DIY Compass Adjusting and Repairs

How to do compass adjusting using GPS | How to replace the compass fluid | How to build a compass deviation table

Summary: Repair a cracked compass dome, replace the compass fluid, adjust the compass using a simple GPS-aided method, and create a deviation table.

You may want to take a look at the book "Boat Navigation for the Rest of Us: Finding Your Way by Eye and Electronics", or one of our other selected books on navigation for more information.

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Repair the cracked compass dome

How to replace the compass fluid

How to adjust the compass using GPS

How to Build a Deviation Table

Two seasons ago, our main steering compass gradually developed an air bubble that continued to grow. Of course, that meant that the compass fluid was leaking out, and it didn’t take too long to find out why.

After 25 years or so, with a number of them spent in the tropics, the Plexiglas dome had developed some crazing and even some deep-looking cracks. One of them was slightly damp, showing where the compass fluid was leaking.

Replacement parts for our old compass weren’t available, and after looking at prices for similarly-sized new compasses, I decided to try fixing the cracks in order to get at least another season or two out of it (costs for a similarly-sized new one were in the $1000 range, and I wasn’t prepared to spend that kind of money at the time).

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Repair the cracked compass dome

The compass was easily removed from the binnacle by undoing two screws. Then the compass fluid needed to be drained so that the dome could be dried, cleaned and glue applied to the cracks to seal them up.

It may be impossible to do this on some smaller fully-sealed compasses, but, on this one at least, there was a drain plug screw which was easily undone. I emptied all the fluid into a clean bottle, trying to capture as much of it as possible in case I needed to reuse it.

After a thorough cleaning and wipe down and leaving the dome overnight to completely dry off, the next task was to try to seal up the leaking crack and strengthen the deeper ones so that they wouldn't start leaking.

It’s now been about three years since I made this repair, so I'm very happy with how it's work out so far. The method I used was to apply some super glue (of the “Crazy Glue” variety) to the affected areas so that it entered the cracks as much as possible, and let it cure. I then applied some two-part epoxy glue over the areas as well.

Since the cracks were fortuitously positioned along the sides of the dome away from the normal viewing position, the glue doesn’t impinge on reading the compass.

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How to replace the compass fluid

Once the cracks were sealed and the glue had plenty of time to cure, it was time to replace the fluid.

Compasses are generally filled with one of two different types of fluids: alcohol-based and oil-based. Since the compass card is printed with paint specifically designed to get along with the type of compass fluid in which it’ll be living, it’s a good idea to refill with the same type. Otherwise you may wind up with the compass card paint flaking off, turning your finely crafted precision navigation instrument into a snow globe!.

But how do you tell which type of compass fluid it is?

The method I used was to take a small piece of paper towel (1cm x 1cm, maybe), dip it into the fluid, put it into the sink (anywhere safely non-flammable would do), and set it on fire.

If it is alcohol, it’ll burn with a bluish or even an invisible flame. If oil, it’ll burn yellow and give off black soot. Mine was oil.

Since the fluid I removed originally from the compass wouldn’t be enough to fill it completely back up (because some had leaked out causing the bubble), I decided I’d rather go with all new fluid rather than have a mixture. This was just in case the two fluids weren’t compatible, or the viscosities were slightly different, leaving different strata in the bowl.

You can order compass fluid from a number of different companies over the internet, but mineral oil, also commonly sold as unscented baby oil at drug stores, will also do. I wound up picking some up from the local drug store, and it’s been fine – about the right viscosity and is still perfectly clear over two years later.

The only challenges in filling the compass back up were getting the fluid into the rather tiny drain hole without making a giant mess, and getting the compass completely full so that no air remained inside at all.

We wound up building a thinly-necked funnel out of an eye dropper (for the neck), and some paper for the bell of the funnel. It worked nicely for filling it back up.

Getting the last bubble of air out of it proved to be a bit more challenging. We filled the compass as much as we could until there was only a fraction of an eye dropper’s worth of fluid to go. We then disassembled the funnel arrangement and used the eye dropper as it was originally intended.

The idea was to hold the compass so that the drain/filler hole was at the top dead center, and have the fluid-filled eye dropper plugged into the hole. With a lot of tilting of the compass, we managed to get everything but the tiniest bubble out of it. After quite a few tries, we never got it any closer to being 100% full, and eventually gave up, deciding to just live with the pinhead-sized bubble.

But the next morning, we were delighted to find that the tiny bit of air had dissolved into the fluid, and the compass actually was completely bubble-free, and it has been so ever since, even during the coldest winter days.

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How to adjust the compass using GPS

When removing the compass for the dome repairs, I had noticed that there were a number of mild steel screws in the underside of the cockpit table (the table is built around the binnacle and compass) that would definitely interfere with its operation.

