COG and Heading differential

[Gyrfalcon]

Hi-
We have an SCX-20 and a TZT3 chartplotter. When we are underway, the COG seems to always be a few degrees (1-4) to starboard compared to the heading. Data sources for both are set as SCX-20 and there are no calibration factors entered. This seems a bit odd--and it seems like, even when the wind should be pushing us to port, we have the same offset of Heading/COG. We'll continue to watch and see if this behavior continues, but I wondered if I'm overlooking a setting.

 

[Johnny Electron]

No, if the SCX20 is mounted square on the boat, the COG and heading should be good. Ensure you are using the COG data as your primary source from the SCX20 in your data source list. Both heading and SOG/COG should be set to the SCX20. I almost sounds like someone has picked a different source of COG than the SCX20. If both are the SCX20, then any deviation is normally caused by currents and like you said, you should monitor that but I have never seen one require an offset be entered.

 

[hiccup]

Totally normal for heading & COG to not match. Your example of the wind pushing you is a good one, as is current. Essentially the boat is travelling in a slightly different direction than it is pointing. COG doesn't know or care where the boat is pointing and Heading doesn't know or care which direction the boat is traveling. The most extreme example would be when you're drifting--Heading & COG could be off by a full 180 degrees.

Now, if it's always off by the same amount & direction, then that starts to sound like a heading sensor installation alignment, which can be tweaked--but you'll be setting yourself up for disappointment if you expect them to match.

 

[Gyrfalcon]

Hi--
We have been watching our heading and COG for the past 300 miles (wind on both sides of boat, various currents), and while underway the COG is slightly variable as expected but ALWAYS 3-8 degrees (~5 degrees average) to the right of the heading line. The only time when it isn't is when you would expect the COG to be to the left of the heading (ie we make a sharp turn to starboard, and the COG lags the heading by a bit, or when we passed an outflow of a fjord). We have a Garmin which is attached to the same backbone and thus the SCX20, and it shows the same differential. We also have a Bad Elf connected to Navionics on an ipad which reports COG. Its COG matches the COG on the Furuno.
The SCX20 is mounted squarely on the boat.

Johnny E--if you think that calibration factors should not be needed, what do you suggest we do?
Thanks
Nancy

 

[Johnny Electron]

Based on what you say, you believe the COG to be correct and the heading to be off? How does the boat steering compass read in comparison to the mag heading of the SCX20 sitting still? COG is common to be off of heading as has many variables pushing it off like currents, winds and the boat itself. Some boats don't go straight because transducers and items pulling it to one side or another. We see this all the time in setup of an autopilot where you need counter rudder to have the boat go straight. I have never seen a properly mounted SCX20 to need any offsets but first before even considering changes, try to find out exactly what you think needs to be changed. Is it heading or COG that needs adjustment? If heading and you verify it with other sources that it is off then you can simplify apply a fixed offset by the amount needed and then it should stay correct.

 

[Gyrfalcon]

Our boat weighs 167 tons and we move slowly so I doubt the transducer affects our course. When the rudder is centered via the rudder indicator, the boat tracks very well when in a calm area without currents. We have been trying to determine which if heading or COG seems correct by sighting on buoys, and most of the time it seems to be COG pointing to the chart object. As a side note, the setting for rate of turn is 10s. I'm not sure if that's correct or what impact this has.

 

[hiccup]

I'm not sure you (or anyone else) can accurately determine by eye whether or not the SCX20 is aligned to the boat within a couple degrees. I was going to suggest using a buoy to help determine if the scx20 is aligned, as you mentioned. One step further would be to get quite close to the buoy to confirm its position on the electronic chart, dropping a waypoint if needed, then moving away from the buoy, and pointing the boat directly at it. Compare your heading to the bearing from the electronic chart (buoy symbol or waypoint if the buoy symbol was off). Do this several times and average the results and use that value to determine the offset to use in the SCX20 installation menu. This approach can be helpful determining radar alignment offset as well.

