The Offshore Voyaging Reference Site

You Still Need an Accurate Compass

Finding a home for all our provisions before heading off on a cruise is always a challenge since I tend to overstock on the basis that we will use everything up in the end but running out of something vital in some out of the way place would really inhale. So, as always, there was a last-minute scramble to put everything away before we left last week. And then off we went for our 200-mile overnight.

It quickly became apparent that our fluxgate compass, which runs our autopilot, was off by at least 15 degrees. At first we assumed that it was due to current pushing us off course. However, because we don’t have our autopilot connected to our plotter and we have a magnetic compass on the binnacle that we have swung regularly, it didn’t take us long to realize that the magnetic compass and our course didn’t correspond to the autopilot heading and we were able to compensate for the error without any great difficulty.

Then we started trying to figure out why the compass, which had been working fine 5 weeks ago, should suddenly act so strangely. What had we done in the meantime?

Why, stored a pile of provisions in the locker directly next to where the compass is mounted below. What kind of provisions? Why, tins of peanuts and porridge! Yup, when tested with a magnet, the tins were definitely ferrous. So a big shakeup of provisions ensued and the compass miraculously straightened itself out.

In most circumstances this would be no big deal. However, given the wrong situation:

  • plotter dies,
  • datum way out on the chart against the GPS,
  • intricate navigation;

this could have been a problem.

Keeping our electronics separate (separate radar, separate autopilot) and incorporating redundancy into the system:

  • magnetic and fluxgate compasses,
  • second stand-alone GPS,

all contributes to maintaining our situation awareness—knowing where we are.

Old-fashioned, maybe, but it could save the day given the right (or wrong) circumstances.

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Paul Mills

Hi Phyllis,

Sounds like a classic! . I had a similar one two years ago when a ‘helpful’ crew decided to put my sandals into my cabin for me, only problem being that they have magnetic straps, and the fluxgate is under the head of my berth…… .

Redundancy is agreat asset on a boat, in my case the error was obvious because we were heading fo the headland rather than around the end of it, however in other circumstances it might have take a while to notice the 25 degree error.


John Harries

Hi Paul,

Magnetic shoes, who knew. The number of ways that one can get messed up in the game never ceases to amaze me.


If you had it integrated with the plotter you would have noticed this as well!
If you would have had a waypoint set in the plotter you would immedeatly seen the difference between the course the plotter say it is to the waypoint and the compass course from the fluxgate. Your system would have been taking you safely to the waypoint even with the wrong course from the fluxgate. But yes I agree redundency is the key. And a reliable magnetic compass is still a must on a boat.

John Harries

Hi Roland,

You are, of course, right. That is as long as the plotter continued to work. But, like you, we still like to have a well adjusted high quality magnetic compass as the ultimate check. Does not need and electricity either.


Roland—You and I have a religious difference with John and Phyllis: We believe in sometimes letting the chart plotter and auto-pilot steer the boat (while, of course, keeping a careful watch), but they don’t. About 2 decades ago a cruise ship bound from the Caribbean to Boston, Mass., ran aground on Nantucket Shoal. Whatever instrument she was using to navigate was off by 15 miles. The U.S. Coast Guard District Commander was quoted as saying: “Electronic navigation instruments are fine, but you still should look out of the window from time to time.”

Brian Lockett

Your storage tale reminds me of a delivery where I, as navigator, finally figured out that the substantial compass error was due to a crew member storing flashlight batteries in the binnacle cover.

John Harries

I trust you strung him up by his thumbs?

Flemming Torp

Yes, it can be pretty problematic, when we forget how some of the installations in the boat works.
Before the holiday trip to Norway this season, I wanted to carry one more anchor og some more chain. I put it in the locker close to the cockpit … You can image the consequences, bacause the fluxgate compass, was placed just below …
The other day, I removed an “car like” old antenna with a magnetic “foot” … and I just put it on top of my basket … where my wallet happen to be … the consequense? All data om my credit gards, my gasoline cards, insurance card, my personal ID card etc. were erased … I did not learn it until my Visa Card did not work … Oh, what a lesson …

Best regards Flemming Torp
pt. at anchor in Nerdvikvågen – SE of Smøla – Norway on the way to Lofoten …

Colin Speedie

Hi John

We just had a brief reminder of this issue with the charter boat we used for our latest survey, when we noticed a huge error on the AP compass, traced to our two mobile phones (switched on) sitting 1/2 a metre away on top of the locker – and I never had heard of that before.

