Next message: Trevor J. Kenchington: "Re: Refilling a compass, was: Compass tilt"
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?
Thanks;
Robert Gainer
>From: RSPeterson <>
>Reply-To: Navigation Mailing List <>
>To:
>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
>mess.
>
>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)
>>models
>>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
>>other,
>>I am not in a position to evaluate its performance against other
>>compasses,
>>though I have not had cause to complain: not even under rough conditions
>>in
>>a 26-footer. I know that Sestrel have made other, very different,
>>compasses, some really big for large vessels, flat disc types with
>>external
>>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
>>day
>>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
>>up
>>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
>847/697-6491
>Compass Adjusting & Repair for Lake Michigan Navigators Since 1985
>Physics @ Bartlett HS
>e-mail: rspeterson(at)wowway.com
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