Magnetic Compass Compensation

My thanks to Capt. Wil

By Wil Andrews
Wed Apr 15 00:40:01 EDT 1998

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 traveling, but it does not.

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. 

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 Fiberglass 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.

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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 it to remove the influence of 2 and 3 so

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. 

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What Before How

CaptnWil's second law is, "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.

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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 canceling or removing the other

magnetic influences on the compass.  We will concentrate on canceling 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.

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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 friendly with what we just did.  It will help you on the

boat if you are completely friendly with this procedure.  If you are not

friendly with this, do it again until you are.

 *******************************

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.

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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 center 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 center 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 (I wish I could

underline that last sentence because it is so important).  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.  Extra Emphasis!  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.

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.

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Problems

 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 fiberglass 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 fiberglass 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.

CaptnWil

Wil Andrews

Sail on

John Danicic - CD36 - Mariah #124- Lake Superior

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