What? - before How?
You must answer 'What'
questions before you can entertain 'How' questions.
Here's what we're going to
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
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
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
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.
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!