Best wishes


John Harries

Hi Colin,

Wow, a cell phone at 50 cm. I would have bet that the compass safe distance would have been much less than that. Really worth knowing.


Beware handheld VHFs too! The new one got me first time out, fortunately in home waters where I just knew the compass was lying…
Thanks as always Colin for reminding us to be ever vigilant.


And thanks John and Phylis too!

Erik Snel

We installed our new autopilot last year shortly before a 2 week cruise and it worked flawlessly at installation. Magnetic compass and fluxgate showed similar readings and auto steering was a breeze.

Right before the cruise we loaded the yacht with an enormous supply of food etc, so much that we had to relocate some stuff from one locker to another. When we sailed we engaged the autopilot and again it worked fine. Then all of a sudden the boat lurched to a 30 degree different course. Being in the middle of a not so wide channel between sand banks, I took over to manual steering until the channel widened. At that point I once again engaged the autopilot on the course we were steering manually. And it operated just fine! About 15 minutes later a big lurch to another course.
We than re-calibrated the whole autopilot and fluxgate compass system. Only after that I discovered a pan that had slipped under a backrest in the cabin, left there when we were shifting stuff from one locker to another. The pan would shift in a bigger than usual wave to another position on the couch, next to a board on which the fluxgate compass sensor was installed on the other side. The difference between the pan resting right next to the sensor or 30cm away was enough to totally disrupt the operation of the fluxgate!

By the way, in the areas we cruise in, magnetic compass course and the course to a waypoint over ground are almost never alike, as current usually sets us of in one or the other direction. Therefore the tip to use the chart plotter to check your fluxgate compass can be ill advised. I tend to use the information the other way around: comparing compass course and bearing to a waypoint as shown by GPS gives me a good indication of the current we’re dealing with.


John Harries

Hi Erik,

Yikes, than pan had it in for you!

And a really good point about the affect of currents. This in one of the reasons we, like you, make sure that we have a really accurate master compass and refer to it often. A practice that I suspect is becoming less and less common as people just blindly steer to the plotter with no understanding of the effect that current and tide is having on them.


Chart (noun from Latin charta): A map of the sea that tells you what object you just hit.


Hi John and Phyllis,
I have a somewhat different question regarding Aids To Navigation : Drones or maybe called the new hi-teck crowsnest. Have been thinking about these and can be very usefull especially entering new and or tricky situations.
Does any one have any experience with these and recommend or avoid some??

As for compass errors, I have another unusaul experience, not at sea, but Sahara dessert.
My truck needed a small repair and had sent my crew ahead and I would follow in an hour or so. Had a compass, not that they are very accurate in a truck, but helpfull. We would meet about 85km south of basecamp, but after 100km still no sign of the others. Stopped and climbed on top of truck to have a wider look, when I noticed a the machanic had left a magnitised screwdriver near the compass. So had no choice to drive back to basecamp and start all over. Many drivers were dragging a piece of chain thru the sand which tells you what tracks are yours, but I had decided to dragg a piece a frayed steel cable which clearly indicated what my tracks were. When ¨lost¨ at sea you still have resources you dont have in a 50´C dessert, reason the truck cooling systems are water filled only, to make it drinkable.

John Harries

Hi Rene,

I really don’t know much about drones. I have done a bit of research and the biggest issue is that if they land in the water they are toast. That said, I did hear of a company that was working on a waterproof one that could even take off from the water.

Ernest E Vogelsinger

Hi John,
I’m somewhat late to this party – but do you have a recommendation for a binnacle compass on a steel boat? Currently I only have an electronic fluxgate with the sensor mounted on a mast 2m above the steel hull, but I’d rather have a “normal” magnetic compass as this works even when everything else breaks down. But I’m unsure about which kind of compass would work in this case?

John Harries

Hi Ernest,

I don’t. But the little I know about compasses on steel boats says that it’s not about the compass itself, but rather the installation that must include compensation magnets and steel balls, and then getting someone really good to swing and adjust it. Might be best to just install a good quality analog read out where you can see it from the helm, and maybe a second flux gate for backup, although that won’t help in the event of a power failure.

All that said, I do like having a mechanical compass as the final arbitrator of heading and as a backup:

Rene Blei

Remember what happened when Captain Bligh, s/v Bounty, was put in a row/sail boat to fend for themselves?
He was given a compass and made it back home.

