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It is difficult to find information which is of value to yachtsmen on this subject..  The compass manufacturers tell you to find ranges and adjust your compass from that - those instructions are useless.   Professional maritime texts tell you how to construct a deviation table -this procedure does not correct the compass.  What follows is a method to actually correct the compass, written simply and in layman's terms and I found this descriptive article to be the most useful.

Magnetic Compass Compensation 

There are at least three things that make a magnetic compass in good repair not read true north. 

1. Variation: This is the offset between the geographic North Pole and the magnetic North Pole (yes I know this is a little generalization, but it's close enough for gov'ment work). 

2. Deviation: This is the result of magnetic disturbances on the boat that have an effect on the compass. 

3. Misalignment: This really doesn't make the compass read incorrectly, but it makes our interpretation of it incorrect. Misalignment means that the lubber line is not parallel to the keel.  When this happens, we think the compass is showing us the course we are travelling, but it does not.

We can only correct for Items 2 and 3.   Item 3 is the simple, but sometimes difficult, task of making sure the lubber line is parallel to the keel.  This should be checked on every vessel.   It is a very common and unfortunate cause of magnetic compass problems. This epistle will not deal with this subject, but don't overlook it. 

All magnetic navigation compasses provide a means of making adjustments to compensate  for deviation, and vessels constructed of different materials require more or less means of adjustment. Steel vessels are a special problem and will not be discussed here. Vessels constructed of fibreglass or any other non-magnetic material present few problems so that they can almost always be solved by making adjustments to the compass, or by relocating equipment and/or wiring. 

If after going through a complete compensation procedure, the compass is still not compensated, the most likely problem is that the magnetic deviation is caused by electrical equipment installed too close to the compass.  In that case, either the compass or the offending equipment must be moved.  The offending equipment could be electrical wiring. Just remember that all of the 12 volt DC wiring close to the compass should be of twisted pairs.  That's another problem that we won't cover here.

The Nature of the Beast 

There's nothing wrong with your compass.  It is aligning the "needle" with the magnetic fields it finds itself in.  It can be influenced by three magnetic fields: 

1. The magnetic fields of the earth.

2. The magnetic fields generated on the boat.

3. The magnetic fields of the adjustment mechanism in the compass. These are really magnets set 90 degrees apart.

The whole compensation exercise is to remove the influence of 2 and 3, so that only the magnetic fields of the earth operate on the compass.  If we do that, the compass will display the correct magnetic directions.

It is important for you to know that no compass, to my knowledge, comes from the factory with the adjustment magnets in any but random positions. That means that your compass will almost certainly read incorrectly when you first get it, even if you have no magnetic disturbance on the vessel. All new compasses should be compensated. 

What? - before How? 

You must answer 'What' questions before you can entertain 'How' questions.

Here's what we're going to do:

1. Take the compass off the vessel.

2. Put it in a magnetic free area

3. Remove all the effect of the adjustment magnets

4. Return the compass to the vessel

5. Using the adjustment magnets, counteract the magnetic disturbances produced by the vessel, which act on the compass.

Then your magnetic compass will tell you the truth about magnetic directions. 

The General How?

The compass itself will be used to make corrections to itself using the following fact. 

    If you turn the compass 180 degrees physically, it should show that 180 degrees change on the card. 

If it does not, some magnetic influence is affecting the compass besides the earth's magnetic field. Our job is to make the compass reflect the 180 degree physical turn accurately by cancelling or removing the other magnetic influences on the compass.  We will concentrate on cancelling those influences.

The very first time we do this, we must first perform this exercise off the boat and then on the boat. After the first time, you'll just need to do it on the boat.  The procedure on and off the boat will be different, but the result will be the same. It is important for you to fully understand that in both cases, we are going to make the compass card reflect a measured physical direction change of 180 degrees. That's all there is to it. Any procedure which does that can be used to compensate a compass.  In our case, our only problem is how to measure the physical 180 degree direction change we will force upon the compass.

Off The Boat

We take the compass off the boat to remove all of the existing adjusting forces so that our job on the boat will be easier. We will have much more control, and the exercise will be much easier, on land than on the sea. 

