Greetings,
Our new boat has a magnetic compass mounted near enough to our Garmin GPS/radar screen to effect it's accurate operation. I would really like the compass as a backup in the event of electronic failure.
Is there any way to tune out the close effects of the electronics to increase the accuracy of the compass without moving either the compass or the electronics?
Both are mounted in the helm area in such a fashion that moving either will create a hole in the dash that will be both unsightly and difficult to patch.
Greetings,
Mr. ps. Not sure of the answers to any of you questions. I only remember that last year the indicated heading on the magnetic compass was off a significant amount as compared to GPS readings. We've only been on the water about 5 hours with her so she's REALLY a new boat to us and the memory dims.
I'm assuming that there ARE adjustments as most marine compasses do have them.
I can check if the compass is affected when the GPS is on or off.
As per any new boat owner we have "the list" and this was just one item that popped into my dimming mind as I sit here on TF. In this case, perhaps I'll defer the question until I have more information. Thanks.
Would like to get some items crossed off before we head away from the dock.
Get a satellite compass such as Garmin MSC10 or Furuno SCX20. They calculate heading based on the signals from multiple GPS units.
Aside from solving your problem, they have the advantage that the boat won't change course going through a bridge or over a tunnel. They also take out the effect of rolling. Highly recommended.
Back in the dark ages, I worked for Captain's Nautical Supply in Seattle. Probably one of the last businesses that would swing a compass. We usually did it on Lake Union, where sighting down Seattle's north/south streets above Gas Works Park helped get things aligned. A full deviation card was 24 headings (15 degree increments) and would require running back and forth, so 48 bearings. Took several hours.
I haven't bothered to do that on my own boat. It is on my to do list, but it ends up so far down that I only think of it once in a while. Even though it would be simple to do given the Garmin GPS also on the helm. Speaking of which, my Garmin had a little door that covered the mini-SD card slot. It snapped shut in a way that made me wonder if it was a magnetic latch. Yep. Made about a 10 degree difference on the compass (on some headings) whether opened or closed. Moving the compass further away is the only thing I've done to date re compass deviation.
The problem with relying on a compass in case of electrical failure is that one's "charts" are likely on the failed MFD (paper charts now being obsolete regardless of what the CG claims). Better than a compass (recently swung), accurate deviation card, and recently updated charts would be a second navigation app on one's phone, laptop, etc. That's why my compass has turned into something of a relic. Were I planning a circumnavigation (of the earth, not Vancouver Island), I would have reason to worry about the accuracy of my compass. I no longer keep track of local variation.
We gave the customer a nice nautical-looking frameable deviation card to be posted on the helm. Looked real salty. Like having one's ship station license posted by the VHF (and probably just as useful).
Mr. MF. I, for one, will always have paper charts aboard. Call me old fashioned but rather than zooming in and out of the electronics, paper gives me a better, immediate overview of location(s). With touch screens in a sporty seaway I find it VERY difficult to not multiple-touch to a screen/function I didn't want. It's also very possible that our compass has NEVER been swung.
A deviation/variation of 3 or 4 degrees will be quite sufficient for my purposes in the event of an electrical failure.
One change I plan on doing is modifying the GPS screen to display analog readings of engine conditions (temp, rpm, fuel etc.) in spite of the fact I have digital readouts of all necessary numbers. I really much prefer analog over digital.
Not long after I got my boat, I replaced the compass with a model that had compensation capability. A Plastimo model, so it didn't break the bank, but it provided what I needed in the space I had available. There were a couple of very useful documents that I downloaded at the time. I used one of them to guide me through creating my very first deviation curve. I attach them here, in case they might be of any help to you. Doesn't matter what make your compass is - but hopefully, it will have compensation screws.
P.S. They say you are supposed to check/update your deviation curve every year, as the magnetic field of the boat can change while in winter storage, for example. I confess I have not done this, though I have done a few cross checks to confirm that my original curve is still "good enough" for the coastal cruising I generally do.
Can you post a picture of your helm layout? These days everyone has something within a foot of their compass. If you can’t adjust the deviation out then make a deviation table. TVMDC add West.
__________________
Archie
Irish Lady
1984 Monk 36 Hull #46
Currently in Stuart, FL
Greetings,
Mr. HW. I'll try tomorrow IF I can get aboard. It's raining buckets here in FLL. 4" yesterday at the airport. Probably similar today. Good idea on the table.
I strongly suspect there are adjustment screws. In order to swing the compass I would have to be away from the dock and given the weather AND the forecast it may not happen for another 2 or 3 days.
I might be able to use the compass on my phone to get rough directions. That and observing compass readings when the electronics are on and off should give me a better idea of the situation.
Thanks
Before I would get into swinging the compass I would query Garmin and see if they have anything to put on the power cables to suppress the interference. Also maybe put a shield on the cabling and see if that helps. Then if all else fails swing the compass. It can be a long process especially if you haven’t done it before. We used to be required to have a deviation table on our boats as CG Facilities. It would usually take all day and we had done it multiple times before.
__________________
Boat Nut:
If you are one there is no explanation necessary.
If you aren’t one, there is no explanation possible.
