Navigation the Rite Way.. or Is GPS Reliable???

[starcrest]

the artificial horison is something I never tried .it is not as easy as you think.the suns image needs to be brought down to the reflected image in the water...I just dont think that  method is very accurate compared to bringing the image down to the actual horison.plus the azimuth....that is to say the true direction to the sun ...thats the direction that you look...I dont think its a very accurate method.the shadow given off by the pintle of a sundial....delineates passage of time....that shadow when followed to its source.....thats azimuth.some how I think the shadow given off by a bucket of water is too ambiguous to be relied upon.any way I am not in N.C but I can and have taught the stuff

[Captain Smollett]

I use an artificial horizon for sextant practice.  It is very tricky, and takes a lot of patience.  I was once trying to teach a lady to use the sextant with it, and she just could not fix the sun's image in the water.  I think it's a useful technique to know about, and good for practice, but it does add some difficulties that a newbie might well find overwhelming.

[starcrest]

I once used an aviation sextant that was modified for marine use.the bubble had to be removed,and it was only good for the sun and moon but it worked.I understand the actual use in an aircraft was quite a stunt.that height of eye correction would have been phenomenal

[hearsejr]

 bet it was feat back in the 30's to learn that stuff from an airplane  then getting sent off the South Africa in ww2. I would hate to have to do it. look up the story of the B24 called "Lady Be Good" . they got lost after a bomb mission and over flew their base by 400 miles and bailed out thinking they were over the med, but they were realy over the dessert. I think they walked like 350 miles trying to get back to the base, and 1 by 1 they all died. the plane was in such good shape  when they found it that some of the radio equiment was used to reapair a busted unit on a hellicoptor they using during the search for the remains of the crew. they found it in like the late 50's. very inteeresting reading for anyone.
I was looking at the set from Ebay, the guy said he was willing to take like $150 for the sextent with case and manuels. I backed out after he said he did not know if it had everything or not and he would not refund the $$ and would not check to see if anythingwas missing or broke..he claims to already boxed it up for shipping.
  I still thinking Ebay is where I'll get the first one from then I'll thinnk about getting a nice one from the Latt's and Att's ad I saw.....somethinng $500 though, and it may take a while to save it up. lol.
Bill

[CapnK]

This is a good thread - thx guys. :)

Re: the Lady Be Good story - I've read about that. Amazing that they found the plane, out in that desert. And even later, the remains. Interesting story.

I wonder if there are still a lot of miltary surplus sextants around? Based on the eBay price, doesn't look like many...

[hearsejr]

I wish
I had snatched the old navy one up back when I had a chance. the guy had the whole nine yards, the compass in a wooden box, the sextant, the pertractors, the ruler, and survival book from a life boat. all for $50. oh well should've ..could've..would've's are the stuff dreams are made of right? lol.

bill

[Pixie Dust]

NOAA has a free online chart service offered.  It has posted all 1000 and some charts on line.  These are all up to date with most recent Notice to Mariners.  Recommended use is for a planning tool or quick access.
Site is below
http://www.nauticalcharts.gov/viewer/
Click on your water area and then needed chart numbers.

Zen, I did not see anything listed as "ducking coast"  ;D

[CharlieJ]

Connie- if you are taking about the ENC charts, they are pretty much worthless for navigation purposes. Laura and I had some aboard last year, loaded on our laptop. They don't give you the info you need.

Looks like the new release of charts may be much better but we haven't had time to check 'em out yet.

[Pixie Dust]

Charlie, These are supposed to be the actual Govt. Charts, updated with the Notice to Mariners and to be used for planning purposes etc.  It did recommend that you use your digital charts or purchased charts for actual cruising.
These may be good to use when doing global pre-planning or using them to update your hard copy charts. 
Problem I had, looking at it on small computer screen, you zoom in and you only see a small area, when backed out, I could not read the depths etc.  May be my 40 something eyes too.  :)
Check it out when you get time Charlie and give us some feedback on your thoughts. 

[CharlieJ]

lol- "when we get some time" ;D

SOON I hope.

Back in November I began a new building here at the house to move my business into (woodshop/boat building). We have moved but are now neck deep in clearing the old building and trying to set up a "garage sale" to rid ourself of as much "JUNK" as we possibly can. HOPEFULLY by next month we'll be clear of that city, we won't have to drive there any more and we WILL have some time to look over the new charts and MAYBE even sneak in a sail.

Gee I hope so ;D

[starcrest]

one more thing I jus' remembered.this celestial stuff is all predicated on a falacy.it really should not werk at all.it is based on the geo-centric universe theory.in other werdz,the oith iz da centuh uvduh  yooneevois,an' evry tink revovles around it.iz dat tru? iz da oit da centuh of da yooneevois?theres that nooyawka in me comminz out.sorta like the hulk when he gets mad

[NANP]

Utterly newbie question here . . . in scanning eBay, I see really, really expensive sextants, and then---I see ones like this:

http://cgi.ebay.com/HENRY-BARROW-Working-6in-Sextant-w-Box-Excellent-Gift_W0QQitemZ7384912685QQcategoryZ37971QQrdZ1QQcmdZViewItem

So, the question is---do the "brass working reproductions" do the job, or are they just "for pretty?"  ???