Marc Dacey

Correct. We have a Ritchie Globemaster inside our mild steel pilothouse (but with an aluminum roof) and it sports the amusing named “compensator balls”. We pay attention to deviation and variation where we are and note in the log when it’s a nice easy number (Canadian Maritimes are 20 degrees West, so easy to do) and we compare it to GPS and fluxgate inputs. It was clearly set up well whenever it was last “tuned”, but I have to be careful with changing any devices nearby, of course.

If I remember correctly, it will need to be “reswung” in the Southern Hemisphere.

John Harries

Hi Marc,

Not only will you need the compass swung again if you go south, you will also need the card changed to a southern hemisphere one that is properly balanced for the different dip angle.

Also, if you have never had your compass professionally adjusted I would recommend you do rather than relying on the checks you list, particularly since, if memory serves, you had a bunch of welding done in the area. There are, I’m pretty sure, good professional adjusters operating in Halifax, so now would be a good time.

Marc Dacey

Thanks, John. No welding done that close, but I could call into Halifax and make some inquiries.

Rob Gill

Hi Marc,

I think changes may not need to be that close to have an effect in a steel vessel. Essentially your whole boat is one big combined hard and soft magnet and a complex resulting magnetic field, which your compass sits in, and is adjusted for.

You will then most likely have a deviation card, being the remaining uncorrectable compass deviation to add / subtract to your magnetic course / bearings to give you compass course / bearing.

From memory of ship’s magnetism forty years ago, any welding, hammering, heating etc on a steel structure can change your induced soft magnetism. And any new structure if it is not stainless, will probably introduce new permanent magnetic change.

And over time, steel naturally demagnetises so changes occur in a steel vessel, even if you do nothing.

On ships we would re-swing the vessel after any major works / repairs and I think every five years for survey (it was 40 years ago).

We have an aft cockpit fibreglass yacht. We even have alloy propane tanks. This gives a neutral ship’s magnetic field and we only need to worry about things in the immediate area around the compass.

Things like swapping an all plastic hand-operated foghorn to a compressed-air in a can operated one, near the helm station… but what professionally trained navigator does that?

John Harries

Hi Again, Rob,

I’m guessing, but do not know, that Richie being more of a commercial outfit, are just more picky about this stuff, rather than just putting in a “Universal Card” and calling it good. It might be, (again I do not know for sure) that a universal card will have more turn lag than one that’s specifically balanced for the zone.

John Harries

Hi Rob,

Thanks for the fill on steel boats.

Arne Mogstad

Hi. I am servicing my steering system, and as such the compass is also taken out. I want to have it adjusted as good as I can, and then make a deviation table for the remaining. But I have no one that can provide a compass “calibration” service anywhere near me. Did anyone have any good suggestions as to what I can use as a reference heading?

I figured if I use a digital compass (iphone), and a handheld mechanical one, move them around onboard, and using a distant landmark as a reference point, I should be able to get a decent “average” between my compasses, and use that to note the deviation against? Any suggestions is highly appreciated!


Ernest E Vogelsinger

Hi Arne, I’d suggest not to use the compass apps of mobile devices, as the accuracy of them may vary greatly, depending on the way the device is held. If you have any GPS based navigation app on your mobile phone, or tablet, wait for a calm day and smooth water, and go for the GPS compass while making straight way at slow speed (no sail). Note that most GPS apps give you true north, so for calibrating your compass you need to add the local deviation.

Arne Mogstad

Hi. Thanks for the tips. I don’t believe the iPhone compass is so great, but it was more to have another independent source that can be moved around the boat to try to get away from any magnetic disturbances.

The GPS-based “compass” I do not believe to be of much value however, as they show course, and not heading. This becomes exacerbated on a slow boat like a sailboat. If I am motoring at 5 knots, and I have even just a 0,5 knot sideways drift due to current/wind, we are talking quite some difference between the heading and course. I haven’t done the math, but it will be so inaccurate that the whole point of the calibration is gone. Especially as I would not know the exact speed and direction of the drift.

I am thinking that using a paper map (or digital maybe), and then on a clear and calm day I’m going to a few known locations where I can see mountains that are far away (40-50 miles away). Then I can find the magnetic bearing to those mountains from my location (taking any variation into account). Since they are so far away, even a slight drift or error will be quite small. Then while motoring straight towards those points in the distance, I can visually aim the boat, and try to adjust my compass. And when I have adjusted what I can in as many directions I can, I take the remaining difference and plot it in a deviation table.