Usually for magnetism I thought it was twisting wire pairs (parallel DC wire cause magnetic fields) to solve/prevent issues....less about shielding.
Nowadays with GPS, swinging a compass to reasonable specs is pretty quick if you can find an open body of water where you are not subject to current or wind leeway. The instructions are with new compasses or probably online.
Twisting the wires can help, using a ferrite core may help and sometimes shielding may help. The problem with using a GPS to swing the compass is that if the GPS is interfering with the magnetic compass how do you know the compass is swung properly. Maybe turn off the GPS and swing the compass. Then turn on the GPS and see how much difference there is.
__________________
Boat Nut:
If you are one there is no explanation necessary.
If you aren’t one, there is no explanation possible.
Twisting the wires can help, using a ferrite core may help and sometimes shielding may help. The problem with using a GPS to swing the compass is that if the GPS is interfering with the magnetic compass how do you know the compass is swung properly. Maybe turn off the GPS and swing the compass. Then turn on the GPS and see how much difference there is.
It depends on what causes the deviation. I need to swing mine after a chartplotter replacement. But the new plotter causes the deviation just by being mounted close to the compass. Turning it on or off makes no difference, so using GPS COG for compensation would be fine.
It depends on what causes the deviation. I need to swing mine after a chartplotter replacement. But the new plotter causes the deviation just by being mounted close to the compass. Turning it on or off makes no difference, so using GPS COG for compensation would be fine.
The whole point of swinging a compass in my book....
Isn't it GPS heading really what you want versus COG? Of course the instructions usually say no find an area without set or leeway so your COG/heading are the same.
I have never heard that shielding affected magnetic fields on power cords and the other wires usually have shielding. I would like to see some data on that as we never learned about it in my factory marine electronic installer's certification courses.
If I wanted to see the difference between my chart plotter both on and off for swinging purposes, I would just use a compass app on my phone...good enough for small boat navigation. Either way, one is just wasting time doing compass swings the old way compared to good enough with a GPS.
In the last 35 years of recreational and commercial boat operation, the only time I used a magnetic compass was to make a turn on in limited vis or high speed at night. The actual heading was not important. I still always had one, and swung them (with GPS after it came out)...but never really needed one to navigate so can't get excited about them anymore.
The whole point of swinging a compass in my book....
Isn't it GPS heading really what you want versus COG? Of course the instructions usually say no find an area without set or leeway so your COG/heading are the same.
I have never heard that shielding affected magnetic fields on power cords and the other wires usually have shielding. I would like to see some data on that as we never learned about it in my factory marine electronic installer's certification courses.
If I wanted to see the difference between my chart plotter both on and off for swinging purposes, I would just use a compass app on my phone...good enough for small boat navigation. Either way, one is just wasting time doing compass swings the old way compared to good enough with a GPS.
In the last 35 years of recreational and commercial boat operation, the only time I used a magnetic compass was to make a turn on in limited vis or high speed at night. The actual heading was not important. I still always had one, and swung them (with GPS after it came out)...but never really needed one to navigate so can't get excited about them anymore.
If you have a good source of heading info that you know is accurate like a sat compass, yes, that will be best. But otherwise, CoG in an area with as little wind and current as possible will get you close enough that you probably can't read the compass any more accurately anyway.
I agree, about the only time I use the compass is to give the admiral a course to follow when hand steering.
What some instructions recommend, is to use a bearing between a known charted object where you start and another one you head for. I never really needed that as I usually found a calm wind moment with no current so for me it was easy to swing those compasses.
Having a compass showing cog and heading simultaneously is sometimes of benefit. It allows you to calculate set. Particularly helpful in open ocean when there’s no land visible to detect effects of currents. An example is entering and leaving the golf stream. You’re commonly at right angles to current so speed isn’t always effected . You want to make use of breakouts and swirls to increase days work. Also coastally when crossing repetitive headlands such as you see along Maine coast. Especially when using AP running off waypoints you make not appreciate the presence of set.
To my understanding - Any expanding and contracting electrical field will be associated with a comparable magnetic field. AC or DC both do this. It’s what makes electric motors (generators ) work and induces current passively in previously current non bearing wires. It screws up compasses, antennas (especially SSB) and other electronics. Goal with magnetic compasses be they for APs, hockey puck bearing or fixed location for navigation is to have little or no such magnetic fields near by. With the inverse square law minor increases in distance can be quite helpful. When using a handheld to get a bearing will do it twice in two locations. The small difference in location distance should not change bearing much if the object is far away.
Can you mount your magnetic compass on the overhead away from radios and the like? If so and adding few ferrites to decrease field size may make a huge difference. Given magnetic field strength and distance strongly effected swinging the compass may not be sufficient by itself as field intensities may vary. Look at everything. Even a small fan or windshield wiper motor can contribute.
So would place your compass where it’s least likely to be effected. Watch it with various things on and off. Ferrites or shielding or moving offending objects as necessary. Also pay attention to magnetic and non magnetic iron pieces as those can cause field distortion. Only then to the very important work of swinging it. Swinging it is key but can only give you a steady reading if disturbing things are eliminated or minimize first.
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