You will need:

1. Some means to make the compass sit flat on a surface. The shape of the compass may well allow this, but if not, anything non-magnetic that will hold the compass will be fine. 

2. Some flat object with square sides to hold the compass. A large book, a piece of wood cut square, a small cardboard box, etc. 

3. Another flat object to place Item 2 on. 

4. The adjustment key that came with your compass.  What, you don't have it?  You must find a non-magnetic blade of some kind to turn the adjustment screws.  Some electronic stores have plastic screwdrivers.  But it must be non-magnetic.  I've made such a tool from a penny with a file.  If you do this, be sure to wash the finished tool to take off any steel residue.

5.  A straightedge:  A yardstick or non magnetic carpenter's level  will do. 

Item 2 should be smaller than Item 3.  We're going to rotate Item 2 on Item 3, guided by the straightedge.  A book would serve well for Item 2.  A wine case would serve well for Item 3. Just don't drink the wine till we're finished compensating the compass.

Work on any level surface away from any magnetic influences. Indoors is generally bad because of magnetic influences. Remove your watches, knives, belt buckles, or any other material that may be magnetic. 

Place Item 3 on the level surface.  Then place Item 2 on Item 3.  Then place the compass in Item 1 on item 2. You now have a compass on a "book" on a "box." I'll use the book and box identifiers for what follows.

These are the steps:

A.   rotate Item 2 (the book which holds the compass) until the lubber line of the compass is either on East or West. 

B.   Place the straightedge along side of Item 2 (the book). Then, being careful not to disturb the compass on Item 1, and holding the straightedge firmly, turn Item 2 (the book) 180 degrees. The straightedge lets you do this perfectly.  Presume you started with the lubber line reading 90 degrees.  It should now read 270 degrees (90 + 180 = 270).  If it does not, you have a compass error. 

C.   Correcting the compass error will always be done in the same way.  Using the adjustment key, remove ONE-HALF of the error.  If your compass reads 280 degrees, you would adjust it while keeping it firmly in place so that it reads 275-degrees.  When you are making adjustments for the E/W direction, adjust the screw adjacent to the N/S lubber line.  The adjustment magnet is always 90-degrees from what you want to adjust for. 

D.   Repeat steps A thru C until there is no error.  Notice. you are changing where the compass says East is every time you make a correction. The wonderful compass is telling you about its errors itself. You will be able to adjust it until you get no error if there are no magnetic disturbances and you do the job properly. 

F.   Now go through the steps A thru D again, except use North (0 degrees) and South (180 degres) instead of East and West. 

G.   When all the error is gone, make a final check. You'll find that you have a compass that tells you the truth. 

Make sure you are familiar with what we just did.  If you are not familiar with this, do it again until you are - it will help you on the boat.

On The Boat 

We now have a compass that reads the correct magnetic directions when not under the influence of magnetic disturbances.  When we mount it on the boat, we must check it to see if we have placed it in any magnetic disturbances.

If we have, we'll make adjustments to the adjusting magnets and cancel those disturbances.

What We Will Do 

We'll use the compass itself again to check itself.  Remember, all we must do is prove that the compass card accurately reflects the 180 degree physical turn we make to the compass.  Instead of turning the "book" on the "box" 180 degrees to turn the compass, we will turn the boat 180 degrees, and that will turn the compass 180 degrees.  Our main problem is what to use for the straightedge we used with the "book" and "box."  We'll use the SUN and the shadow it casts as our straightedge.  You can buy the tools necessary for this work from navigation supply places, but a device you make yourself is easier to use and much easier to store. 

The Tool 

We're going to end up with a piece of wood about 3 inches square and about 1 inch thick - the exact dimensions are unimportant.  On one side, you scribe a line from one edge, through the centre to the other edge.  This piece of wood will have a pin attached to it at the mid-point of the line about 3 inches high and perpendicular to the piece of wood.  The pin is called a SHADOW PIN, and the piece of wood is called the SHADOW BLOCK.  When the shadow block is level, we can use the shadow cast by the sun shining on the shadow pin as our straightedge.  The only requirements are that the shadow block is level and the shadow pin is perpendicular to the shadow block, and that the shadow block remains motionless during our work. 

