NavList:

Oh, good. My last posting seems to have stirred up some argument.

About corrector magnets, and whether they are, or are not, a Good
Idea, I had written-

| > If you're
| > an ocean voyager, it's likely that big changes of latitude will
render
| > those corrections invalid.

and Lu replied-

| Whilst I can not directly speak to issues with change in deviation
with
| latitude (I assume this would have something to do with card dip),
the
| majority of us are coastal voyagers.  And here I must strongly
disagree
| with George.

From George-

I, too, am a coastal voyager, and have never crossed an ocean in my
life. But there are those of that do cross oceans, and we have to be
careful not to proffer advice that is inapplicable to them, without
making its limitations crystal clear.

If you could treat a magnetically perturbing object, such as an
engine, as just a permanent magnet, then you could compensate for its
presence, to a large extent, by other permanent magnets. But there's
another effect, and a serious one. Even if the engine block isn't
actually magnetised itself, the Earth's magnetic field, dipping down,
doesn't pass through that steel unaffected. It causes its own
magnetisation in the block; the overall effect being for the field
lines to gather more closely, in line with the block, strengthening
the field, and more weakly to the side of it, where the field has been
bypassed to some extent. This effect is induced magnetism, as opposed
to permanent magnetism. It's often referred to in texts, incorrectly,
as the effect of "soft iron" and "hard iron", but all iron and steel
shows both characteristics, to some extent. Induced magnetism is of
major concern if you have a steel hull, and is the reason for those
large iron balls, placed either side of the binnacle. But even with
wood or glass hull construction, the magnetic perturbation from the
engine shows both effects, and the relative amounts vary with the dip,
and therefore with the magnetic latitude. You can compensate, using
permanent magnet correctors, around one magnetic latitude only; the
further you travel from that latitude, the worse the correction will
become.

Not only that: for a sailing vessel, hard on the wind, at a constant
heel, the induced magnetism will differ from what it was on a level
keel, and any correction will suffer accordingly.

Lu assumes that it has something to do with card dip, and it has; in
that both are influenced by the slope of the magnetic field.

Of course these effects, of varying magnetic latitude and of heel,
apply just the same whether correctors have been used or not. In
truth, a deviation card, just like a corrector adjustment, will apply
only to one latitude and to a level keel. What I am pointing out is
that you can't simply rely on having once adjusted the correctors, and
leave it at that. If you don't correct at all, but get into the habit
of checking compass error whenever a known azimuth (terrestrial or
celestial) comes into view, you will become familiar with its
behaviour, to your advantage.

A mistake made by many (but not Lu, I think) is to assume that compass
correctors are there to compensate for errors in the compass itself.
They are not. They are there to compensate for field distortions in
the ENVIRONMENT of that compass. If you shift the compass to another
position, any correction (or deviation card) will be inapplicable, and
the job will have to be done again. That's why correctors (or
deviation card) are applicable only to a steering compass, that's
always in the same place, and never to a hand bearing compass, which
can be used anywhere. Indeed, for such a bearing compass, if it's
taken high on deck well clear of any steel objects, deviation can be
assumed to be zero on a non-steel vessel. If you have a good bearing
compass, such as a Mini-compas or Opti-compass (same thing, really)
and reasonably calm weather, you can use it high on deck, and read
your heading to the nearest degree. That will give you a reliable
bearing, to compare with the steering compass and  get its deviation
on any heading, with no need for landmark or seamark transits.

As for the problems that Lu expects, in adding or subtracting a few
degrees, and getting the right answer, well, each to his own
prejudices. I like to think it out, and if necessary make a little
sketch in a corner of the chart, using commonsense and not (as some
still do, I know) a bit of doggerel verse. I don't find it a problem,
but others may.

Lu suggests that
| ....electricity ... can badly affect a compass.

Well it can, and some care has to be taken, but it shouldn't be
overstated. It's only conductors carrying high currents, in the
vicinity of the compass position, that can be serious. As long as the
flow and return leads are kept close together, their magnetic effects
are small. In particular, you have to be careful obout the main feed
between battery positive, via its isolator, and the alternator|,
returning to the battery usually via the engine block and an earth
braid, and keep the area enclosed by that current-loop as small as
possible. Lower currents, for example lighting wires, present no
problem if they are run in flat twin-flex. The compass light itself
draws such small currents as to present no magnetic problems, but
because of its sensitive position most such lights are fed with
twisted-pair, which certainly does no harm.

I had suggested pointing the bow along a known transit to check
deviation, and Lu responded-

| Respectfully, George, using a GPS is a LOT easier!   Trying to
transit a
| known course works only if you can stay on the course line, ie, not
get
| set wind age or current.  And it involves finding meaningful course
| lines every 45 (or, better, 30) degrees, often hard to do.  A GPS
will
| give the bearing to a waypointed object from any location.

Well, that shows up the difference between our attitudes, Lu's and
mine. All I need to do, when on a known transit, is to point the bow
in that direction, read and note the steering compass, and the job's
done (for that heading). It's the work of a few seconds. If there's a
cross-tide, trying to take me away from the transit, it doesn't
matter; there's not enough time for it to act.

