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Old 10-24-2014, 08:30 AM   #1
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Satellite Compass Questions

I'm considering adding a satellite compass to make the autopilot track better and had a few questions. I have a very recent Garmin plotter system with a nmea 2000 network. The autopilot is an older Simrad AP20 with a standard fluxgate compass. Simrad is of no help and basically offers no support for their older equipment. The autopilot functions properly and I have a remote docking station and a remote steering control that I'm very happy with, so I really don't want to change autopilots if I can help it.

Questions:
If I add a satellite compass through my nmea 2000 network, does the faster more precise heading sensor make my plotter more precise which then makes the autopilot more precise, or is the autopilot rate of response controlled by it's fluxgate compass?

The Simrad AP20 is nmea 0183. If the satellite compass is nmea 2000 and running through my plotter to the autopilot, will that work or does it require a converter of some type?

If a satellite compass will significantly improve my autopilots tracking performance, which one do you like? Have been reading information on the different makes and models and am suffering from information overload. Was hoping for a relatively simple plug and play with my nmea 2000 network. Was hoping not to have to add another display to the console. As I am a simple man, I need some simple answers.....this stuff has my head spinning.

Ted
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Old 10-24-2014, 09:10 AM   #2
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I installed Furuno SC-50 primarily for improved AP (Simrad AP28) performance. Not worth the cost IMO.
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Old 10-24-2014, 10:13 AM   #3
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Hi Ted,

The main difference between running my Autopilot on the Fluxgate (FG) versus the Satellite Compass (SC) is how much the boat wanders along the course I have set. It's a much straighter path running the SC (almost straight) and much more like a drunken sailor driving when running off the FG.

Sea Eagle has two Simrad AP-50's with a switch installed that allows switching heading sources between the SC-60 or RFC-15R fluxgate.

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Old 10-24-2014, 10:14 AM   #4
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I was kinda wondering that too. I believe if you got a N2k/ 0183 compass (Believe sitex makes one) that you have to connect It too the one of the four 0183 ports on the J300x brain box. If you get an all N2k compass you'll have to get a converter box, I believe Furuno and Actisense sell them. Im curious to see the replies myself because we've been thinking of getting one to help with tracking in rough weather, and overall having a more accurate setup.
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Old 10-24-2014, 10:56 AM   #5
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I'm considering adding a satellite compass to make the autopilot track better and had a few questions. Ted
You are not alone. Often however the electronics are not the problem.

Other items at play include the type and size of pump, hydraulic ram size, line size and design slop in the rudder system. If you can, take a look at a reasonably new Nordhavn from a mechanical and hydraulic standpoint. My take is Nordhavn and Dashew do it the best - their APs are setup for long haul no fuss crossings. The one KK 52 I looked at awhile ago was another good design. Not easy to find a Dashew to look at though.

So putting in a Furuno compass may be similar to waxing your car, it esoterically feels and looks better but it is still the same old wheels.
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Old 10-24-2014, 11:32 AM   #6
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Ted,

It won't make your chart plotter any more accurate, it will just make the heading more accurate if you can display heading from an external compass on the chart plotter. The AP is looking for a specific NMEA 0183 message containing heading data coming in on its NMEA 0183 input port. To get that message into that port from a NMEA 2000 satellite compass requires some type of NMEA 2000 to NMEA 0183 such as that made by Actisense (NGW-1 about $180). The more precise the heading the better the mechanical part of the AP can follow it depending on its mechanical capability.

The big advantage to a satellite compass as I understand them, is they aren't affected by magnetic anomalies. When I see those annotations on the chart, you can watch the heading change on my display when I approach but the course over ground stays the same. If the AP is on, you have to constantly adjust the desired heading on the AP to compensate. In Coastal Explorer you look like you are traveling in a 20 degree crab with the boat heading one way and course over ground another. With a satellite compass that wouldn't be an issue.

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Old 10-24-2014, 11:50 AM   #7
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You are not alone. Often however the electronics are not the problem.

Other items at play include the type and size of pump, hydraulic ram size, line size and design slop in the rudder system. If you can, take a look at a reasonably new Nordhavn from a mechanical and hydraulic standpoint. My take is Nordhavn and Dashew do it the best - their APs are setup for long haul no fuss crossings. The one KK 52 I looked at awhile ago was another good design. Not easy to find a Dashew to look at though.

So putting in a Furuno compass may be similar to waxing your car, it esoterically feels and looks better but it is still the same old wheels.
Yep big redundant Accusteer pumps, along with dual oversize cylinders.
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Old 10-24-2014, 12:26 PM   #8
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Ted,

If your AP20 is equipped with an RFC35 Fluxgate Compass, you can upgrade it to an RC25 (or RC35R), which is a much more modern and accurate heading sensor. It is in essence a combination of a solid state rate sensor and a fluxgate sensor, where the rate sensor is slaved to the fluxgate sensor. The Rate Compass gives a more stable and precise autopilot steering on any type of boat. It also eliminates the effect of the distorted horizontal magnetic field at high latitudes and 'S-ing' when on autopilot steering.

