NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Deviation Card with GPS
From: George Huxtable
Date: 2006 Jul 24, 15:23 -0500
Robert Eno has told a salutary tale about his compass and its
misbehaviour. It would be interesting to know the whereabouts of the
Pike-Resor channel, and Lapointe Rock. I've tried, but failed, to
identify those as geographical place names in the gazetter of my Times
Atlas. I don't even know which side of the continent they are on, but
clearly, they're way up North, and in a region of high magnetic dip.
The high dip seems likely to be the key to the matter, and also to the
high values of magnetic deviation he has found.
He has described a worrying situation, way outside my own experience,
and that of most of us, I suspect. It calls for some hard thinking. It
would be interesting to learn more, but in asking questions I am not
casting any doubt on his account.
In the fog, presumably he couldn't see his surroundings, and I am
interested in the details of his wildly spinning compass card. How
could he tell that it was wildly spinning, and not the boat sailing in
circles? At a guess, because its spin was far faster than the boat
could possibly be turning, but about how fast was that, I wonder? Was
he by that time steering by other means, radar or GPS, to maintain a
straight course?
Was he using engine at the time, or under sail, to be doing his 7-9
knots? Was the sea calm or rough? What's his hull, and his ballast
keel, made of?
The symptoms seem to point toward a local magnetic anomaly; perhaps
some iron-ore deposit, enough to deflect the local magnetic field,
already dipping steeply, into a straight up and down direction or
nearly so. In which case, there would be no directive force on the
compass from outside, and it would be free to spin (depending somewhat
on the residual magnetic field from any of his steel, mainly the
engine and , if iron, the keel). Or perhaps any small remaining
horizontal component is exactly nullified by that field from his
steel, at one heading, in which case, correctors might indeed provide
the answer. However, with such correctors fitted, but adjusted for
another magnetic latitude, they might provide the only remaining force
on the compass. Then the card would be likely to turn with the boat,
giving no information about heading; but not the wild spinning he
refers to.
I wonder if the wild spinning can result from engine vibration, at a
point when the magnetic forces have fallen to near zero?
Robert mentions a lighthouse, which implies that this channel may be a
frequented one. In which case, I would expect the chart to contain a
warning, as ours do here and there, such as "local magnetic anomalies
reported in this area". Surely, he can't be the first skipper to have
noticed such an effect. Here, the Hydrographic Office are keen to get
reports of such matters, and it might be useful to all concerned for
Robert to report his experiences to his charting authority, and ask if
anything else is on file.
I wonder what type of steering compass Robert is using, Doesn't matter
much which make, as long as it's not a Danforth. The new Danforth
compasses have a patent system for suspending the magnet, for which
somewhat extravagant claims are made. It might indeed work well enough
at mid-latitudes, but by my reckoning will be seriously defective at
high angles of dip.
Robert has reported enormous magnetic deviations of up to 25 degrees,
which ought to be a serious worry. What is the pattern of these
deviations? What does a deviation card look like?
There's no chance of doing anything to reduce the magnetic impact of
Robert's engine or keel (if these, rather than a local loudspeaker
magnet, are the dominant influences) except to reduce their effect on
the compass by moving it away. Perhaps a good place to install a
master-compass (as opposed to a steering compass) might be up on the
taffrail, or on a pole above the taffrail. Yes, it would be most
awkward to steer using that compass, but it could well provide a
reliable direction to which the steering compass course could be
referred, at times when it mattered. A hand bearing compass, used high
up, to get the heading of the bow, would serve the same purpose. In a
special situation such as Robert reports, there may be a good case for
installing a remote-reading fluxgate compass, mounted up the mast, out
of local magnetic influences.