Not sure it would work, but it is the best I have managed to think of. Although it will be a lot of work and time.

John Harries

Hi Arne

Professionals either use a very expensive gyro reference compass (think thousands) or swing the compass using fixed point on land with known bearings from the chart. These days this can be more easily done using the bearing function to a fixed point on the plotter. Using that function you should be able to swing the compass yourself given that I have, back in the day, swung a compass quite successfully using paper charts.

In short the procedure is to: use the plotter to:

  1. Determine the the bearing to a fixed point on land
  2. Steer toward that point
  3. Check the compass and write down the difference.
  4. Repeat for at least N,E,S,W
  5. Adjust the compass to get rid of as much error as you can
  6. Repeat until you get the errors to an acceptable level (2 degrease is good)
  7. Make a deviation table

This is likely to be much more accurate since it’s using the real world and does not rely on any other compass.

That said, you do want to be sure that the plotter is relying on a modern survey with really good datums in the area.

Arne Mogstad

Hi John, and thanks (again!). That is pretty much what I was planning on doing. I would like to use paper charts, as I struggle to find the data for electronic charts (dates and so on). That information seem to be impossible to find sometimes. Besides, I don’t currently have a plotter onboard, as my old Raymarine gear is very obsolete and impossible to find maps etc. Hence a huge desire to upgrade my full electronics suite (NKE, TimeZero, and Furuno). The MEMS compasses does seem to be fairly good though. And I suspect it would be at least as accurate as I could ever get my mechanical compass. I guess a good satellite compass could also function as a reference.

Rob Gill

Hi Arne,

Something you might check before adjusting your compass and whilst it is out of the boat. Have your compass sitting in its normal horizontal orientation and note the compass heading. Approach with a magnetic source. Does the card move freely, or jerkily? Does it return to the same heading when you remove / reintroduce the source?

Try this again, rotating the compass by 90, 180 and 270 degrees relatively. If the card has an irregular / jerky movement or doesn’t return to the same heading, you will need a new card.
Or more likely these days, a new compass.


John Harries

Hi Rob,

Good point. The other possibility, although rare, if the card does not move easily, is that it’s the wrong card for the zone and so binding because of dip. When we bought our McCurdy and Rhodes she was just back from a circumnavigation of South America. It took me a while to figure out why the compass was binding: the previous owner had put a southern hemisphere card in. In those days I just sent it back to Richie for a northern card and all was good. No idea what happens these days.

Rob Gill

Hi John, your comment surprised me a little as our magnetic compass is from Plastimo, and I can attest it works just fine and our boat was bought in the Med and shipped to NZ.

It has the spherical bowl construction like the Ritchie, with a Bimini style mount. It’s 20 years old, but a little research shows it has what Plastimo calls “universal balance”, so operates in either hemisphere.

Ritchie compasses don’t appear to have such an option and are Northern (Zone1) as standard according to their website. Changing the card would still be a factory only modification I’m sure, for safety / survey reasons.

The lack of this option is surprising (it may be there) given that Ritchie seem a more specialist marine instrument supplier, with more commercial / adjustment options.

Especially since ocean going vessels regularly steam North/South and back in the one trip. In my time, we never adjusted the magnetic compass crossing the equator, so these must have had universally balanced cards.

Rene Blei

Interesting, and I didnt know about universally balancing act and wonder how Captain Bligh, s/v Bounty, got home with his life-boat compass?
Or was the universality factor already known at the time? How much of a difference does it make?

John Harries

Hi Rob,

If memory serves, universally balanced cards work in the normal shipping lanes, north and south. The problem comes if a southern card has been fitted and then the boat goes to higher latitudes in the north. This was the situation in my case since the previous owner had gone as far south as Cape Horn and was considering Antartica and so was advised by Richie to change cards to a Zone 6. When I got the boat, that card still worked in Maine, albeit with a visible dip to one side, but, if memory served started sticking around 50 North, it was also slower to respond than the right card (Zone 1).

As to changing the cards, at the time there were plenty of competent compass techs around who could change the card without it going back to Richie (we had a great guy in Portland Maine, although I’m guessing that’s no longer the case, and Richie now say that.

You can read about compass zones here:

Anyway, in these days of flux gate and satellite compasses it’s pretty esoteric stuff. Still I do like having functioning and properly specified magnetic compass so worth knowing about, particularly for cruisers who wish to visit both the northern and southern high latitudes.