Conference vicki::boats

    Lets see if i can clear up some of this confusion.
    
    First thing, find a convenient spot on your console, away from things
    that are magnetic, like speakers, and other metal objects, although
    stainless is not a problem.  Orient it in line with your keel, it
    does not have to be right over the keel. The magic spot refers
    to a location that is away from inteference. (Chapmans should have
    a lot of info on this) Once mounted you need to run the boat,
    
    From: North to South
          South to North
          West to East
          East to West
    
    You'll need to find 2 sets of buoys that will accomodate these
    directions. Make the runs, note the deviation, turn the compass
    adjustments screws until all or almost all of the deviation
    has been corrected. Any deviation remaining, should be written 
    on something that is waterproof, and kept near the helm.
    
    When you plot a course, you would add or subtract these deviations
    from your calculated course.
    
    Its a project, and it will take some time.
    
    Hope this helps ,
    
    JIm.

RE: .0

>> It implied that I shouldn't secure the compass to the console until I 
>> had calibrated the deviations

This doesn't sound right.  When you compensate a compass, one of the things
you're compensating for is magnetic interference from other hardware near
where it's located.  I suspect that most compasses are pretty much "right 
on" when sitting on your workbench or front lawn.  It's interference from 
your radio, ignition switch etc. that you're compensating for.

I'm pretty sure the procedure is to mount the compass in its final location
and *then* adjust it.

Rick

    Usually, compasses are neutrally set, out of the box. But if it is not,
    you will make life much easier on yourself by doing preliminary
    adjustments on land away from all metal.
    
    I did mine on a sheet of plywood, with a book for a square, in a field
    away from all metal. Simce my boat has no ferrous metal within 6-7
    feet of the compass, it seems to be near perfect.
    
    Since compass adjusting is such an inexact science, you should check it
    at E-S-W against something square like a book that has been aligned
    to north, and adjust out any errors, before you put it into an
    environment that has deviation. I mounted my bulkhead mount compass in
    a cardboard carton for this.
    
    Bill
    

Woops, re (.1) I think I created the confusion.

I sort of understand how to correct the deviation, and callibrate the 
residual, but the problem is how to orient the compass relative to the keel
.

re (.2)
The instructions and the Eldridge write-up imply that you should:

	a) locate the compass on the boat as you suggest, 
	b) attach the compass to the boat with ONE screw,
	c) "correct" the deviation as suggested in (.1)
	d) ROTATE the compass ( around the one screw) based on some
	   observed "systematic" deviation - this is the part that is 
	   not clear to me
	e) THEN "nail it down" to its final resting place

Its the d) above that I don't understand.  How to eliminate the systematic 
error, allegedly caused by the misalignment of the compass with the keel,
that is the rational for d) above.  Unfortunately I couldnt follow what the 
example in Eldridge was trying to do.  

Sorry for the confusion.

Any help?  

    Methinks Eldrige is making sure the compass is indeed PARALLEL to the
    keel.
    
    Let's review the steps quickly (you seem to understand them):
    
    	1) On land away from all metal check the compass against N-S-E-W
    	   & adjust as necessary (This is called boxing the compass as it
    	   is usually done by attaching the compass to a square box,
           placing one edge of the box at a time against a straight edge
           pointing due N-S.  The error is best removed by eliminating
           50% at each 180 degree interval)
    
    	2) Put the compass on the boat in an 'appropriate spot' away from
           all the bad stuff (this is the magic spot - minimum deviation)
    	   ONLY ATTACH ONE SCREW at this time - allowing the compass to
    	   pivot.
    
        3) Take the boat some place where there is a range, set of buoys,...
           to insure you know what heading the boat is indeed on.
           NOW pivot the compass around the one screw until it reads the
           heading you are indeed on -- & fasten it down with the other
           screws.  This has insured the compass is in parallel with your
           keel.
    
        4) Now make the appropriate runs & adjustments to minimize
           deviation.  (It may already be as good as it'll get if the
           'magic spot' really = the useable spot for the captain.
    
        Hope this helps,
                         Use a brass screwdriver,
                                                   Lou

re (.5)

Thanks Lou, Why couldnt the instructions say that.  

I appreciate the clarification.

Roger

Roger,

I don't agree with everything you have beed told to do.

1. You should Zero or "BOX" the compass as mentioned
   in a couple notes. This makes sure the only deviation
   you will get is due to your boat.