Obviously make sure you are always comparing correct units (magnetic or true).

SCX20 is the only piece of Furuno gear I have that has been zero trouble--I find its performance to be amazing.

 

[Gyrfalcon]

Good idea about the buoy--getting close. We will do this when we get the opportunity in the next week or so (some buoys are not too accessible).

 

[Gyrfalcon]

Hi, we put a go to course and followed the line. I assume the degrees on the banner for using a go-to point is based on the chart. When we were aligned with the GoTo point, our heading matched the degrees in the goto banner. But for the last 1247 miles, our COG is irritatingly to starboard (second pic).

 

[hiccup]

Did you perform scx20 alignment check as discussed above? What were the results?

What was your CTS in the above scenario? My guess is that your scx20 is slightly misaligned but more to the point you're under the false impression that COG and HDG should be matching. The autopilot is not trying to match COG & HDG but rather HDG & CTS. CTS is a calculated metric intended to match COG to BTW. Understand these terms and what they mean and as your understanding goes up, your anxiety will go down.

While it's certainly possible that your scx20 is slightly misaligned (which is easily checked and corrected as discussed in previous posts), there is nothing in the screen shots that you show that indicates any problem.

 

[wkearney99]

You're not alone with some SCX20 and TZT3 weirdness. Ours was "just fine" last season but since spring of this year something's "not right" with the COG/heading alignment. I get that there are always going to be slight variations between them, but recent use of the system makes me wonder what's gone wrong. Our SCX20 is mounted up mast, and that gets folded down for winter storage, so I suppose there's potential for it to have gotten knocked out of alignment, but it's never had trouble in past seasons. It's on the same hardware as the radar array and that's not showing any similar misalignment. I confirmed the data sources are all pointing at the SCX20.

During a trip today it seemed to lose heading entirely for about a second. The red heading lost banner appeared briefly with it. The boat icon showed a 90 rotation to port when that happened. We do not have autopilot.

I'll try the buoy/marker alignment check next time we're out. But I'm thinking that's not going to be the problem and wonder what's else is really going on.

 

[Cfc1215]

I am not sure if I have a similar problem or something different. I have three TZt 3s with 330 gps sensor. I have a precision 9 simrad compass for heading. My heading set from precision 9 and the COG set from 330 GPS is about 22 degrees off give or take a few degrees. I have tried everything, the problem is my charts are about 22 degrees off actual my actual heading. When using the radar overlay, you can clearly see they are misaligned and the radar is accurate, it is what is in front on me. Is there a way to offset the chart plotter to align the actual heads up in the charts?

 

[Quitsa]

I am surprised no one has suggested a very simple check, which is to run reciprocal courses and compare the values. That will neutralize the effect of wind and current. If you head 180 degrees based on heading value from SCX-20 and COG reads 182, then make a U turn and steer 0 degrees heading, COG should read 358 degrees. If the difference isn't an identical amount, you probably have an alignment error in how your SCX-20 is mounted. This can be corrected by going into the SCX-20 settings and inputting an appropriate offset value. Using my example, if the COG on the reciprocal course were 350 degrees and not 358, you would set a 2 degree offset. Then when you run the reciprocal courses, the difference between COG and HDG would be identical. Running with no wind or current, your heading and COG would then match.

Even a small alignment error will affect the performance of an autopilot when operating in NAV mode. The other thing that is very important to get right for optimal autopilot performance is the rudder zero point, which can also be checked running reciprocal courses to eliminate impact of wind and current.

 

[Gyrfalcon]

Hi--I apologize for the delay in responding to some of the recommendations; we had charter guests on board, and the last thing they want to do is go back/forth along a course (unless of course, there are bears and whales all along).

The tests below make me think that the SCX-20 is behaving correctly. Here's what we'v

We have done several tests in the past few weeks. Going at a given heading in 2 different directions (eg N then S in the same bay), the COG is always to starboard of the HDG (it's not exactly the same amount off, but that's likely due to error in our navigating). Unfortunately, these are fairly short runs, so difficult to assess.