Whereas Lu needs to stay on a constant known heading for long enough
to twiddle away at his correcting magnets until he is sure that he has
got the compensation just right. Then do the same on a different
course. Then go back to the original course to ensure that the
compensation is still right on that one.

So, Lu has to make a meal of it. It becomes a little project. For me,
in contrast, it's a casual business, done for a particular heading
whenever a suitable transit or bearing presents itself in the course
of a journey.

And why are meaningful course lines required at every 45, or 30 degree
intervals? You can plot a point, on a deviation graph, at any heading,
and having accumulated a few, draw a curve between them.

Lu advocates using a buoy for such purposes. It depends on his sailing
waters. In our part of the world, (though not in my home port) the
tidal range can exceed 45 feet, and floating marks tend to be in deep
water on a very long chain. They can wander far from their charted
position.

Lu adds
|  Rather
| than having your boat in one place and swinging it towards multiple
| charted objects,  sail your boat around the single charted object
and
| take a bearing (GPS and bow bearing by steering compass) to it every
45
| or 30 degrees as you drive in a circle around it!!

I don't understand this. In that circle, how do you relate the GPS
bearing TO the buoy with your bow bearing approximately (but how would
you make it exactly?) tangential to it?

I made a simple practical suggestion-

| > You can rig a simple device, using a protractor with sightvanes on
the
| > coachroof, to set the bow at a known angle to an identifiable
| > direction.

and Lu has scoffed-

| Yes, these are all traditional methods.  So are Lunars (sorry,
Frank).
| But just as Lunars (and steam locomotive engines and horse-drawn
canal
| boats) are wonderful bits of history but not something I'd use
today,
| using a pelorus to determine deviation is likewise something that is
| better consigned to history than touted as 21st century technology.

Notice that Lu hasn't presented a single reason against the use of a
pelorus. Until recently, every ship of the US Navy would carry at
least one such device, and as far as I know they still do.

I had suggested using celestial azimuths.

| Again, a tried and true method -- if one is at sea and there is no
haze
| or fog and can see low-to-the horizon celestial bodies and there are
| enough of them to provide a meaningful deviation table.
Unfortunately,
| God doesn't always cooperate...

You don't need to be at sea, just to have a view of a lowish Sun. With
a simple pelorus, you can do the whole job with that one view. No need
for "enough of them".

I ended

| > On my boat, I do my very best to keep things simple.
|
And Lu ended

| Me, too.  We apparently just have different definitions of "simple"
(for
| example, my boat does have an engine!).

So does mine, but it doesn't get used much. A little one-pot BMW
Diesel, that I installed back in 1980, to replace a useless
two-stroke..

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

I had mentioned the difficulties in relating chart positions with GPS
positions, and Lu replied in another posting-

"Are we running with out-of-date charts, George?  <g>   Surely the new
ones are  WGS-84"

Two answers to those questions. In many parts of the World, none, or
few charts have been updated to GPS, even if they have in the US and
(rather recently)around UK.

Second, I certainly am running out-of-date charts. Many, indeed most
of mine predate the adjustment to GPS. Some of them even predate the
change from fathoms to metres. With 60-odd Hydrographic Office charts
in my locker, I couldn't consider replacing them each time they happen
to bring out a new edition. I keep them updated, as far as possible,
from weekly notices (or, to be more accurate, my wife does). But each
time a new edition appears, there are changes that I will miss.

What I deplore is the current practice of bringing out a new edition
for the most trivial reasons. Time was, when a chart would go for 50
years or more between editions, accumulating dozens of corrections
over its life. It seems to me that the new approach is a purely
commercial one. With compulsory carriage of the latest chart by
commercial vessels, each new edition becomes a licence to print money.

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

Commenting on my preference for making a deviation card rather than
using corrector magnets, Robert Eno wrote-

"I agree in principle on this George, however, there are conditions
under
which it is wiser and more sensible to use corrector magnets to bring
the
deviation down to as close to zero as possible. A case in point would
be
when the deviation changes by large amounts from one sector to
another. To
quote a book I have, "as the boat yaws and pitches, the poor compass
will
try to follow its errors". The result is a compass that spins
uselessly as
one changes heading."

Well that situation may occur, as Robert says, when there are extreme
changes in deviation, such as he is suffering from. But I am not sure
the comment from that book makes complete sense. I can't see pitch as
making any significant contribution at all. Yaw motions are not
usually so great or so sudden as to cause the effects described. The
main effect upsetting a compass is that of the horizontal
accelerations due to motion in a seaway. A compass becomes more
sensitive to such accelerations if its centre of gravity has been
lowered to make it less susceptible to dip-tilt. The only time I have
ever seen a compass behaving as described in that quotation is in a
boat on which, at certain engine speeds, vibration would cause the
compass to rotate steadily anticlockwise. Perhaps Robert can tell us
whether he has experienced the behaviour described in his book, of a
"compass that spins uselessly as one changes heading". There's a lot
of nonsense written in books, just as there is in navigational mailing
lists.

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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