It seems the RC25 was recently discontinued by Simrad so you might have to be creative in locating one. While not cheap, they are much less than a Satellite Compass, simpler to install (no NMEA 2K to 0183 converters) and designed specifically to give the best performance with your autopilot. When I upgraded, the difference was amazing. There is an RC25 listed here, and an RFC35R listed here. They are often available on Ebay.

Obviously, if you are already using a Rate Compass and it is not working well enough for you, a satellite compass or new autopilot might be the best solution.

This chart is from Simrad and shows the performance difference between a standard Fluxgate Compass and a Rate Compass.
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Old 10-24-2014, 01:13 PM   #9
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Autopilots steer to headings and sat compass's track heading changes faster than gyro compasses. Just because a heading changes faster doesn't mean you are steering a better course. Quite the contrary actually. How straight is your wake when you look behind you (hand steer vs auto)? Some boats just track better and require less rudder correction. Try tweaking the pilot settings a bit first. Fwiw I have 2 ap50's one uses a sat compass and the other uses the gyro. I have them set up for different sea states so I haven't really done an ab comparison just changing the heading sensor. In fiords and around tall buildings the sat compass has lost it's bearings a few times.


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Old 10-24-2014, 02:05 PM   #10
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Second that about using a rate sensor. On my ap35 it does work better.

Also second that about tuning the AP parameters. If AP is not tracking well, rudder and counter rudder settings may be non optimal. The autotune procedure rarely gets it right, IMHO.
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Old 10-24-2014, 02:32 PM   #11
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Second that about using a rate sensor. On my ap35 it does work better.

Also second that about tuning the AP parameters. If AP is not tracking well, rudder and counter rudder settings may be non optimal. The autotune procedure rarely gets it right, IMHO.
Ski,

You are absolutely right about tuning the parameters. I even keep a small notebook with settings and observations I have tried in the past. The autotune didn't work for me on either of my boats, and not even close on the Fales. It took a while to dial in, especially the counter rudder, but it now steers like it's on rails. I also tweak the seastate filter to suit conditions, but get the tightest heading control with it off, particularly in following seas. It just takes some patience and trial and error, but it's really worth the effort.
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Old 10-24-2014, 04:21 PM   #12
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I have the comnav Vector G2 satellite compass, same as the Si-Tex, both are made by Northern Hemisphere. I have mine connected to an old Robertson and I have found that it greatly improved performance. I'll pit my old Robertson against anything on the market today using a fluxgate or rate compass but as noted by others it does not negate the need to set up the AP accurately to sea conditions. My system is similar to Scott's, I also have an RC35R connected in.

If I add a satellite compass through my nmea 2000 network, does the faster more precise heading sensor make my plotter more precise which then makes the autopilot more precise, or is the autopilot rate of response controlled by it's fluxgate compass?
I say yes but depends, you are talking about sending two heading sentences into the data stream at once. The experts here with nmea 2000 might answer better but as I recall when I set up mine on 0183, "you can" feed two heading sentences into the nmea stream alright but it made no sense to do so, darned if I can recall which is actually read/used, I believe it was the last Hdg sentence but likely wrong. In light of this I am set up to input either the Satellite or the rate compass but not both. No real good reason for doing so other than it provides some redundancy. My situation was quite complicated because my pilot uses sine/cosine (not nmea) and satellite compasses require an interface to play nice.

The Simrad AP20 is nmea 0183. If the satellite compass is nmea 2000 and running through my plotter to the autopilot, will that work or does it require a converter of some type?
I agree with Tom you will need a 2000 to 0183 converter but I disagree it will not make the plotter more accurate in that it will position your boat on the chart more accurately due to their phenomenal accuracy and very rapid refresh capability (up to 100Xper sec versus 1Xper sec. I can set the baud and refresh rates to near anything I like to suit various pieces of equipment.

If a satellite compass will significantly improve my autopilots tracking performance, which one do you like? Have been reading information on the different makes and models and am suffering from information overload. Was hoping for a relatively simple plug and play with my nmea 2000 network. Was hoping not to have to add another display to the console. As I am a simple man, I need some simple answers.....this stuff has my head spinning.

The Vector G2 (Nmea 2000 version?) is touted as literally plug & play, no other hardware required but you can keep it as simple or complex as you like. They have the capability to provide a multitude of outputs both RS232 & RS422. By connecting a computer to it each of can be individually tailored to different baud & refresh rates and also control of what nmea sentences get sent.

What are my real world experiences, I like it but it's so advanced I'm certain it will be obsolete before I can replenish the bank account it drained. God forbid it ever breaks. Was it worth the cost? Yes & no, no because who really needs accuracy in inches unless your using the Sat compass to dock your boat. Yes because in my situation it gave me the ability to tie together a hodge podge of older/newer equipment with little brain drain however mostly it's just an expensive delicate toy.
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Old 10-24-2014, 07:04 PM   #13
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I appreciate all the imput. As far as rudder and steering, my boat turns on a dime and returns change. Pretty sure the problem lies in the autopilot compass. When steering to a waypoint on flat water the pilot corrects usually after a degree off heading. When I tell the pilot to hold a heading, it can go off course more than 2 degrees before the pilot makes a correction. In short, it holds a straighter course running off of imput from the plotter than when I set it to maintain a specific compass bearing.

