NavList:

----- Original Message -----

From: David F. McCune

To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM

Sent: Saturday, April 29, 2006 9:16 AM

Subject: Re: When is it Good?

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.

 

 

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