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Re: When is it Good?
From: David F. McCune
Date: 2006 Apr 29, 18:16 +0200
From: David F. McCune
Date: 2006 Apr 29, 18:16 +0200
Well,
as a purely practical matter at sea in a small sailboat, the definition of
"good" has many parts. Here's how I look at it when
navigating:
1. How far from land am I? The closer to
land, the more I want a feeling of accuracy. When I'm 1,000 miles from
anything that I might run into, then I'm pretty relaxed. If I get a
"cocked hat" that's 15 miles on a side, say, then I just plunk down a fix in the
middle of it and don't worry about it until tomorrow's sights. If I'm
approaching a coast however (especially if there were any risk at all of hitting
the coast during the coming night) then I try a lot harder. That
means repeated sights, running fixes, and as many bodies as I can possibly
shoot. I do running fixes every three hours the day or two before
landfall. And if I am lucky enough to have the moon at a good angle
to the sun, then I feel much more secure. And I would never miss a dawn or
dusk set of stars under those conditions. Even far at sea, however, I
usually take five rounds of sights each day (dawn, mid-morning, noon,
mid-afternoon, dusk). Partly that's because I always sail alone and it
gives me something to do. And partly that's because I never know when I
might get my next sight. After all, it could be cloudy for the next five
days - in which case those last couple of sights I didn't take really
hurt.
2. In my experience, the single biggest source of
inaccuracy is the movement of the boat. So if I'm in the middle of the
Pacific High and the sea is flat calm, then I would be very disappointed by a
15-mile cocked hat. But punching close-hauled into a 15-foot
tradewinds swell, let alone running down 25 or 30-foot waves, I'd be pretty
happy with any fix that even remotely looked consistent. The hard part of
celestial navigation is learning to take reasonably accurate sights in those
kinds of conditions. I typically stand with my legs straddling the cockit,
one foot on each locker, my back resting against the dodger rail, harnessed to
jacklines on each side of the boat. I have the sextant, a clipboard, a pen
and stopwatch around my neck. Eventually you learn to judge the waves and
grab a sight just as the body is kissing the horizon. I typically do three
or four of each body in fairly rapid succession before going on to the next
body. I get a gut feel for which of the sights just felt right and
put a check mark next to that one on the clipboard. It's hard to describe
what I mean by "felt right," but sometimes just at the moment that I grab the
sight and look at the watch, I just know that I got it right on the distant
horizon and not on a nearby wave top. Usually that instant is accompanied
by some exclamation of pleasure. "Yes!" or "Gotcha" or some such
thing. And then other times I just feel I missed the sight. Maybe
the boat rolled, or I lost my balance. That is usually accompanied by
"Shit!" or some such thing. Once I get the sextant put away and am sitting
at the nav station, I look over the sights. I typically pick one for
each body and then reduce them. Sometimes, after reduction, I'll find that
one of, say, four bodies seems way off. If so, I often
go back to the other two sights I took of that same body and reduce them and see
if that adds any clarity. And then again, sometimes I'm tired, sore and
lonely and I just say "fuck it, let's open a can of beans and have
dinner."
3. I've never yet worried about significant
sextant error. I check for index error before I get started with a round
of sights and then don't think about it after that. Any sextant of even
half-assed quality is accurate enought to find Hawaii or the Azores or any
landfall with a hill or 500 feet or so. Hell, I could probably do that
with a protractor and a piece of string. You just have to treat your
sextant right. I am as loving with mine as the pope is with a
crucifix. Don't drop it or bang it against anything. But as a
practical matter, I'm just trying to get to my destination, not survey the
ocean. I don't really care if there is a second of arc too many or too few
here or there. If someone told me that I had to make my next voyage with a
Davis Mk 3 sextant, I would feel perfectly safe. (I wouldn't feel content,
of course. But that's more like dancing with a woman whose body you don't
like versus one whose body you do like. Once you like a given sextant's
curves, it's hard to give it up.)
4. I am a fanatic about keeping my DR
updated. Assuming you trust your compasses and have a good taffrail log,
assuming your wind-vane is in good order, assuming you know the local currents,
and assuming you note your course and speed every couple of hours, then you
would be very surprised by a fix that didn't fit your DR
position.
5. My experience is that most errors in celestial
navigation are so large as to be very obvious, even before the plotting
begins. Either I made an arithmetic error in the reduction process, or I
misread the sextant (scribbling down 47 deg instead of 37 deg while standing in
the cockpit), or I scribbled down the wrong star name, or I got the
same/contrary name bit wrong, or something like that. Those errors are
pretty obvious even before you get to the intercept of "487 away"
stage.
5. Remember that in small boat navigation you are
never going to be more accurate than to within a few miles. If you need
better accuracy than that, get a GPS. So if you need to thread the needle
between awash reefs not more than a mile apart, don't do it with a
sextant. A sextant is not as efficient as a GPS. But then again,
sailing is not as efficient as United Airlines, either.
Anyway, just my two cents worth as a sailor who,
once at sea, mainly just wants to get off the damn boat as soon as
possible and on the right continent.
Good
luck,
David
-----Original Message-----
From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Guy Schwartz
Sent: Saturday, April 29, 2006 2:38 AM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: When is it Good?Hello:In the book 100 problems in Celestial Navigation. In the answer section problem 1-2 it says "The LOPs have more spread than we would like, but we rate the reliability as good"In the perfect world all LOPs would cross at a given point, however the system is not perfect, therefore when they say it has more spread than they would like, how much spread is to much? Is there a certain distances that relate to excellent, very good, good, fair and unuseable?Do these distances relate to the reliability of the sights.Thank you.