How does the autopilot work?

[dunedin]

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?

It’s a forum. And as you said in the OP is an entirely theoretical question, with no practical application.

But if you really wanted to know you, it will likely differ based on the plotter make and version, and probably on the individual contract software engineer at the time. Ask B&G, Navionics etc.
PS Having worked in the IT industry for a long time I choose carefully what is integrated and what is not, as pros and cons (even in 2020) - often the least informed are the most enthusiastic about integration

 

[AntarcticPilot]

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?

The answer is that if it isn't specified in the chart plotter's manual, you don't know. Based on my knowledge of mapping and charting, I'd be surprised if it was anything other than the rhumb line, but over the distances that a chart plotter is useful for, it probably doesn't matter.

If you have a scenario where it matters, I'm sure the chart plotter maker would tell you.

 

[lustyd]

PS Having worked in the IT industry for a long time I choose carefully what is integrated and what is not, as pros and cons (even in 2020) - often the least informed are the most enthusiastic about integration

I've worked in the IT industry for a long time too and knowing how the various integrations of yacht stuff works there is just no downside to connecting them, it's effectively passive and if one thing breaks then the others downgrade to the state of being standalone.

@AntarcticPilot yes I figure there will be differences but hoped someone at least would know which sentences are passed generically on NMEA which would give a clue. I think you're probably right in your various guesses though. As I said in the OP I don't think there will be a situation where it would matter, I just sometimes like to know these things. It comes down to more than just great circle though, if a waypoint changes that may or may not be communicated to the AP, and that might be important.

 

[ip485]

Autohelm

 

[lustyd]

No, autopilot. Autopilots

 

[Buck Turgidson]

XTE and Heading. Some plotters allow RL or GC as a selection in the autopilot NAV mode.

 

[Skylark]

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

When was the last time you turned the pages of Norries Tables ?

Straightforward way to derive a DR from a days run.

It’s equally simple to calculate dLat, dLong and Departure using Log Tables, if you kept them from your halcyon school/college days ?

 

[AntarcticPilot]

I've worked in the IT industry for a long time too and knowing how the various integrations of yacht stuff works there is just no downside to connecting them, it's effectively passive and if one thing breaks then the others downgrade to the state of being standalone.

@AntarcticPilot yes I figure there will be differences but hoped someone at least would know which sentences are passed generically on NMEA which would give a clue. I think you're probably right in your various guesses though. As I said in the OP I don't think there will be a situation where it would matter, I just sometimes like to know these things. It comes down to more than just great circle though, if a waypoint changes that may or may not be communicated to the AP, and that might be important.

I'm not guessing about it being XTE that is passed to the autopilot, that appears to be the usual method. That implies that the chart plotter determines the track to be followed. There's no reason (beyond additional computation) why that can't be a great circle - but it's an additional complication that gives no added value in most cases, so I'd doubt whether the developer would bother. Further, we draw rhumb lines on charts to determine safe clearances - if the plotter followed great circles, it might put you on an unanticipated course over a danger

 

[Neil_Y]

Last time I used one I just had an autohelm and either set it on a course or a wind angle to maintain whilst I slept. It worked. Electric supplied by towed generator so we had enough to steer with and keep the fridge cold.

 

[lustyd]

@AntarcticPilot in that case that’s the answer as I know the plotter can do a great circle it was just the how it tells the AP to change course I was stuck on, thanks

 

[prv]

There is an NMEA0183 autopilot sentence, which includes the following items:

• XTE
  • Magnitude
  • Direction
  • Units
• Arrival flags
  • Circle entered
  • Perpendicular passed
• Bearing to destination
  • From origin
  • From present position
• Heading to steer to destination
• Destination waypoint ID

Pete

 

[[163233]]

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

Yes obviously I plan a route, it's the application of the route to the autopilot I was wondering about.

 

[KompetentKrew]

Not sure if this is still relevant, but I've just noticed this option in my B&G's menus:

 

[laika]

There is an NMEA0183 autopilot sentence, which includes the following items:

This would suggest that OpenCPN creates a series of waypoints for a great circle:
Great Circle Sailing [OpenCPN Manuals]
And it uses RMB per prv's post:
Great Circle Sailing [OpenCPN Manuals]
..and autopilot sentence "B" (APB)

I note that there are separate bearing to waypoint sentences for rhumb line (BWR) and great circle (BWC). I just looked at what my whizzy new evolution recognises but of course it's all N2K which I'm less familiar with and for all I know rhumb line / great circle may be covered by the same pgn with an indicator to say what applies to the waypoint.

Which is to say, I don't know but it does seem from the above like OpenCPN may do it like we would and construct a route as a series of rhumb line waypoints.

 

[ip485]

No, autopilot. Autopilots

I know it is totally pendatic, with which I usually dont engage but ..

when was the last time you referred to the skipper as a pilot (except maybe on a pilot boat)?

I think this has evolved from aviation where of course we have autopilots, but I prefer the person on the wheel or tiller to be the helm, hence autohelm, just as for me people on trains are passengers, not clients or customers.

 

[Buck Turgidson]

As an aside all the commercial aircraft I'm familiar with use great circle between waypoints.
L1011
BAE146
EMB145
EMB170/190
CS100/300

So that's everyone bar Boeing but I know they do too.

 

[Buck Turgidson]

I know it is totally pendatic, with which I usually dont engage but ..

when was the last time you referred to the skipper as a pilot (except maybe on a pilot boat)?

I think this has evolved from aviation where of course we have autopilots, but I prefer the person on the wheel or tiller to be the helm, hence autohelm, just as for me people on trains are passengers, not clients or customers.

Back in my Nimrod flying days the Captain was referred to as Skipper. I got some odd looks when I moved to the transport fleet ??

 

[TernVI]

I know it is totally pendatic, with which I usually dont engage but ..

when was the last time you referred to the skipper as a pilot (except maybe on a pilot boat)?

I think this has evolved from aviation where of course we have autopilots, but I prefer the person on the wheel or tiller to be the helm, hence autohelm, just as for me people on trains are passengers, not clients or customers.

If you're going to be pedantic, the 'helm' is the wheel and the bloke is a 'helmsman'.
Like an oar and an oarsman.
Autohelm was a trade name so probably belongs to Raymarine now?

If the 'steering computer' is calculating CTS by looking at great circles and XTE and all that, maybe it's more than a mere 'helmsman'?

 

[lustyd]

I provided the link to show that they are called autopilots. It's not up for debate, if you go to Raymarine or B&G they list a bunch of autopilots, therefore I bought an autopilot because that's what they sell and therefore the thing I own is called an autopilot. Call it what you like, but the people making them call them this so that's their actual name as far as I'm concerned.

If I Google autohelm it returns a bunch of tiller based solutions which are actually called Tiller Pilots so from the look of it the term helm is the only one not used by anyone in the industry!

Edit to add - my autopilot is nowhere near the helm and doesn't drive the helm ?

 

[lustyd]

There is an NMEA0183 autopilot sentence, which includes the following items:

• XTE
  • Magnitude
  • Direction
  • Units
• Arrival flags
  • Circle entered
  • Perpendicular passed
• Bearing to destination
  • From origin
  • From present position
• Heading to steer to destination
• Destination waypoint ID

Pete

This is perfect, thanks Pete. Looking at this it would seem possible for the nav to send the occasional bearing from present or heading to steer rather than relying on XTE. Presumably there must be differences in implementation but using either of those it would certainly seem likely that it could do the routing any way it wanted to without changing waypoint.