Autopilot Confusion

[Tristan]

Let me modify my question. Which device is able to calculate XTE ?
Is it a plotter (OpenCPN) or a GPS ? If it is a GPS where does it take the current waypoint from ? Can you load to the GPS a whole route of waypoints and then turn the plotter off ?

[transmitterdan]

Most autopilots have one or two basic modes. The first mode is steering to a compass heading. Nearly all pilots have this mode. For heading mode all they need is heading information. The heading can be supplied from a dedicated device connected just to the pilot computer or it can be from a bus such as NMEA. Then on some pilots this mode can be modified to "wind" wherein instead of using a magnetic heading it steers to an apparent wind angle. Again all it needs is the wind angle information but often they also like to have a magnetic heading so they can create an alarm if the wind shifts too much.

The other mode is cross-track error (Xte). This mode is what is used to steer to a specific waypoint. Typically this works by having some other computer on the boat compute a line from the current position to a desired position. Then the computer compares the present GPS coordinate to the computed line and sends the autopilot periodic updates as to the distance away from the line (XTE) and bearing to the waypoint. As the distance from the line increases the pilot computer tries to reduce this error by steering toward the waypoint (and thus toward the line). It will often follow a S shape course and it travels back and forth across the line.

In many (most) systems the autopilot computer is not actually dealing with any GPS data at all. It either deals with heading (or wind angle) or cross track error and bearing to waypoint. In the case of the latter some other computer has to provide these data values. OpenCPN can provide this data as can many chart plotters. You can probably read the manuals of your particular pilot to see exactly what data it needs.

[Tristan]

Then there is no point to run a high power consumption PC on the long passage .
One can prepare the route before the trip and feed the waypoints to some low power computer (raspberry Pi or smaller ) which in turn can calculate cross track error based on the current waypoint and the GPS messages and generate APA, APB sentences to the AP . Obviously the operator can modify the track considering the current circumstances.
Just trying to save a few amps.

[osirissail]

As mentioned by a few, the key to your question is the interface between the autopilot and a navigation device - gps or nav program or plotter. You can install the system so that any of the navigation devices can feed the autopilot - if - the navigation devices can output the nmea sentences that the autopilot needs.

If you want the flexibility of selecting the navigation source to your autopilot and it uses a NMEA 0183 interface then normally a separate little box is needed to "line up" the outputs of the navigation devices and send their information to the autopilot along with separate switches (on-off or selector) to only allow only one navigation device to talk to the autopilot at a time.

But the interface language of the navigation device must match the autopilot's. I have not heard of a "translator" to change NMEA 0183 to 2000 or vice versa.

[colemj]

Originally Posted by osirissail

I have not heard of a "translator" to change NMEA 0183 to 2000 or vice versa.

These are very common, relatively inexpensive and made by quite a few manufacturers.

Mark

[osirissail]

Right on, a Google search shows Mareton and Simrad make bi-directional translators. Price was shown to be about $100. So a system of selector switches could be put together to feed the autopilot from whichever navigational device the OP wants to use. But it might be easier if all the units "spoke" the same.

[twistedtree]

Originally Posted by Tristan

Let me modify my question. Which device is able to calculate XTE ?
Is it a plotter (OpenCPN) or a GPS ? If it is a GPS where does it take the current waypoint from ? Can you load to the GPS a whole route of waypoints and then turn the plotter off ?

A plotter can do it, and I "think" certain high-end GPSs can as well. A lower cost GPS that just blabs out position etc and has no user interface will NOT work. But I think some of the higher end GPSs that have control panels can be loaded with one or more waypoints and can act as the nav device. But I've never used one so I'm not sure.

The documentation for most electronic components will list the sentences/ PNGs that the device emits and listens to, so that should tell you.

[Pelagic]

I think there is also the basic philosophical question as to whether you “should” allow your autopilot to steer to waypoints.

My own opinion is NO!
I believe it dilutes the watch keeper’s sense of the ever changing dynamics of wind and tidal currents upon the boat.
Because it numbs their sailing senses and absolves them of a basic responsibility to stay on course I think it helps to promote an environment of poor seamanship which leads to a failure to maintain a sharp lookout.

At watch change, there is a certain pride when you handover with zero XTE.
This is something I do not want to lose in a shipmate thru automation.

[DotDun]

Originally Posted by sy_gilana

The APA sentence uses XTE to get on course
The APB sentence uses BTW to get on course, both have advantages and disadvantages

Both APA and APB contain XTE information.

The AP requires connection to a compass, traditionally a magnetic fluxgate, which could be internal to the AP.

In nav mode, the AP receives from a chart plotter: Bearing to Waypoint (BTW) Magnetic or True, Distance to Waypoint (DTW), and Cross-Track Error (XTE).

Most, but not all APs require Speed over Ground (SOG) or Speed Thru Water (STW).

If BTW is sent = True, the AP most likely will require an external source for magnetic variation. (FWIW, this issue was recently addressed by OpenCPN, they added the option to send BTW as magnetic.)

The AP uses BTW to initially steer the boat towards the destination waypoint. The AP then uses XTE to make adjustments right or left in order to stay on track. The rate-of-turn the AP calculates for XTE adjustments is dependent on vessel speed, the slower speed may allow for more radical steering adjustments.

A good source for those wanting to dissect NMEA0183 sentences is:
http://gpsd.berlios.de/NMEA.txt

In the NMEA2000 world, the same principles apply, just the packaging/packetizing of the data has changed.

