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    Re: Automatic deviation calculation by electronic compasses
    From: Lu Abel
    Date: 2009 Dec 09, 20:34 -0800

    George:
    
    Why do you assume that an area of low deviation is required as a
    location on a steel vessel for a mounting place for a fluxgate compass?
    
    It seems to me that ANY spot, regardless of how esoteric its deviation
    table or Napier diagram, would be a suitable mounting place so long as
    the deviation (table) does not change with vessel heading or activation
    of machinery.
    
    It seems to me that the joy of an autocompensating compass is that it
    will have an internal table of deviation at least as accurate as could
    be drawn by a human (before you object, I'll admit that I'm assuming
    that a proper constant-angular-velocity turn can be executed to
    compensate the compass) and that the MDC calculations will occur at
    computer speed and not at tired, seasick human speed.
    
    Clearly a spot of low deviation would be desirable, but I can't
    understand why you think it would be a necessity.
    
    Lu
    
    George Huxtable wrote:
    > Thanks to Joe Schultz for his peacemaking response, which is accepted.
    >
    > He gives a reason for being unable to answer my specific question, which is
    > fair enough. The question was-
    >
    >  "Can a suitable place for a fluxgate compass always be found below decks on
    > a steel warship, in which an automatic correction algorithm alone will
    > provide sufficient heading accuracy for navigation, with no need for
    > correction magnets or balls?"
    >
    > And my question applies to any other steel vessel, not necessarily a
    > warship. Without any such specific assurance, one way or another, I have to
    > say that I remain unconvinced that it can. Good enough for an autopilot,
    > maybe. For a steering compass, I would like to see evidence. The fact that
    > in the end, such fluxgate installations didn't get adopted as steering
    > compasses, tells us something.
    >
    > As for his phrase about a "sweet spot", for placing such a compass, I, too,
    > took a look at Wikipedia, but I could not relate that definition to the
    > positioning of a fluxgate compass. I don't accept that below decks, on a
    > steel vessel, there's some point at which magnetic perturbations somehow
    > cancel out, or become specially small. Of course, some locations may be
    > somewhat more affected than others, but it seemed to me that "sweet spot"
    > was trying to imply more than that. Such are the difficulies in mutual
    > undestanding, when woolly phrases are used.
    >
    > I followed Joe's suggestion to look at the James Husick articles, but didn't
    > find anything helpful to my enquiry there.
    >
    > George.
    >
    > contact George Huxtable, at  george{at}hux.me.uk
    > or at +44 1865 820222 (from UK, 01865 820222)
    > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    > ----- Original Message -----
    > From: 
    > To: 
    > Sent: Wednesday, December 09, 2009 1:03 PM
    > Subject: [NavList 11034] Automatic deviation calculation by electronic
    > compasses
    >
    >
    > George, a gentle "heads up": I don't live in my computer.  It may be days or
    > weeks before you see a response from me.
    >
    > I get it, regarding language.  I pick up a wrench and hand over a spanner.
    > Sometimes I forget and hand over a wrench.  I'll try to improve my
    > performance in language.
    >
    > No promises regarding how my writing "sounds."  My background isn't in
    > authoring romance novels.
    >
    > I'll try to answer your questions and comments (post 10912) in order.
    >
    > 1. "Can a suitable place for a fluxgate compass always be found BELOW DECKS
    > [emphasis mine] on a steel WARSHIP [emphasis mine], in which an automatic
    > correction algorithm alone will provide sufficient heading accuracy for
    > navigation, with no need for correction magnets or balls?"
    >
    > I have limits, and I apologize if it appears that I've strung you along.  In
    > simple terms, the "stay out of jail" card reads like this: I can talk about
    > unclassified-unlimited distribution items.  The legal precedent has also
    > been set so that I can talk about items that have been "leaked" into the
    > press or published elsewhere, and that leads to the ethical dilemmas that
    > "me and my kind" get to deal with.  That question is out of bounds, in my
    > opinion.  Sorry.
    >
    > However, I have already stated that they were installed on all-steel ships
