[Rhodes22-list] Compass

[Bill Effros]

Slim,

The idea behind swinging a compass is that if you point the boat so the 
compass reads 0 degrees, and then swing the boat so it points in the 
exact opposite direction--the compass should read 180 degrees.  When it 
doesn't (not "if it doesn't" -- boats aren't that easy to exactly point 
in the opposite direction, and the magnetic properties of the earth vary 
from place to place) you split the difference with the adjusting 
magnet.  Then you do the same thing for 90 degrees and 270 degrees.  
Then with 45 and 225.  Then with...  You get the idea.

It's a pretty discouraging exercise, and when you are all through, your 
compass won't have the slightest idea of which way you are pointing.

But you are lucky.  Your compass is already screwed up, and it is 
mounted on a board you can remove.

So you can get discouraged in far less time.  Take the compass on it's 
mounting board as far away from any magnet as you can think of.  Point 
it at 0.  Make a chalk line, or scratch a line in the earth, or on a 
beach.  Turn the board 180 degrees.  Adjust magnet.  Continue until you 
are totally frustrated.

Alternative method.  Take 3 compasses.  Point them all in the same 
direction.  Add the readings together and divide by 3.  See if you can 
adjust any one of the compasses to read the average reading.  Continue 
until totally frustrated.

I find that the fluxgate compasses we can afford work no better than the 
magnetic compasses we can afford.  I find that the larger compasses we 
can afford are more sensitive to all the metal and magnets we have on 
board.  The GPS tells you only what direction you  have gone, not what 
direction you are now pointing. 

I find the consistently most accurate compass is the mini-vion "hockey 
puck".  It has no adjusting magnet--a quality you will come to treasure 
if you actually start to adjust larger, more expensive compasses.

A 4" to 5" name brand compass straight out of the box (not adjusted by 
you or anyone else--kept away from large magnets) should provide 
consistent readings for accurate steering--although you should navigate 
to determine the direction you are actually traveling.

A Peloris (sometimes called a "dumb compass" -- it has no magnet) can 
also be used to determine the error in a magnetic compass.  You can also 
steer a true course using a peloris and charts.  And you can use a 
peloris to correct for calculated errors in your steering compass.  A 
GPS can perform the same tasks, but I find the peloris more interesting.

I've never been on a boat our size that had a properly calibrated 
compass or a deviation chart that was even close to the mark.  There 
used to be people who earned a living by calibrating compasses, but I 
wouldn't know how to find them anymore, and the compasses we use cost 
less than the adjusters.

The really interesting aspect of your original question is learning that 
almost no one knows what direction their boat is headed, and few know 
that they don't know.  With GPS and computerized charting people just 
learn how to adjust their course to get where they are going without 
having any real understanding of how they got there, or, in the case of 
our boats, how they could arrive at the same place much faster if they 
could steer a true course, and weren't zig zagging all over the place.

Bill Effros



Steve Alm wrote:

>The previous owner of my boat had a removable six inch high board that sat
>across the companionway.  On it he mounted a compass AND an auxiliary
>speaker for the radio.  An ill-fated feat of engineering since the compass's
>proximity to the speaker (magnet) rendered the compass useless right from
>the get go. Needless to say, I've never used the thing and it's been stored
>in my garage all these years.  But I pulled it out the other day and removed
>the speaker.  The compass now seems to point north OK but I'm wondering if
>it's damaged or compromised by sitting next to the magnet all this time.
>It's a Ritchie B-80.  It has what I assume to be a calibration screw on the
>front, but I don't know how to calibrate or test a compass.  Any advice?
>
>Thanks,
>
>Slim
>
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