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    Re: Automatic deviation calculation by electronic compasses
    From: Joel Jacobs
    Date: 2009 Dec 10, 11:08 -0500
     
    It is rare that George makes a mistake or is imprecise in his sentence structure which can be confusing to some of us. George said "Kelvin (then William Thomson) wrote a very readable account of this in "Terrestrial magnetism and the mariner's compass", collected into vol 3 of his Popular lectures and addresses" of 1891. This is the volume on "Nivigational Affairs", which I thoroughly recommend. Kelvin owned a fully-crewed 130-ton schooner, and did a lot of sailing aboard her."
     
    Just to clarify his syntax, William Thomson was a renown late 19th Cent physicist teaching at the University of Glasgow, who worked with James White an English instrument maker. The Kelvin-White brand was in recognition of this partnership as were many other companies that arose through a series of mergers. He was first knighted by Queen Victoria, and became Sir William Thomson. In 1892, he was elevated to Baron Kelvin of Largs. His accomplishments are so numerous, I wont repeat them here other than to say he is best known for helping lay the telephone cable between England and the U.S., designing the improved steering compass of his invention that was adopted by the Royal Navy, and the shipping industry at large. It is his design of the ship's binnacle which is most recognizable today with the quadrantal correctors (soft iron balls) on each side. The interesting, dry card, threaded compass is also of his invention. The Wilfrid O. White Company in this country was his representative.
     
    Enough said,
     
    Joel Jacobs
     
    ----- Original Message -----
    Sent: Thursday, December 10, 2009 10:29 AM
    Subject: Re: [NavList 11064] Automatic deviation calculation by electronic compasses

    Lu Abel asked a fair question, which I will do my best to answer-

    "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. "

    But first, let's dispose of this one-
    "(before you object, I'll admit that I'm assuming that a proper
    constant-angular-velocity turn can be executed to compensate the compass) ".
    I  ask Lu to examine my posting [10799] of 23 November. There, I pointed out
    (after belatedly realising it) that there was no need at all for constant
    angular velocity. As long as the vessel passed through every point of the
    compass, and didn't turn too fast, all the necessary information was there
    to deduce deviation, at ANY inconstant speed-of-turn.

    Now, back to Lu's question. First, lets deal with the deviation of the
    compass as it affects most small-boat-owners: those with fibreglass or
    wooden hulls, in which the deviation is the result of odd bits of iron and
    steel in its vicinity; particularly an engine. In that case, deviations are
    small, both the sine(heading) or "hard-iron" component, and the sine(2 x
    heading) or "soft iron" component. It can be handled by the algorithm we
    have discussed, simply making a 360-degree turn. That would also apply to
    the rare stainless-steel hull, and perhaps even to a construction of "very,
    very, special steel", if it was a Nickel alloy of low magnetic permeability.

    But an ordinary ferrous hull is a different matter. Its magnetic effects can
    be enormous, and there's nowhere within the hull they can be escaped. The
    history is informative here. In the 19th century, it was a powerful and
    valid argument against iron and steel construction; that it wasn't possible
    to correct a compass properly. Mariners knew how to make a deviation table,
    but that wasn't sufficient. Deviation varied, not just with heading, but
    with (magnetic) latitude, with heel-angle, and with cargo, and with time, in
    ways which were not understood. There were countless accidents as a result
    of compass errors. The work of Airy and (particularly) Kelvin helped to sort
    it out. It resulted in the familiar polished binnacle, placed as high as
    possible above the hull, in solitary splendour on the bridge so there could
    be no undetected interfering objects. That carried permanent magnets to
    correct for the ships "hard-iron" component, soft-iron hollow balls placed
    thwartwise to correct for the "soft-iron" component of induced lengthwise
    magnetism, and the Flinders vertical iron bar to correct for vertical
    induced magnetism. Even after all that, there was the need to create a
    deviation curve, and a prosperous trade of compass-adjuster to bring it
    about.

    Kelvin (then William Thomson) wrote a very readable account of this in
    "Terrestrial magnetism and the mariner's compass", collected into vol 3 of
    his Popular lectures and addresses" of 1891. This is the volume on
    "Nivigational Affairs", which I thoroughly recommend. Kelvin owned a
    fully-crewed 130-ton schooner, and did a lot of sailing aboard her.

    =====================

    Let's try a thought-experiment to demonstrate the sort of problem that can
    arise, as simply as possible. We will be at the (magnetic) equator, so
    there's no dip, and we can choose a spot with no magnetic variation, so the
    field points North. Represent our ship by a length of board, with a compass
    mounted a foot above it.

    As we turn our ship, the card will ride over the lubber-line
    correspondingly, and indicate true heading with no errors. But next, attach
    a little bar-magnet to the board, directly below the compass, pointing
    lengthwise, such that when the ship points North its field enhances that of
    the Earth. That magnet represents the "hard-iron" component of a ship's
    hull; we are ignoring the "soft-iron" component here, for simplicity. It's
    clear that it causes no compass-error on a Northerly heading, nor (as long
    as it's effect is weaker than the Earth's) on a Southerly heading, and the
    maximum error will be on Westerly or Easterly courses. The error will vary,
    more or less sinusoidally, as long as the field from the magnet is small
    compared with that of the Earth. However, in a steel ship, that can no
    longer be assumed. If the magnet happened to produce a stronger field than
    that of the Earth, it would override that of the Earth when the heading was
    South, so the compass card would turn with the ship and still indicate a
    North heading. It would then be impossible to create a deviation table. All
    you could say, if the compass read North, is that the ship was pointing due
    North or due South!

    That may seem an extreme case, but the effect of a steel hull can be so
    great that it is by no means far-fetched. The binnacle components compensate
    so as to reduce the hull effects to manageable proportions; single-valued at
    least, that can then be handled by a deviation card.

    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.

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