A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Plastic sextants and prejudice. was: GPS shortcomings.
From: Dave Weilacher
Date: 2005 Jun 10, 08:03 -0400
From: Dave Weilacher
Date: 2005 Jun 10, 08:03 -0400
Consider: The only thing a sextant does is measure an angle. (unless you are John Wayne) On a small boat (or a prudent navigator), you take a bunch of shots in order to determine the likely best. You combine this with other lines of position and a DR and all of it, as a whole, determines where you are, not the composition of the sextant frame. They navigated for centuries with a gadget that you had to eyeball the horizon and the sun (etc) with alternating eye movements. If you never use an angle closer than one minute and drop all precision in table angles that is less than a minute plus exclude the z-correction and the correction for minutes in declination. The resulting lines of position you achieve are still close enough to navigate offshore with. On a small boat, you can even leave off the correction for dip and always take the semi-diameter of the sun as 16 minutes, ignoring parallax entirely. (This is how I find my sight reduction errors when something goes amiss. It amazes me at just how close the result tends to be compared to a proper reduction. I am NOT advocating this in place of using any given precision. I'm just arguing that the quality of the cheapest plastic sextant is adequate to the task and likely far superior to what was once used) I would far rather have a Davis Mark 3 than a backstaff. BTW, I truly like my Astra 3b. Compared to my Davis Mark 25, the optics are great. Dave -----Original Message----- From: Robert Gainer
Sent: Jun 10, 2005 5:48 AM To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM Subject: Re: Plastic sextants and prejudice. was: GPS shortcomings. Peter, My first sextant was a 13 dollar plastic Davis, the one with a solid frame and it was made before they even blackened the marks and numbers. I used that on my first Transatlantic on a 22-foot boat and made my landfall at Bishop Rock Light about ? mile to a mile north of my intended track. Used with care it is more then accurate enough to navigate within the limits obtainable from a small boat given the height of eye and movement of the boat. The gray plastic English made Ebbco (East Banks Boat Company) is even better and more accurate then the cheapest Davis. The Ebbco even has a micrometer drum that the original Davis did not. I have not used the more expensive plastic sextants except to just hold them and look at them, so I can make no comment about them in use. I bought my first metal sextant because it was embarrassing to do a delivery or be the navigator during a race using a plastic sextant. Of course, I would never go back to plastic, but the addition of a metal instrument to my kit did not add anything to the precision of my landfalls. To me, this means either I am terrible at navigation or the Davis is equal to the job. I am not saying that the plastic is as good as the metal. Just that it is adequate and I would not be uncomfortable aboard a boat were the skipper used one as the primary means of navigation. My only question is about the skill of the user, not the accuracy of the instrument. Robert Gainer >From: Peter Fogg >Reply-To: Navigation Mailing List >To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM >Subject: Re: Plastic sextants and prejudice. was: GPS shortcomings. >Date: Fri, 10 Jun 2005 07:16:35 +1000 > >From George, with claws showing ever so slightly: > > > > I don't pretend that such sextants are perfect, and we have discussed >some > > defects more than once on this list. But Peter Fogg's comments read more > > as > > a rehearsal of prejudice than a reasoned evaluation. > > >There was no claim to a "reasoned evaluation", just a report of my limited >experience, given somewhat reluctantly after being specifically asked. Yes, >it was a Davis, I don't know which model - grey with a solid arc of >plastic. >I'm prejudiced about very little and certainly not sextants - the doors of >my mind are blocked eternally wide open! > >However, what I've just read here inspires even less confidence; it all >runs >too much along the lines of: ' yes, they may have this and that and the >other shortcomings, but they're really not so terrible after all; given >this, that, and the other caveats'. > >Much too defensive. Nevertheless, I'm not really all that into sextants as >such, and think it entirely appropriate that each navigator choose his own >instrument after careful reflection. > >If I needed a hammer I would probably buy the cheapest on offer, why pay >more? If a carpenter chooses a hammer it is done with considerably more >care, and he may pay several hundreds of dollars. It makes sense from his >point of view; such things as the balance and the composition of the steel >are very important to him - just as a crude blunt instrument is all I need >from the same tool. _________________________________________________________________ Don?t just search. Find. Check out the new MSN Search! http://search.msn.click-url.com/go/onm00200636ave/direct/01/ Dave Weilacher .IBM AS400 RPG contract programmer .USCG Master lic. 100 ton .ASA Sailing Instructor Evaluator