NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Compass tilt. and other compass issues
From: Robert Gainer
Date: 2004 Jun 21, 17:35 +0000
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? Thanks; Robert Gainer >From: RSPeterson>Reply-To: Navigation Mailing List >To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM >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 _________________________________________________________________ Make the most of your family vacation with tips from the MSN Family Travel Guide! http://dollar.msn.com