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    Re: Compass tilt. and other compass issues
    From: Robert Gainer
    Date: 2004 Jun 21, 17:35 +0000

    I joined this list today at the suggestion of a friend. Wow my head is
    swimming. (That�s a pun in the boating world) The reason I joined this list
    is I am looking for a base to a Walker Cherub Taffrail Log and he thought
    the list might have someone that knew where to get one. I didn�t expect a
    discussion in such detail about the compass. I still use only a sextant and
    old style chronometer for my navigation. I think I will enjoy this bunch.
    But back to the subject. Does any one know where I can find that base?
    Robert Gainer
    >From: RSPeterson 
    >Reply-To: Navigation Mailing List 
    >Subject: Re: Compass tilt. and other compass issues
    >Date: Mon, 21 Jun 2004 11:57:01 -0500
    >I'll use George's ?s to respond to all:
    >1)  Sestrel does make a range of models.  The model ready for world
    >traveling and several magnetic latitude zones is the "Major" (has approx
    >5 1/2" diam dome).  They make a smaller (approx 4" diam) model called a
    >"Minor" and a strange goose-neck mounted model called a "Moore".  Those
    >I do not know about though I'm sure the "Moore" will NOT change zones.
    >George, sounds like you have a "Minor".
    >2)  Careful with the baby oil.  Without stabilizers in the fluid the
    >stuff tends to jell up with time and exposure to air.  The real issue is
    >pulling the entrained gases out of solution.  Need a vacuum pump for
    >that though there is a heating technique in the oven.  Not recommended.
    >The manufacuterer has designed the compasses for a particular damping.
    >(Remember those dynamic effects?) Overdamping is not always the answer.
    >Baby oil is almost for sure too viscous.  The fluids used today are
    >variations of Exxon Isopar though Odorless Mineral Spirits was used in
    >the past.  Still,  they all must be treated properly before filling.
    >3)  Before filing an old compass (pre WWII) be careful to determine if
    >it was oil filled or alcohol filled.  Getting this wrong will destroy a
    >perfectly good instrument.  Oil has a high expansion coefficient with
    >temperature.  It wants to move, so a large expansion mechanism must be
    >built into the compass.  If you see a rubber expansion diaphragm or a
    >bellows with many (more than two) ridges, it is probably oil filled.  If
    >you see only a flat plate, you can be damn sure it is alcohol filled.
    >Alcohol (plus water) doesn't require the volume range.  But here's the
    >real trick.  Alcohol compasses are painted with special paint!  It's egg
    >tempra paint 'cause nothing else will with stand the alcohol (It is 190
    >proof with some water.  Martinis anyone?).  But this paint takes a
    >witchdoctor license to mix it up and apply.  So, don't mis-fill your
    >compass with the wrong fluid!  And especially don't fill it with
    >anti-freeze!  The damn stuff is toxic and makes a complete mess of
    >things.  I've had to throw away perfectly good compasses because someone
    >filled it with anti-freeze.  Also the orings/gaskets on an alcohol
    >compass must be gum-rubber.  Use buna-n and you will end up with a tarry
    >4)  Because most modern compasses are oil filled, it is important that
    >they be removed for winter storage.  When the compass is filled (new or
    >repaired) the ambient temp is approx 70 degrees.  In  Chicago, the run
    >up on the high side during summer is 30 degrees (--> 100 degrees).  But
    >the down side during winter is 90 degrees (--> -20 degrees) so they take
    >a  much harder beating during winter.  There simply is not enough temp
    >range for the expansion mechanism to accomodate and the compass is
    >trying to implode.  Something will give.  Then an interesting cycle
    >starts.  Usually the exp diaphragn tears to relieve the pressure which
    >makes the compass happy at the low temp.  But then the temp rises on the
    >next warm day and the fluid expands except it now has a place to go -->
    >out.  Which it does and evaporates.  Next cold night?  Repeat cycle
    >until the compass is empty.  Hmmmm?  Wonder were all the oil went?
    >5)  Back to compass dip:  Indeed the old sea captains trick that Trevor
    >referred to would work, but you have to know how much.  There are tables
    >in HO226 (now absorbed into Bowditch)(BTW,  I think Ken at Celestaire
    >still has some last copies of HO226) to calculate "how much"  but again
    >there is nothing like a measurement (data) to build confidence.  It
    >would not be the Flinders Bar (which is the vertical "soft-iron"
    >corrector along with the quad sphere for horizontal soft-iron
    >correction) rather the vertical field corrector. This is a vertical tube
    >mounted dead center below the compass for a vertical magnet to be
    >raised/lowered and thus to "rebalance" the  dip angle.  So it all comes
    >full circle.  BTW, all these details were worked out by a very bright
    >Brit, named Lord Kelvin who gave us lots of other physics.
    >6)  Trevor -- If you can find the S/N on your Ritchie flattop, I can
    >find out when it was built.  Ritchie has complete log books of every
    >compass they ever built.  The books are hand written and go back over
    >150 years showing date of build and who it was sold to and when
    >rebuilt.  Amazing!  They built compasses with both quarter points and
    >degress.  I have heard here in the States that the Merchant Marine
    >wanted to use quarter pts and the Navy preferred degrees as a result of
    >the quality of the recruits.
    >I think that covers it.  Thanks. -- Bob Peterson
    >George Huxtable wrote:
    >>I can see that changing the distance between the pivot and the COG is very
    >>likely to affect vulnerability to horizontal accelerations. There are
    >>Sestrels and Sestrels, so I am interested to know which (if not all)
    >>he is referring to. For more than 30 years, I have used the common
    >>spherical type of Sestrel with a 3-inch (or so) card, and an internally
    >>gimballed cage carrying lubberlines and pivot-socket. Having used no
    >>I am not in a position to evaluate its performance against other
    >>though I have not had cause to complain: not even under rough conditions
    >>a 26-footer. I know that Sestrel have made other, very different,
    >>compasses, some really big for large vessels, flat disc types with
    >>gimballing. So it would be interesting to learn whether the instability
    >>problem Bob identifies applies to all Sestrel compasses or to specific
    >>types, such as mine.
    >>I have had reason to refill an oil-filled compass with new oil, and have
    >>found Johnson's baby oil, straight from the bottle, to be perfectly
    >>satisfactory. After 5 years or so, it's just as crystal-clear as on the
    >>it went in, without a hint of a bubble. And the amount of damping seems
    >>just right, to me.
    >>For compasses that use a spirit-water mix, I think removal of dussolved
    >>gases may be more of a problem. After refilling such a compass, I put the
    >>assebly into a vacuum chamber (with the filling-plug removed and its hole
    >>at the top). The amount of bubbling surprised me, and I needed to top it
    >>a few times. Even after that, the compass developed a bubble later. So I
    >>would be reluctant to undertake a spirit refill, but with baby-oil, there
    >>seems to be no such problem.
    >Robert S. Peterson
    >31 N Alfred, Elgin IL  60123  USA
    >Compass Adjusting & Repair for Lake Michigan Navigators Since 1985
    >Physics @ Bartlett HS
    >e-mail: rspeterson(at)wowway.com
    Make the most of your family vacation with tips from the MSN Family Travel
    Guide! http://dollar.msn.com

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