I made a thorough inspection of the area and was fairly horrified to find a number of steel bars used to brace the table (right under the compass!) as well as quite a lot of rusty steel screws. That definitely wouldn’t do (because the magnet in the compass would want to point at the iron bars rather than line up with the earth's magnetic field!), and I removed all of that stuff and replaced them with non-magnetic materials.

That also meant that the compass needed adjusting for sure – chances are, it had never been adjusted in the past, but even if it had, with all the steel I removed from around it, it would certainly need it now.

Compass adjusting really isn’t difficult at all and is well within the abilities of the DIY-sailor, especially if you use GPS to help.

Most of the complication of traditional compass adjusting stems from having to use a pelorus and a known/charted landmark or buoy a fair distance off in order to set your boat on a stable and precise North/South and then East/West course.

With GPS, calm water, and no current, this is easy. If your unit has the option of automatically showing magnetic bearings, all you need to do is check that the magnetic variation correction factor which it uses, matches the local variation shown on the compass rose on your chart. Otherwise, you’re best off setting it to show true bearings and apply the local variation yourself, taken from the chart.

Then by following the steps below and using the North/South and East/West deviation adjustment screws on the compass, you’ll be able to get it adjusted nicely and build a deviation table so that you’ll know what compass course to steer for any magnetic heading you desire.

Let’s assume you can set your GPS to magnetic, and that it matches your chart, you've got a nice, calm day, and no tidal flows or currents to contend with (GPS shows your course over the ground (COG), and the COG will only match your magnetic heading in still water):

1. Motor along (you can do this under sail as well, but it will be harder to steer the courses you need, depending on the wind direction and strength) on a course exactly North according to the GPS.

2. Maintain the North course as per your GPS and use the North/South adjustment (with a non-magnetic tool!) screw on the compass to make it read exactly North.

3. Turn the boat around and head exactly South according to the GPS.

4. Use the North/South deviation adjustment screw again to remove half the error. That is, if you are reading 6 degrees off South, half the error is 3 degrees, so turn the adjustment screw until the compass reads 3 degrees off South.

5. Turn back to a North heading and remove half the error again - you are expecting it to read 3 degrees off North due to your adjustment in step 4, so if it reads 5 degrees off North, you've got an error of 2 degrees. Adjust it by half that error (1 degree) to make it read 4 off.

6. Do this a few times until you’ve minimized the deviation readings and are getting the same compass reading North and South each time you turn the boat onto that heading (it likely won’t be exactly 0 and 180 degrees – that’s ok as long as it’s the same each time).

7. Repeat the above procedure for East and West – steer East by GPS, adjust the compass to point exactly East, turn to West by GPS, remove half the deviation error using the East/West adjustment screw, and repeat until the East/West deviation is minimized and you get the same readings for each heading.

8. Re-check North/South just to be sure everything is stable.

That’s mostly it for the adjustment unless you have a steel boat and a compass equipped with quadrantal spheres. Quadrantal spheres are hollow iron balls, affectionately known as “Nelson’s Balls” to Navy folks (named with a somewhat crude sense of humour in honour of Admiral Horatio Nelson, 1758 – 1805).

You can use Nelson's Balls to manipulate the magnetic field around the compass by moving the balls closer to or farther from the compass. Use them after adjusting the compass for N/S and E/W via the adjustment screws has been completed. Put the boat on NE/SW and NW/SE headings and using them in a similar manner as the adjustment screws to minimize deviation on those headings.

The iron balls are mounted to port and starboard of the compass and are often painted red and green to match and to double as a handy reminder to the helmsman as to which colour goes with which side (red to port, green to starboard of course!)

If you are finding huge amounts of deviation, start looking for nearby magnetic metals or electrical circuits that could be putting out a magnetic field. They’ll have to be removed or relocated.

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How to build a Deviation Table

Once you’ve got your compass reading happily and reliably on North/South and East/West headings, it’s time to turn a few doughnuts.

Steer in a circle, stabilizing the course in 15 degree increments, and note the compass’s heading compared to the GPS on each heading. What you’ll wind up with is called a Deviation Table, which will tell you what compass course to steer for any desired magnetic course.

You’ll probably find that the deviation figures form a rough sine wave pattern, with numbers running from negative, through zero, into the positive range, and back again.

My deviation table looks something like this:

Steering Compass

For Magnetic Course of:	Steer:	Deviation:
0	5	5
15	19	4
30	33	3
45	48	3
60	63	3
75	76	1
90	90	0
105	105	0
120	121	1
135	136	1
150	150	0
165	164	-1
180	177	-3
195	193	-2
210	209	-1
225	224	-1
240	240	0
255	257	2
270	274	4
285	289	4
300	305	5
315	320	5
330	335	5
345	350	5
360	5	5

While I was at it I also created a similar table for the autopilot’s fluxgate compass and have two copies of each laminated and posted them at the helm and down below in the navigation station.

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If you have any questions about Compass Adjusting using GPS, building a compass deviation table, or how to replace your compass fluid, please contact us.