only difference a great circle has made in my experience is a few miles difference in the 'distance to home' and a few degrees in 'bearing to waypoint', which was completely academic as it's either been a beat or we were heading off the direct route looking for more wind. In the second case, the autopilot stopped working anyway.

 

[AntarcticPilot]

Does anyone actually set them up with such complex routes anyway? Even on longer trips I just get the boat on a course and review it every few hours. In essence just holding a bearing. I don't think I've ever contemplated setting up any kind of complex track for the autopilot to follow even if I've put one in the plotter for my own benefit.

Not that I've done anything where a great circle route would make any difference of course.

I find it useful for passage planning over unfamiliar waters; I have used it extensively for that.

As you say, Great Circle routes are of academic interest for sailing boats. Over short distances (a few hundred miles) the difference is insignificant and over distances greater that 100 miles, it's unlikely that a vessel propelled by the wind could stay on one long enough to worry about.

Over oceanic distances, the Great Circle route is very unlikely to be the most effective sailing route - Atlantic crossers, for example, head south to pick up the trade winds, and then west. Even those heading for the eastern seaboard of the USA will usually find it faster to follow that route than the Great Circle route.

 

[mjcoon]

Secondly, even some quite ancient GPSs can do great circles.
Try putting in a waypoint on the same latitude but with a much different longitude.

Quite. My ancient (and barely still functional) Garmin GPS II+ not only has scores of built-in co-ordinate datums to choose from, it provides the ability to set a "user datum" by inputting five parameters. If you have looked at the maths involved in datum transformations you would not doubt all these devices capacity to calculate great circles! (NB datum choice is independent from choice of projection, which is a matter of display on a flat surface.)

 

[johnalison]

Does anyone actually set them up with such complex routes anyway? Even on longer trips I just get the boat on a course and review it every few hours. In essence just holding a bearing. I don't think I've ever contemplated setting up any kind of complex track for the autopilot to follow even if I've put one in the plotter for my own benefit.

Not that I've done anything where a great circle route would make any difference of course.

Yes. However complex the route I will normally set it up in advance. An average day’s sail will only take a few minutes with a modern plotter. One quick way of doing this is by setting up the start and finish, and then adding the intermediate points one at a time. Twenty waypoints could well be needed for a coastal trip. Whether the autopilot is used depends on various things, but I have the options of either asking it to follow the track, or just to set a course and adjust this as the leg proceeds, which of course is what I would do in a cross-tide leg.

 

[dunedin]

Even on coastal trips, setting up a route can be useful.

Yes, setting up a route on a chartplotter can be very helpful - eg sailing the Swedish rocky leads.
But that doesn’t mean you need to get the autopilot to alter course based on the route waypoints.

Manual change of course better:
- 5 seconds per waypoint to manually look around the alter course ain’t a big overhead, also
- it is generally better to set waypoints a safe distance off dangers - but if conditions benign and not busy, we will often curve the corner gently in multiple small steps, and often taking slight short cuts that are perfectly safe done under manual control but not under automatic control

 

[pvb]

Yes, setting up a route on a chartplotter can be very helpful - eg sailing the Swedish rocky leads.
But that doesn’t mean you need to get the autopilot to alter course based on the route waypoints.

Manual change of course better:
- 5 seconds per waypoint to manually look around the alter course ain’t a big overhead, also
- it is generally better to set waypoints a safe distance off dangers - but if conditions benign and not busy, we will often curve the corner gently in multiple small steps, and often taking slight short cuts that are perfectly safe done under manual control but not under automatic control

You can always change the course if the planned autopilot route looks wrong, and I agree that taking short cuts works well.

 

[lustyd]

Lovelly to have all of these opinions on whether to use a plotter or great circle route or whether one should connect the plotter to the AP (in 2020 WTAF!?). I take it nobody actually knows anything about the thread topic for sure then? Post two came close but was unsure but then we got diverted by posts with no relationship to the thread. Should I start a new thread and ask the same exact question?