2. Mount the compass so that a line passing through 
   the lubber line and center of the compass is 
   parallel to your keel. This is the MAIN goal! If you
   move the mounting to adjust to a known course as Lou
   suggests in .5, you are probably compensating for
   deviation due to your boat and WILL NOT get the
   compass parallel to the keel. That may not be a 
   problem if the deviation is small, but could cause 
   you significant grief if it's large. If there is no
   deviation from your boat -- no problem, but you 
   don't know that until you get it mounted and then 
   check it. That's the whole idea of going through the
   zeroing process first, and then mounting it parallel
   to the keel.

3. Now compare you compass readings to as many known
   ones as you can find. The difference between you 
   reading and the known one is now deviation that is 
   totally due to some magnetic interference on your
   boat. My suggestion is to NOT try to compensate for
   it!! Simply write it down and make up a "Deviation
   Table" to keep on the boat. Since a deviation 
   pattern through 360 deg is almost never symetrical,
   trying to compensate for it by adjusting the compass
   will get you into more trouble than it's worth. 
   Leave the compass zeroed, and work from there.

There is also a method of calculating deviation using
a flat piece of wood with a compass rose on it and a 
verticle wooden peg, and the sun (sort of like a 
sundial), but it takes two people and is very time
consuming. If you want to know how, let me know and
I'll send you the instructions.

Do you know how to apply your deviation to steering a
course? ie, if someone (like the Coast Guard) tells
you to steer a particular course, how you would
translate that to the actual compass heading you would
use? The CG will always give you a TRUE heading.

(T,V,M,D,C,+W)

Don

Thanks, (.3) and (.7) I've now learned about the need to "box" the compass.

re (.7) 
<Mount the compass so that a line passing through the lubber line and>
<center of the compass is parallel to your keel. This is the MAIN goal!> 

Yes that is the goal, and the (real) subject of my original note
but I'm still not clear on how you go about achieving it. 

In some book (Dutton's, Chapman's ???) I've read since I posted the
original note, it explained that you should plot the deviation against heading, 
and smooth it to look like a circular function (e.g. sine wave). 

If there is NO systematic error in the compass reading (i.e. the lubber
line/compass pivot is aligned with the keel) The AMPLITUDE of this Deviation
/Heading curve should be symmetrical, i.e. the MAX (+) deviation should
equal the MIN (-) deviation !!!  

If they are not equal, the explanation went on, then the systematic error
due to the misalignment of the compass and keel is represented by that
difference.  The compass's ORIENTATION should be reoriented (east or west)
by HALF the difference, making the deviation's amplitude symmetrical. 

Does this sound reasonable ??  

<There is also a method of calculating deviation using a flat piece of wood>
<with a compass rose on it and a verticle wooden peg, and the sun (sort of>
<like a sundial), but it takes two people and is very time consuming.>

I believe it's called a sun compass.  I saw a Pelorus at BOATS/US which 
proports to do that job.  Chapman says it's not easy.

BTW I got the TVMDC bit, but what does +W stand for (WAG???)

Don

    re: .7
    
    Deviation due to anything "in a permanent position" on the boat is
    indeed very symetrical -- it will be a sine wave.
    
    I agree that if when you place the already 'boxed' compass on the boat
    there is any significant movement/deviation - you must re-check things
    like major sources of deviation, location, etc..; however, if the
    deviation/change is initially small - I would try to correct as you/I
    stated by running reciprocal courses at all N,S,E,W -- as you reverse
    course only correct for half the deviation then do the next 90 degree
    check.
    
    If the deviation table is not a sine wave something is very wrong.
    If the table is a sine wave and within 5 or so degrees I'd be
    comfortable.
    
    Using the 'SUN' method requires a lot of co-ordination & a responsive
    boat as you anly have about 15 min. to do the whole thing.
    
    Other methods such as a plesourous (damn I mispelled that one..) are no
    more accurate or simple to do than using some 'reliable' ranges.
    
    Also keep in mind that many a compass has been grossly mis-adjusted
    by tring to get it -- too perfect.
    
                                   Fair seas,
                                               Lou

    I just re-read .7, .8 ---
    
    re: My comment on pivoting around a single point & lining up the
    compass:
    
    When you first do this treat it as deviation --
    Face north - line up the compass (pivot)
    Face South - repivot for half the error.
    Do the same for East/West
    
    At this point you have compensated for most of the N/S E/W deviation AND
    established being parallel to the keel ---- attach the other screws.
    
    If you still have any major deviation -- something is drastically
    wrong.   Any minor deviation will tune down readily.
    
    Sorry for any initial confusion --- we'll see if this causes any more
    conversation......
    
                     Lou_who_does_1200nm/yr_without_a_deviation_table
    
    

    The +W means to "add Westerly" (variation etc).
    
    Bill