NANP

[starcrest]

I have seen those the problem will be reading the arc...it may work fine...as long as you use the rite numbers...but the arc is the vernier type with that magnifying lens the  read out may be difficult.I prefer the micrometer drum type by all means.and yes...sextants are great fun when you know how to use them.it aint hard at all...simple addition and subtraction in groups of 60.....35min+35min=1hr 10min.ya shood b'ableta getta good yoozd metalone fora boutz 200 bux at a marine consignment place ore jusgetta astra 3b ifya like chinese food...ifyaz wanna go allout get da won wally krankite has its silver an' costs 35 gozillion billion bux.but all kidding aside...definately learn what its about.its a real discipline.....it doesnt take long....just acoupla days of intensive practice an'yul havit memorized.if you are indeed really serious about the celestial stuff then start with the davis mk 2...dont worry about the fancy see thru horison mirror..it wont change the fact that it is plastic and vulnerable to serious index error they are good to learn on....then when you see the difference a metal bodied one makes you will take tha plastic one and seriously wrap it in its box and save it for emergency use only

[Lars]

Nanp.. Don't waste your money on those  Ebay reproductions.They are stricktly display items with some moving parts. If you must have a metal sextant go with the AstraIII  for the money.. you can catch them on ebay for around 500. Personally  for use on a sailboat i recomend the davis 15 for the light weight. Plastic sextants have good accuracy if you adjust them after each use. I can get 6 mile intercepts with my 20 dollar davis mark 3 ..

[NANP]

Thanks, Lars, that's exactly what I wanted to know!

As a vintage-inclined gearhead in general, it's a bit hard to make myself consider anything with the word plastic in it . . . but the difference between $500 and $20 might just force the issue!  ;)

I mean . . . that's like, a box of Cuban cigars and a case of Scotch, right there. 

NANP

[Joe Pyrat]

You might try these sites:

http://www.starpath.com/catalog/courses/1701c.htm

http://www.irbs.com/bowditch/

[NANP]

Wow . . . great links, Joe! Awesome!  :o

I've already started transferring the Bowditch manual to my hard drive . . . and poking around the rest of that site. 

Good karma to ya; this'll keep me busy awhile!  ;D

NANP

[starcrest]

actually the intercept is the distance that the resultant line of position is plotted perpandicular to the azimuth ....either to or from the assumed position and has no bearing on instrument accuracy at all.there is a "long intercept" method that can be used when the proper sight reduction tables arent available.'INDEX ERROR" is what the difference is between plastic and metal instruments.this needs to be corrected before the sight is taken and is one of the first corrections introduced when figuring the Ho,or height observed.even with the best plastic instrument  this can be up to plus or minus 30 minutes.which basically means an error of 30 miles. Even so,the first sight,or the Hs,(sextant height) under the most optimum conditions can be perceived differently between 2 or more different people.even your own two eyes my get a different readout at the same time.this is a neurologic thing.with the ww2 david white I used it was never more than -.5,or less than 1/2 mile basically the width of a pencil line on a plotting sheet,so I just left it at zero.I used the davis ..the one with the traditional  split horison mirror and have seen the index error change rite b4 my eyes.basically I set the micrometer to zero,sighted the horison,and b4 I knew it there were 2 horisons.these things are ok,but actually my garmin 12 channel gps is far more accurate and prefer to use it even more than walter cranshafts silver bullet

[Captain Smollett]

'INDEX ERROR" is what the difference is between plastic and metal instruments.

ALL sextants, plastic or metal or made of cheese will have a finite Index Error.

this needs to be corrected before the sight is taken and is one of the first corrections introduced when figuring the Ho,or height observed.even with the best plastic instrument  this can be up to plus or minus 30 minutes.which basically means an error of 30 miles.

No it does not.  Not if you know what the index error is, and you correct for it.  If you subtract (or add, as appropriate) the IE from your Hs, the IE introduces NO ERROR to the calculation.  Every CN book I've every read, and the CN exercises in Ocean Navigator (a PROFESSIONAL trade mag) have included correcting for the IE as part of the process.  Therefore, the finite IE of the instrument itself is corrected out, so, a IE of 30 minutes (which would be very very large, by the way, my Davis Mark 15 typically has an IE of about 2-5 minutes, and again, I KNOW what it is before taking a measurement) absolutely does NOT introduce an error in the resulting fix of 30 miles.

With the Davis 15 plastic sextant, it is trivial to zero the mirror to 0 IE before you take a reading.  Even if you don't want to do this, it is equally trivial to measure the IE by nulling the horizon itself.

You can get quite accurate readings with a Davis mark 15 plastic sextant.  Period.

[starcrest]

before or after makes no difference.how ever a difference between"minus point five" or  two to five  is a huge difference resulting in as much as 2-5 miles.still I will swear by my garmin 12,which yeilds an error of 6 feet,or about.001  nautical mile