NavList:

George:

Great discussion.  We may differ, but it's differing between intelligent
men who understand the problem, just differ on which solution is
personally the easiest to implement.


George Huxtable wrote:


> 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.

You're absolutely right, and this is an informal way I use to
occasionally check deviation.  One thing neither of us emphasized is
that zeroing a compass and/or drawing up a deviation table is not the
end-all of compass accuracy.  A good navigator occasionally checks
his/her deviation card, and just noting compass direction while sailing
between two charted objects is a good way of doing so.

>
> 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.

Not so.  The usual instructions for "twiddling the corrector magnets" is
to FIRST determine a deviation table, then point the boat due west (or
east), adjust the E/W corrector magnet, point the boat due north (or
south), adjust the N/S corrector magnet.  At that point one should
re-determine a deviation table to see if all deviation is out.

>
> 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.

Aha, I think it's what is "a few"  I can create a minimal deviation
table with just two or three deviation values, but I wouldn't try to
draw a deviation curve (often formally expressed as a Napier Diagram)
without deviation at 45 (or, better, 30) degree intervals.  Even with
just a table, there is an assumption that I can linearly interpolate
between recorded values.  That's a dangerous assumption of the intervals
are more widely spaced than about 45 degrees.

>
> 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.

George, you are right, but your idea of running a course between
floating aids is subject to exactly the same problem.   BTW, in the US
aids are usually on a scope of about 1.5:1, so one can reasonably
estimate the swing circle.   To tell a tale on myself, when I first
purchased a Loran 20 years ago and started using its "mark" function to
store the position of floating aids, I noticed I was getting different
positions (by a few hundredths of a minute) for one particular
deep-water buoy.  Took me a while that what I was seeing was accurate
coordinates for a buoy that was swinging on its chain!
>
> 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?

Sorry, I realize my description was unclear.  The idea is to drive in a
circle around the buoy and at 45 degree intervals turn 90 degrees and
point the boat's bow at the buoy.  Reading your compass direction and
comparing it to the GPS's bearing will give deviation.  Swing back 90
degrees and continue circling the aid and repeat.

>
> 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.

Two comments:

(1) A pelorus is useful for a number of navigational tasks beyond
determining deviation.  One of the most useful is determining the
relative bearing of another vessel to determine if a risk of collision
exists.  On my sloop, a glance across rigging or lifeline stanchions
works and I know I have the maneuverability if I'm wrong about the
bearing changing.   It's not quite the same situation on an aircraft
carrier....

(2)  There's a big difference between a pelorus permanently mounted on
the bridge of a large vessel and a portable one placed on the coachroof
of a sailboat.  One of the biggest problems I see with portable
peloruses is how one knows if the axis of the pelorus is aligned with
the keel of the boat.  If it's off by a degree or two, then so is any
deviation measurement made using it.


> 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.

Acknowledged.  I don't know about British charts, but US charts are
outlandishly expensive -- roughly $20 per chart these days.  Used to be
about $2 20 years ago, then the US Congress set up some silly rules
about "cost recovery" and the price has skyrocketed ever since.  Seems
like they ignored the cost of rescues or tragedies that result from
people using out-of-date charts.   The good news is that US charts are
not copyrighted, so there are several companies issuing chart books with
copies of dozens of charts for a particular region for about $50.

And, in an interesting development (for those of us comfortable with
electronics, at least) is that NOAA now has raster-scan versions of all
its charts available for free download from NOAA's web site.  In fact,
you can actually view any NOAA chart on the web at
http://www.nauticalcharts.gov/viewer/   These are supposedly updated
with all corrections issued since the issuance of the latest version of
the printed chart (ie, they're up-to-date within two weeks).
>
> 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.

You're lucky on your side of the Pond -- even as the price of NOAA
charts has skyrocketed, the issuance of updated versions has plummeted.
  Twenty years ago an updated version of a chart was issued every two or
three years.  Now we in the US are lucky if it's every 10...  Commercial
vessels in US waters are not only required to carry the latest version
of charts, but must also have them updated with all updates issued since
the publication of the chart.


George, I may disagree with some of them, but thanks for all your
insights.  You are truly an expert contributor to this list!

Lu Abel

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