    > and they worked.  The sensor assembly makes no real distinction between a
    > steel warship and a steel commercial fishing boat or steel yacht.  Looking
    > for yourself will answer the question, as I have already recommended.
    >
    > 2. "Were such instruments ever actually adopted as the principal heading
    > reference?"
    >
    > Not in "major" warships i.e. frigate and above.  Weapon systems reach out a
    > long ways (even then).  And that's why they went away.  Today's backups are
    > true.
    >
    > 3. "Joe has shifted the discussion toward self-steering, but that's a
    > different matter, as Lu has pointed out, not requiring any great precision."
    >
    > Nope, I'm encouraging you to look for yourself.  I thought "who in the
    > civilian marine world uses this stuff?"  Autopilot users, and folks with
    > integrated systems.  Standardized NMEA/ISO output signal leads to integrated
    > systems.  You'll see that if you look.
    >
    > 4. "De-gaussing."  I won't copy-and-paste your entire comment (this post is
    > long enough).
    >
    > Magnetic influence mines are still in use.  Relating to the fluxgates, two
    > deviation equation/tables were created: degaussing on and degaussing off.
    > You can (and do) change the ship's magnetic signature "on the fly."  It's a
    > good way to stay alive.
    >
    > 5. "If Joe's "very, very special steel" happens to be a stainless or other
    > high-nickel alloy, effectively low-magnetic, I hope he will make that
    > clear."
    >
    > I inadvertently made a Navy school instructor temporarily famous.  Had a
    > fellow student who was fairly intelligent but had absolutely no common
    > sense.  Eventually the instructor's trip wire broke and he made a comment,
    > and immediately apologized even though he was a Commander and the student
    > was an Ensign.  That night I made a sign and hung it on my school desk, and
    > I've hung that sign wherever I've worked.  What did it say? "Xxxxx's Idiom:
    > some things are intuitive; and for that I apologize."  Xxxxx was the
    > Commander's last name.  Reread the paragraphs immediately above and below
    > the hazegray.org link I provided, and the link (where I told you to look),
    > then engage thy brain.  If you still don't get it then I can't help you.
    > Speak up and perhaps another reader will tell you what the very, very
    > special steel is.  And for that I apologize.
    >
    > 6. "Was this equipment used on vessels intended to operate at differing
    > magnetic latitudes? If so, did operating instructions call for a 360-degree
    > turn to be made, for recalibration, as the magnetic latitude changed?"
    >
    > Yes to the first and no (with a caveat) to the second.  Made me think of two
    > very real issues that I can't remember dealing with: 1) the Flinder's bar or
    > unbalanced card effect, as in a "traditional" compass and 2) increased dip
    > at high latitudes which will make the heading more uncertain.
    >
    > Mr. Husick, at the following link, can explain it better than me.  On the
    > left side look for the Electronic Compasses and, ummm..., Autopilots
    > articles.  Pay attention when he gets into the "old technology" fluxgate
    > stuff.
    >
    > http://www.boatus.com/husick/default.asp?WT.mc_id=400098
    >
    > I do remember the "new instructions" called for a recalibration when
    > necessary or when in doubt, deleting the "annual" requirement after they
    > understood the stability of these systems.  Some ships went a couple years
    > between recalibrations.  Believe it or not, the "old instructions" also
    > called for a recalibration when entering restricted waters if the fluxgate
    > was the "active" device (failure of primary).  Bureaucrats and theorists
    > operate in their own world.  Their restricted waters had no traffic and had
    > lots of room to turn a 450ft ship.  Recalibrating BEFORE entering restricted
    > waters didn't count in their world.
    >
    > Now to the thread-specific questions and comments in your post 10936:
    >
    > 1. "Sweet spot."  Hopefully this is clear to you by now, both in term
    > definition and in sensor assembly location.  What do I want you to do?
    >
    > 2. "Armchair term."  I'm not a microbiologist or a brain surgeon, and I
    > won't pretend to be one.  If you've done fleet time then I apologize for
    > sounding derogatory, which wasn't intended.
    >
    >
    > George, I've said all I can say.  You really should go look for yourself.
    > Bumblebees can fly.  Let us know what you learn.
    >
    > Joe
    >
    >
    
    --
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