NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Deviation Card with GPS
From: Robert Eno
Date: 2006 Jul 24, 21:35 -0500
Thanks George,
I will try to reply to your questions but briefly, because I am feeling
rather lazy this evening. I just got off work.
George wrote:
It would be interesting to know the whereabouts of the
> Pike-Resor channel, and Lapointe Rock.
Robert responds:
They are located in Frobisher Bay
George wrote:
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.
Robert responds:
Ah c'mon George. Remember the Monty Python skit where the guy pays to have
an argument??
George wrote:
> 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?
Robert responds:
I may have mislead you by talking about a wildly spinning compass. What was
happening was the compass card was veering by as much as 60 degrees with the
slightest alteration of the helm. As I said, fog is not something new to me
so steering a compass course with low visibility is not something which I
normally have difficulty in doing. Except on that particular occassion. I
kept trying, doggedly, to steer by compass but when that finally failed, we
hugged the NE shore and used the radar. We also had a GPS unit but we
steered mostly by radar.
George wrote:
>
> 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?
Robert responds:
Sorry George. No sail. Only stinkboats up here. Diesel. Flat calm seas. Hull
is is solid fibreglass. Displacement hull. No keel to speak of, at least in
the sailboat sense.
George wrote:
> 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.
Robert responds:
Possible, but remember what I said earlier: I have not ruled out human
error. We were exhausted, beaten battered and just happy to be getting home.
It is entirely possible that we just slackened off because we could smell
the barn.
George wrote:
> 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.
George wrote:
No lighthouses here George. Not within 2000 miles (there is that old
imperial system rearing its ugly head eh?). No official warnings of magnetic
anomolies in this area, however I have heard anecdotal evidence from other
mariners. As for reporting this to the Hydrographic Office, not yet. A one
time only event is not something I would care to report. Repeatability would
be the key here. But this is a main shipping channel into Upper Frobisher
Bay. If there was a serious problem, professional mariners would have
reported it long ago. Remember what I said: I have not ruled out human
error!
George wrote:
>
> I wonder what type of steering compass Robert is using,
Robert responds:
A 4" dirigo alnico flatop compass. Very reliable instrument and in fact,
designed for use on steel boats.
George wrote:
> 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?
Robert responds:
George, I just made one up this past weekend, put it on excel and graphed
the results. I will be happy to send it to you offlist if you want to have a
look at it.
George wrote:
> 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.
Robert responds:
Or just chuck the whole bloody mess and use my GPS. To hell with tradition!
(just kidding).
George wrote:
> 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.
Robert responds:
That is sort of what I was getting at George, although I do disagree with
you that it is a band-aid solution. Seems to me that a lot of professional
mariners do just that. If I am not mistaken, it is standard procedure on
steel vessels.
As always, I enjoy your comments George.
Robert
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From: Robert Eno
Date: 2006 Jul 24, 21:35 -0500
Thanks George,
I will try to reply to your questions but briefly, because I am feeling
rather lazy this evening. I just got off work.
George wrote:
It would be interesting to know the whereabouts of the
> Pike-Resor channel, and Lapointe Rock.
Robert responds:
They are located in Frobisher Bay
George wrote:
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.
Robert responds:
Ah c'mon George. Remember the Monty Python skit where the guy pays to have
an argument??
George wrote:
> 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?
Robert responds:
I may have mislead you by talking about a wildly spinning compass. What was
happening was the compass card was veering by as much as 60 degrees with the
slightest alteration of the helm. As I said, fog is not something new to me
so steering a compass course with low visibility is not something which I
normally have difficulty in doing. Except on that particular occassion. I
kept trying, doggedly, to steer by compass but when that finally failed, we
hugged the NE shore and used the radar. We also had a GPS unit but we
steered mostly by radar.
George wrote:
>
> 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?
Robert responds:
Sorry George. No sail. Only stinkboats up here. Diesel. Flat calm seas. Hull
is is solid fibreglass. Displacement hull. No keel to speak of, at least in
the sailboat sense.
George wrote:
> 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.
Robert responds:
Possible, but remember what I said earlier: I have not ruled out human
error. We were exhausted, beaten battered and just happy to be getting home.
It is entirely possible that we just slackened off because we could smell
the barn.
George wrote:
> 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.
George wrote:
No lighthouses here George. Not within 2000 miles (there is that old
imperial system rearing its ugly head eh?). No official warnings of magnetic
anomolies in this area, however I have heard anecdotal evidence from other
mariners. As for reporting this to the Hydrographic Office, not yet. A one
time only event is not something I would care to report. Repeatability would
be the key here. But this is a main shipping channel into Upper Frobisher
Bay. If there was a serious problem, professional mariners would have
reported it long ago. Remember what I said: I have not ruled out human
error!
George wrote:
>
> I wonder what type of steering compass Robert is using,
Robert responds:
A 4" dirigo alnico flatop compass. Very reliable instrument and in fact,
designed for use on steel boats.
George wrote:
> 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?
Robert responds:
George, I just made one up this past weekend, put it on excel and graphed
the results. I will be happy to send it to you offlist if you want to have a
look at it.
George wrote:
> 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.
Robert responds:
Or just chuck the whole bloody mess and use my GPS. To hell with tradition!
(just kidding).
George wrote:
> 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.
Robert responds:
That is sort of what I was getting at George, although I do disagree with
you that it is a band-aid solution. Seems to me that a lot of professional
mariners do just that. If I am not mistaken, it is standard procedure on
steel vessels.
As always, I enjoy your comments George.
Robert
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