To meet the three requirements stated above: 

1. Get a square or rectangular Tupperware container to put the shadow block in.  This will be called the SHADOW DISH.  The shadow block should fit easily into the shadow dish.  When we use this device, we will float the shadow block in the shadow dish.  This will keep it level.  The shape of the dish will keep the shadow block stable.  If the shadow block hangs up on the edge of the shadow dish while it floats, the shadow block is too large and must be trimmed.  If it lets the shadow block move around a lot it is too small.  It needs to be just right. 

2. The shadow pin is easily made from a piece of brazing rod.  That rod is non-magnetic and serves well.  Just drill a hole into the shadow block using a drill press to accept the pin. The use of the drill press will keep the shadow pin perpendicular to the shadow block. 

Got the picture?  We just have a block of wood floating in a container with a pin in the centre with a line scribed on the block.  This tool is easy to make and will serve you forever. I made mine in 1965. 

The On Boat Procedure

These instructions are for situations where one person can get to both the tool and compass, and the tool is exposed to the sun for 360 degrees.

We're going to do exactly the same thing on the boat as we did on shore.

The boat needs to be at no more than idle speed.  The seas need to be calm.

The sun needs to be 30 degrees or more above the horizon.  Insert the shadow pin into the shadow block, and place the shadow dish with the floating shadow block where you can get to it easily. 

A. Steady the boat up on either 90 or 270 degrees.  We'll assume 90.  Rotate the SHADOW DISH until the shadow from the pin falls on the scribed line.  It is important that the vessel compass reads exactly 90 and that the shadow is exactly on the scribed line. 

B. You have five minutes for this next operation - read carefully.  Look at the shadow block - DO NOT look at the vessel compass.   Turn the boat slowly to cause the shadow to go around the shadow block.  Steady the boat up so the shadow from the shadow pin stays on the scribed line 180 degrees from where it started.  When it does, you have turned the boat exactly 180 degrees. 

C. Now, and only now, look at the vessel compass.  It should read 270.  If it does not, keep the vessel steady so the shadow stays on the scribed mark. The shadow dish must not move until this step is finished.  Observe the compass reading and change the compass heading half of the error by turning the adjustment screw.  For example, if the compass reads 290 degrees, adjust it to read 280 degrees.  Make the adjustment only when the shadow is exactly on the scribed line.  When you are finished, the compass must read 280 degrees while the shadow is on the line. 

D. Repeat A thru C until you have no error, or you can't make the error any smaller. 

E. Do exactly the same thing for the North/South headings. 

F. Recheck it one final time.  Most likely you will find that you cannot read any error. 

You should check it at least once a season, after bad electrical storms, and anytime you add or move magnetic stuff around. 

You can carry this procedure further by checking the compass at every 45 degree interval, but making corrections in directions which are not E/W or N/S is very difficult because both adjustment screws would need to be turned to make any adjustment.  That's beyond what most of us can do successfully, but it does no harm to check as many directions as desired.


Often the adjustment screws on compasses are hard to get to on yachts, and often the shadow dish must be located away from the compass station.  In those cases, you will need some help.  At most, you will need one person to steer and read the compass, one person to make the compass adjustment itself, and one person to tend the shadow dish.  The only problems these situations create is one of communication.  If you work that part out, you'll have no problems with making the adjustments. 

Sometimes you just can't get all the error out.  In fibreglass boats, the most common problems are wiring and electric motors.  In those situations, you must either move the compass or the offending stuff.  I have always managed to fully compensate any compass on a fibreglass boat.  I have even compensated the compass on my dinghy. 

One little known hazard is to have two magnetic compasses too close together.  You can never get them compensated since they present rotating magnetic fields that change every time the direction of the boat changes.

Be especially careful of mounting an electronic flux-gate compass near a magnetic compass.  Neither will work well at any time. 


OK?..........   Go compensate!                         

Accredited to: CaptnWil
Wil Andrews
40 Pier Pointe
New Bern NC 28562
(252) 636-3601


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