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    Compass tilt. Was: Historical Magnetic Variation/Declination
    From: George Huxtable
    Date: 2004 Jun 20, 22:47 +0100

    Thanks to Bob Peterson for his informed explanation about compass tilt etc.
    I hope he will share more compass knowledge with us.
    
    A couple of points arise that may be worth a bit more discussion.
    
    He said-
    >I only know of one compass built to function in many
    >(not sure all) zones that being a Sestrel (from England).  They do this
    >by increasing the distance between the pivot point and the
    >Center-of-Gravity of the card assembly.  This minimizes the dip angle
    >effect, however, not without a price (of course).   These compasses are
    >now susceptible to dynamic effects as the vessel rolls, pitches, yaws.
    >The card can chase all over the place and not seem to stabilize on a
    >steady heading.  This can drive the helmsman crazy as they "chase" the
    >card.  Not good.  So its a matter of striking a balance (so to speak) in
    >design between card tilt and steady readings.
    
    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.
    
    About Bob's comment-
    >This trick requires means and parts to
    >treat the compass oil to remove the entrained gases from the oil
    >solution.
    
    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.
    
    George.
    
    ================================================================
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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