[colemj]

Originally Posted by Pelagic

I believe it dilutes MY sense of the ever changing dynamics of wind and tidal currents upon the boat.
Because it numbs MY sailing senses and absolves ME of a basic responsibility to stay on course I think it helps to promote an environment of poor seamanship which leads to a failure to maintain a sharp lookout.

There, I fixed that for you. It is quite presumptuous to assume that the rest of can not keep a good, safe watch just because the autopilot is steering to a wind angle or a waypoint instead of a compass.

There are those who think that not using a sextant abstracts one from "real" navigating, dulls the senses and promotes poor seamanship. They also take a strange pride in bringing the ship to a precise point and would never want a shipmate that couldn't do so.

Seriously, it is simply a mode of operation for an autopilot. Take a step back from the pedantics and fear and think about just what that means. It means just that. Hardly poor seamanship or sense dulling, and much safer than using a mechanical windvane, the mode under which one operates you seem to conveniently ignore.

Mark

[IdoraKeeper]

My current AP is heading only. I have to manage my own xte. My previous boat was equipped to track courses from the plotter. The proverbial magenta line. That particular boat was light in weight and unstable in heading. In its efforts to keep zero xte it would often take up headings 25 or 30 degrees either side of track. In low visability condition's this was often disconcerting. Contrary to degrading my watchkeeping it raised my alertness even though it repeatedly proved it was right, I never trusted it. I don't miss that function. A good scan on the plotter is essential.

[nigel1]

Originally Posted by Tristan

Let me modify my question. Which device is able to calculate XTE ?
Is it a plotter (OpenCPN) or a GPS ? If it is a GPS where does it take the current waypoint from ? Can you load to the GPS a whole route of waypoints and then turn the plotter off ?

The simple answer is yes, there are GPS receivers that can calculate routes, for example , Furuno GP 80, GP 150.
But, beware, it is very easy to make a mistake inputting waypoint data, especially where data is inputted using the arrow keys to scroll through numbers rather than by a direct number pad input.
So long as you enter the waypoints correctly, and monitor the voyage on paper chart, you should be OK.
It is feasible to output a route from a chartplotter to a GPS receiver, but thats too much of a head ache for me.

If you are concerned about the power consumption of a plotter, turn the backlight down to minimum when you are not viewing the plotter

[Pelagic]

Originally Posted by colemj

There, I fixed that for you. It is quite presumptuous to assume that the rest of can not keep a good, safe watch just because the autopilot is steering to a wind angle or a waypoint instead of a compass.

There are those who think that not using a sextant abstracts one from "real" navigating, dulls the senses and promotes poor seamanship. They also take a strange pride in bringing the ship to a precise point and would never want a shipmate that couldn't do so.

Seriously, it is simply a mode of operation for an autopilot. Take a step back from the pedantics and fear and think about just what that means. It means just that. Hardly poor seamanship or sense dulling, and much safer than using a mechanical windvane, the mode under which one operates you seem to conveniently ignore.

Mark

Thanks for fixing for me Mark.
I can only admit that I am less of a sailor when everything stays on automatic Nav.

But, I am not one of those purists, I love all the modern Nav equipment that I get to play with on Super Yachts but have learned to keep the watch keepers (including myself) engaged in manually adjusting for leeway to keep alert.

Also certain experiences can lock you in to the value of feeling the boat on ocean passages.
A couple of them did that for me:

1 Mid Ocean, one afternoon, I turned off the adaptive pilot to hand steer and something felt funny. We found no problems with the pumps or steering, so back on Autopilot and had an uneventful passage. First opportunity, we dove on the hull and found that one of the rudders had completely disappeared. This was traced back to a recent dry-dock failure to secure the rudder flange with the proper lock nuts.
Now on each watch I always hand steer for a little while, just to feel the boat.

2 Similar experience where in rough conditions off the Australian Coast, the mate told me he had to apply lots of leeway to keep her on track to next waypoint. “Northing Tests” confirmed that both Gyros that had just been serviced in Sydney, were failing in a strange way. Altered course back inside reef towards Townsville and all was fine.
Since this was a possible warranty issue, I had service tech from Radio Holland meet me in Townsville. Turns out their Sydney Tech had failed to top off fluids after assembly and the spheres were hitting the sides in rough conditions creating a variable gyro error in rough conditions. Both Spheres were damaged and luckily replaced under warranty.
Point is…if autopilot had simply been steering to a waypoint, we would never have picked up on that compass problem.

Each to their own!

[goboatingnow]

Originally Posted by colemj

I don't know of any AP that will automatically change course more than 30* without human intervention. Most won't do more than 10*, and they all sound an alarm when doing so.

Simple compass courses can cause more problems because you could be gradually and unknowingly set into a dangerous situation by current or leeway - no daydreaming required.

Mark

Most when set into motorboat mode will execute full authority turns of any degree including 180 s

Dave


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[goboatingnow]

Originally Posted by Pelagic

I think there is also the basic philosophical question as to whether you “should” allow your autopilot to steer to waypoints.



My own opinion is NO!

I believe it dilutes the watch keeper’s sense of the ever changing dynamics of wind and tidal currents upon the boat.

Because it numbs their sailing senses and absolves them of a basic responsibility to stay on course I think it helps to promote an environment of poor seamanship which leads to a failure to maintain a sharp lookout.



At watch change, there is a certain pride when you handover with zero XTE.

This is something I do not want to lose in a shipmate thru automation.

Couldn't agree LESS , the job of helmsman is a lowly position, unsuitable for any skipper. The role of the skipper is to command the vessel, evaluate the situation and make decisions as appropriate. Turning wheels is for monkeys or electrics.

There should be no diminution of responsibility by using an autopilot. In fact an AP aids in situational awareness

Dave


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