NavList:

George Huxtable wrote:

> First, I ask; is it really wise to use corrector magnets to zero out
> the deviation? All that's needed is to chuck out any corrector
> magnets, note what the deviation actually is, at each heading, make
> out a deviation card to correspond, and allow for it by a bit of
> mental arithmetic. Otherwise, you (or worse, someone else) must have
> gone through the exercise of adjusting those correctors, and have Got
> It Right, and presume that thereafter nothing ever changes. If you're
> an ocean voyager, it's likely that big changes of latitude will render
> those corrections invalid.

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.

While we might all hope for a "bit of mental arithmetic," most people's
ability to do it (including my own) is reduced by stress, lack of sleep,
seasickness, and a whole host of things that can and do happen to us at
sea.   As a result I strongly believe in and advocate making underway
navigation as simple as possible.  The fewer the calculations needed
underway, the fewer opportunities for mistakes.

As a result I for years I have advocated annotating course lines on
coastal charts in Magnetic, not true.  I don't want to be out there in a
blow trying to remember whether one adds or subtracts easterly variation
to go from true to magnetic (and what one knows "cold" on shore can
disappear quickly under stress at sea, IMHO).  Similarly, I believe good
(ie, safety oriented skippers) will want their compasses adjusted to
minimize deviation.  Again, in my experience, with a well adjusted
compass deviation can be brought to such a small level it can be ignored
in practice.

> Robert Eno finds his deviations to reach 25 degrees, which ought to be
> a cause for concern. If I were in his position I would go carefully
> around the boat with a hand bearing compass to search out a better
> position for the steering compass, if possible, and to understand why
> it is so badly affected. We have to remember that he is in a region of
> very high magnetic latitude, where the field is nearly straight up and
> down, and the little horizontal component of field that remains is
> likely to be badly affected by any steel around, such as his engine.
> The rest of us, in lower latitudes, have an easier time.
>
> But there are other possible culprits than the engine, and (for
> example) any nearby loudspeaker, even the unit in a VHF radio, should
> be treated with suspicion.

Thanks for the insights, George!  They all make sense.  While I'm not
sure what amenities of civilization Robert has aboard, electricity (and
even worse, permanent magnets in loudspeakers -- and it can be from
something as shipshape as a VHF, not just a stereo) can badly affect a
compass.

> Instead of using GPS to determine deviation, what's the matter with
> simply using your chart, or else celestial bearings?
>
> In harbour, look at your chart. Most places, there will be lots of
> charted objects all around, and any pair of such objects, that you can
> align, gives you a transit with a true bearing that you can measure
> off the chart with a protractor. Get yourself in that line, put that
> transit on the bow, then on the stern, then on each beam, read the
> compass at each position, allow for variation, and already you have
> four points on a deviation card. Find another transit, in a different
> direction, and do it again. Be careful if sighting on  floating marks,
> unless they are very distant, because they can range about so much in
> a tide.
>
> If you can put the boat precisely at an identifiable spot on the
> chart, transits are unnecessary; simple bearings to known landmarks
> will suffice.

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.

Robert, writing this reply brings up another idea:  you need only a
SINGLE charted object such as a buoy if you can sail around it.  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!!

>
> 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.

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.

>
> But why not compare the compass reading with the bearing of a
> celestial body, particularly one that's low, near the horizon? On
> passage, point the bow, or the stern, at a low morning or evening Sun.
> Head off course for a moment, if necessary, to do it. Note the time
> and the compass reading. Later, work out the Sun azimuth at that
> moment, allow for variation, and check the result against your
> deviation card. You can keep on doing that job each day as a matter of
> routine, as the sailing-ship navigators used to do. On a clear night,
> there's a choice of low stars for doing the same thing.

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...

>
> So what's to be gained by invoking GPS, I ask? Certainly, you are
> likely to get into trouble if you compare compass heading with the
> course shown by a GPS receiver, under way. What you need, for the
> deviation, is to know the heading of the bow.  If there's any leeway,
> your course through the water will differ from that. If there's any
> current, your course over the ground will differ, again, from that,
> and that's what the GPS will tell you.

As mentioned in my follow-up, there is a BIG difference between a GPS's
CMG calculations and bearing calculations.

>
> On my boat, I do my very best to keep things simple.

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

Lu Abel

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