The results of the second test are more telling, since we have done this 2x and over a distance of several miles. When we have had a long straight run without significant current or wind (or minimal current/wind only on bow/stern), and have set a route on the furuno and have picked a set the autopilot to follow the route exactly (see below for how we use autopilot), our boat follows the COG exactly, not the HDG, even though the HDG remains parallel to the route. This is consistent with the fact that for the past 2500 miles, our COG has only been to port of our HDG for about 2-3 miles, and has consistently been between 4 and 8 degrees to starboard. I've attached images. This tells me that the boat is going to starboard for some reason and that the SCX-20 is not a problem. What could be causing this is more of a mystery. We have twin screws in synchro. We considered that it could be the rudder setting.

However, we believe that our rudder indicator is appropriately set. When at neutral rudder heading towards a point, the boat tracks towards that point. It may be that if we tested this over several miles that there would be some deviation, but we won't be able to do that until back in fresh water this fall. However, the autopilot doesn't hunt for a course, which makes me think the rudder indicator is set properly.

For the comments about autopilot and CTS--we have a ComNav autopilot which will not talk to either the SCX-20 or the Furuno TZT3, and will not function in NAV mode. We use it basically by heading on the course that we want to take, then putting it in auto mode, and using the dial as necessary to stay on course. We ignore the heading on the autopilot since it's based on a fluxgate compass and doesn't match the SCX-20.

Thanks in advance for helpful comments.

 

[Cfc1215]

I believe your SCX-20 is not aligned. Since you can only do the following procedure at the dock and you can’t get an accurate COG while stationary. Turn on your radar and use the chart overlay function at the dock; see if the radar image aligns with objects on the chart assuming your radar is aligned properly. My guess is the image and the objects will be 3 to 8 degrees apart. Then, if it is easy to get to, loosen the SCX-20 and adjust it ever so slightly to port or starboard, then go back to the display and look to see if the radar image is more closely aligned with objects on the chart, if so, you are moving it in the right direction. I had the same problem, (I wrote about it on this post above) and I found that my gyro compass was not aligned perfectly with the bow of the boat and this procedure fixed it. Hope that helps.

 

[Gyrfalcon]

Hi--
Your reply plus previous posts gave me a great idea this AM. We happened to be in a very large anchorage with a very large rock with an ATON last night. As we were leaving, I turned on the radar and aimed the boat (HDG) towards the ATON. Here are the results.
The heading on the radar and the GPS chart matched.
The radar and the coastline lined up perfectly.
The radar and the GPS location of the ATON matched.
Seems to me, with this info and the fact that the COG predicted where our boat went (see previous post), our boat for some reason goes to STBD...or am I missing something?

 

[Cfc1215]

I would agree with your conclusion assuming the radar is set to use the heading data from the SCX 20 while moving (which it should).

 

[Johnny Electron]

One word of caution as you do your troubleshooting/testing, please ensure no one is sending PGN127237 (Heading/Track Control) on the bus.
The NavNet doesn't like this PGN. I have seen people turn this sentence ON from their autopilot by mistake and cause lots of issues with set course. Normal heading comes from the SCX20 for example and you don't want anything delaying or regurgitating that data.

 

[Gyrfalcon]

Hi,
Thanks for this advice. Our autopilot is independent of the Furuno (we haven't gotten them to speak successfully, but will keep this in mind for the future.
We have successfully fixed our issue, I think. I realized that I could use an infrared thermometer placed against each side of the SCX20 as a laser line, and shoot straight lines from each side of the SCX20 to our bow (which is 20 ft in front of the unit). When I did so, it was clear that the SCX20 was offset to port by a few degrees. Based on the average difference between heading and COG (usually about 5+/-4), we put in a +4 correction factor, and it looks like that solved the problem. Hooray!

 

[Johnny Electron]

Great job!