[Rhodes22-list] Compass

Thu Apr 7 02:45:33 EDT 2005

Bill,

What an amazing and hilarious response... now we all know why no one bothers 
to calibrate them.... which is exactly what the coast guard instructor told 
us in the class after we reviewed the text describing how to do it.... and, 
no doubt which is why I have only the vaguest idea about how to do it... 
however... what I do remember is just about what you described... especially 
the totally frustrated part... I remember asking my uncle who was a career 
USAF officer and navigator to teach me how to use the sextant.... his 
response was... get a gps and have all the backup batteries you could ever 
need...

The book also went into a lengthy discussion on the fact that not only does 
the compass need to be calibrated to itself (the part about taking it out to 
a beach away from etc. etc.) but also needs to be calibrated once 
reinstalled back on the boat to accomodate all the miscellaneous 
interference there... like the electrical panel, etc.

Finally, magnetic north is constantly moving... although not enough 
generally to affect us in our lifetimes... so unless we have another major 
earthquake that jolts the earth substantially, I suppose we can rule out 
this factor.... actually I may be off base on this one... if the earth axis 
slips a tad, does that affect magnetic north?  not sure... seems like it 
would throw something off, but ... hmmmm....

Slim, if you do decide to get anal about this... the pointing the boat one 
direction and then 180 degrees is where the chart range procedure comes into 
play... however you have to find one and then travel there to execute this 
maneuver... I can't remember what lake you sail on, but its not like you can 
find one right in your backyard.

Thanks, Bill, for your insights.... LOL

Saroj

----- Original Message ----- 
From: "Bill Effros" <bill at effros.com>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Thursday, April 07, 2005 1:01 AM
Subject: Re: [Rhodes22-list] Compass

> 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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>>
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