Perhaps, in the end, it may be that the only workable solution is
indeed to null out these perturbing influences on the steering compass
by using corrector magnets. To me, however, that smacks of a
sticking-plaster cure, covering over the problem rather than
addressing it.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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To , send email to NavList-@fer3.com
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From: George Huxtable
Date: 2006 Jul 24, 15:23 -0500
Robert Eno has told a salutary tale about his compass and its
misbehaviour. It would be interesting to know the whereabouts of the
Pike-Resor channel, and Lapointe Rock. I've tried, but failed, to
identify those as geographical place names in the gazetter of my Times
Atlas. I don't even know which side of the continent they are on, but
clearly, they're way up North, and in a region of high magnetic dip.
The high dip seems likely to be the key to the matter, and also to the
high values of magnetic deviation he has found.
He has described a worrying situation, way outside my own experience,
and that of most of us, I suspect. It calls for some hard thinking. It
would be interesting to learn more, but in asking questions I am not
casting any doubt on his account.
In the fog, presumably he couldn't see his surroundings, and I am
interested in the details of his wildly spinning compass card. How
could he tell that it was wildly spinning, and not the boat sailing in
circles? At a guess, because its spin was far faster than the boat
could possibly be turning, but about how fast was that, I wonder? Was
he by that time steering by other means, radar or GPS, to maintain a
straight course?
Was he using engine at the time, or under sail, to be doing his 7-9
knots? Was the sea calm or rough? What's his hull, and his ballast
keel, made of?
The symptoms seem to point toward a local magnetic anomaly; perhaps
some iron-ore deposit, enough to deflect the local magnetic field,
already dipping steeply, into a straight up and down direction or
nearly so. In which case, there would be no directive force on the
compass from outside, and it would be free to spin (depending somewhat
on the residual magnetic field from any of his steel, mainly the
engine and , if iron, the keel). Or perhaps any small remaining
horizontal component is exactly nullified by that field from his
steel, at one heading, in which case, correctors might indeed provide
the answer. However, with such correctors fitted, but adjusted for
another magnetic latitude, they might provide the only remaining force
on the compass. Then the card would be likely to turn with the boat,
giving no information about heading; but not the wild spinning he
refers to.
I wonder if the wild spinning can result from engine vibration, at a
point when the magnetic forces have fallen to near zero?
Robert mentions a lighthouse, which implies that this channel may be a
frequented one. In which case, I would expect the chart to contain a
warning, as ours do here and there, such as "local magnetic anomalies
reported in this area". Surely, he can't be the first skipper to have
noticed such an effect. Here, the Hydrographic Office are keen to get
reports of such matters, and it might be useful to all concerned for
Robert to report his experiences to his charting authority, and ask if
anything else is on file.
I wonder what type of steering compass Robert is using, Doesn't matter
much which make, as long as it's not a Danforth. The new Danforth
compasses have a patent system for suspending the magnet, for which
somewhat extravagant claims are made. It might indeed work well enough
at mid-latitudes, but by my reckoning will be seriously defective at
high angles of dip.
Robert has reported enormous magnetic deviations of up to 25 degrees,
which ought to be a serious worry. What is the pattern of these
deviations? What does a deviation card look like?
There's no chance of doing anything to reduce the magnetic impact of
Robert's engine or keel (if these, rather than a local loudspeaker
magnet, are the dominant influences) except to reduce their effect on
the compass by moving it away. Perhaps a good place to install a
master-compass (as opposed to a steering compass) might be up on the
taffrail, or on a pole above the taffrail. Yes, it would be most
awkward to steer using that compass, but it could well provide a
reliable direction to which the steering compass course could be
referred, at times when it mattered. A hand bearing compass, used high
up, to get the heading of the bow, would serve the same purpose. In a
special situation such as Robert reports, there may be a good case for
installing a remote-reading fluxgate compass, mounted up the mast, out
of local magnetic influences.
Perhaps, in the end, it may be that the only workable solution is
indeed to null out these perturbing influences on the steering compass
by using corrector magnets. To me, however, that smacks of a
sticking-plaster cure, covering over the problem rather than
addressing it.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