Ted
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Old 10-24-2014, 07:10 PM   #14
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I appreciate all the imput. As far as rudder and steering, my boat turns on a dime and returns change. Pretty sure the problem lies in the autopilot compass. When steering to a waypoint on flat water the pilot corrects usually after a degree off heading. When I tell the pilot to hold a heading, it can go off course more than 2 degrees before the pilot makes a correction. In short, it holds a straighter course running off of imput from the plotter than when I set it to maintain a specific compass bearing.

Ted

Per the Simrad Manual. Just have to make sure your firmware is up-to-date.
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Old 10-24-2014, 07:32 PM   #15
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I appreciate all the imput. As far as rudder and steering, my boat turns on a dime and returns change. Pretty sure the problem lies in the autopilot compass. When steering to a waypoint on flat water the pilot corrects usually after a degree off heading. When I tell the pilot to hold a heading, it can go off course more than 2 degrees before the pilot makes a correction. In short, it holds a straighter course running off of imput from the plotter than when I set it to maintain a specific compass bearing.

Ted
Having a responsive rudder doesn't mean your AP is controlling it correctly. At 8 knots, it will take more than 60 seconds before a 1 degree heading error creates a cross-track error of 14 feet. I find that, in normal seas, my AP most of the time my AP maintains a course with less than 6 feet (.001 nm) of cross track error, and probably 90 percent of the time the cross-track error is less than 12 feet (.002 nm), regardless of whether I am running of the sat compass or fluxgate compass. However, the boat's heading, again in normal seas, will move +/- a few degrees but the AP is smart enough not to be continuously moving the rudder. Instead it seems to have a pretty complicated algorithm. As a result, my fluxgate sat compass does not provide noticeably less cross track error than the fluxgate. If you are having big problems with the fluxgate, it is probably because of the way the AP is configured.
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Old 10-24-2014, 08:45 PM   #16
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At my speed, in my waters, and with my old Wagner I look forward to tweaking the heading knob a few times a day.

How much more is needed other than keeping a straight line?

I might be missing something here. I use AP on the lake almost from lighthouse to lighthouse. On the river dodging debris, hand steering is the norm.

How a better heading sensor will help is puzzling me.
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Old 10-24-2014, 08:58 PM   #17
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Capt Kangeroo,

Agree that a satellite compass can improve the accuracy of a chart plotter, but only if you can disable the internal GPS or tell the system to use the satellite compass GPS data. I keep battling my Lowrance system (HDS-8 down below and HD7 up top), as to the source used for navigation. The HDS-8 down below doesn't get a good WAAS signal so I tell it to use the HDS-7 up top. If the HDS-7 is not on yet, the HDS-8 will use its internal GPS but when the HDS-7 comes into the network it won't switch to it unless I tell it to and it is not readily obvious which GPS either unit is using unless you dive into the menus. Not high on the user friendliness scale. Coastal Explorer does a pretty good job of grabbing the best source and showing you what source it is using.

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Old 10-24-2014, 09:14 PM   #18
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At my speed, in my waters, and with my old Wagner I look forward to tweaking the heading knob a few times a day.

How much more is needed other than keeping a straight line?

I might be missing something here. I use AP on the lake almost from lighthouse to lighthouse. On the river dodging debris, hand steering is the norm.

How a better heading sensor will help is puzzling me.

That's the point, the ability to hold the straightest line is with a satellite compass. What you are missing is every one isn't a river cruiser.
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Old 10-24-2014, 09:21 PM   #19
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I've got a Simrad HS70 sat compass, and like it in principal, but it has issues at the moment. I also have an RC42 rate compass as a secondary, and have needed it. I'd say the AP steers slightly better on the HS70, but not a ton better. The HS70 outputs roll, pitch,and heave which is nice. I think there is a ibg difference between a FLux Gate and a Rate compass, though the terminology may differ a bit. The key I believe is whether the device outputs rate of turn, or just heading. I think the rate of turn helps the AP a bunch.

The HS70 has a bunch of N2K problems adn they are supposedly fixed in v1.6 of the FW, but I haven't been successful in loading it so far. I managed to get a contact in Simrad engineering who has been helping me, but simrad's normal tech support is less than useless.

That said, my installer has done a bunch of Furuno SC30 installs and has had no end of trouble with them. They sound even more problematic than the HS70. The Furuno SC50 seems to have a better performance record, but has an internal battery that needs to be replaced as some unknown interval, and replacement involves return to factory or use of a soldering iron, not to mention pulling it down off the mast.

If you are OK with a single heading source then I'd just get an RC42 and be done with it. It's a less elegant solution, but much more reliable assuming it's installed away from magnetic sources.
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Old 10-24-2014, 09:38 PM   #20
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Don't want to derail the thread, but for the life of me I can't find out how to edit the sea state filters. Anyone know how to?
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