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Boat Forum / Cruising / November 2004Astra IIIB Sextant
To any celestial navigators still out there: The recent improvement of the Canada/U.S. exchange rate is persuading me that the time to buy a sextant is approaching. I plan on world cruising in a few years and just as I wouldn't have an autopilot without a windvane pilot, I want to bring a sextant along to back up the GPS. Besides, it's a seamanly skill to have, gives the person on watch something to do, and will teach my kid real-life math skills. Despite the abandonment of the sextant by most recreational sailors, there does not appear to be a flood of cheap old C. Plaths or Tayama models on eBay <G>. Consequently, I am thinking of getting a new $500 U.S. AstraIIIB sextant from Celestaire.com. Two questions: 1) Which is preferable for noon sights and the occasional star sight: whole horizon or half-silveredt mirrors, and why? 2) Does anyone have long use with both the Astra and "better" sextants? Is going more expensive worth it in a predominantly GPS world (I have two handhelds now, will likely add a chartplotter to the bluewater cruiser...) Thanks, R. >wouldn't have an autopilot >without a windvane pilot, two different pieces of gear with two different functions. >>wouldn't have an autopilot >>without a windvane pilot, > >two different pieces of gear with two different functions. Yes, I know. I was employing an analogy. http://dictionary.reference.com/search?q=analogy As both vane and autopilot can steer ships, so both sextant and GPS can be used to navigate. R. ... >>>wouldn't have an autopilot >>>without a windvane pilot, [quoted text clipped - 9 lines] > >R. Uh-oh...somebody agreeing with the lunatic fringe - that means a mistake is under way. Let me put it this way: small boat pilots use some or all of these 1) a magnetic compass sensor, steer a magnetic course 2) a windvane sensor steer a constant angle off the wind 3) a great circle (CrossTrack error) from GPS to steer a gt circle. Brian W While sailing in the Bahamas and arriving at Rum Cay, my GPS went belly up. My intermediate stop was Colon, Panama. Thought about ordering a new GPS, however, due to shipping and order time delays, I decided to press on to Port Royal, Jamaica doing a DR with windvane and mag compass. No problem. Spend a few days in Port Royal then decided to give Colon a shot. Slid about 10 or 15 miles right of the shipping lane and pressed on until I sighted land. Slid left and sighted a merchant moving right on course. Followed her and proceeded to arrive Colon anchorage O'dark thirty, dropped anchor and had a couple of midnighters. Point is, one doesn't need a sextant offshore, if one has a accurate compass and recent charts. After all, sailing vessels are not moving that fast and with time coupled with patience one shouldn't have a problem sighting landmarks. And writing of speed, a sextant in a liferaft is laughable. Nothing will replace an EPRIB. Assuming of course, one is interested in living. Always think in terms of having a water maker in the survival bag. By the way, six man liferafts realistically are only suitable for six children. rhys ess monkey, even with a dictionary ready at hand you still don't know what an analogy is, if one is to judgement from your statement. >From: rhys rhys@nospam.com >Date: 11/12/2004 10:45 AM Eastern Standard Time [quoted text clipped - 13 lines] > >R. IMHO, the Davis Model 25 is an acceptable sextant in an emergency. The plastic body is inherently less stable than a solid brass or aluminum sextant. But if you reset the index before each shot, you can get 1-2 mile accuracy from a solid, ie not bouncing perch. If you are taking your sights from the deck of a small cruiser, you are lucky to get 5 mile accuracy. But even 5 mile accuracy will get you back to land in an emergency. David > To any celestial navigators still out there: > [quoted text clipped - 22 lines] > Thanks, > R. >But >even 5 mile accuracy will get you back to land in an emergency. only if you can see the sky. But turning left and going until you get there always works. Not in the southern ocean......... Geoff > >But > >even 5 mile accuracy will get you back to land in an emergency. > > only if you can see the sky. > > But turning left and going until you get there always works. go left in the Southern Ocean will most certainly aim you at a coastline. If you doubt that, check *any* globe. >From: "BHBH" geoff@brighthikers.com.au >Date: 11/12/2004 9:30 PM Eastern Standard Time [quoted text clipped - 10 lines] >> >> But turning left and going until you get there always works. <G> You may want to take another look at that globe you're using, Doodles > go left in the Southern Ocean will most certainly aim you at a coastline. If > you doubt that, check *any* globe. [quoted text clipped - 6 lines] >> >>Geoff help out here, deck rat. what globe should one be using when trying to find any place where a left turn will not get one to a coast. >From: otnmbrd otnmbrd@earthlink.net >Date: 11/12/2004 10:50 PM Eastern Standard Time [quoted text clipped - 13 lines] >>> >>>Geoff > help out here, deck rat. what globe should one be using when trying to find > any place where a left turn will not get one to a coast. Yeah but....he may not want to wait 6 weeks before he sees land. then turn right. surely, it can be assumed he has SOME idea where the hell he is. well, maybe not in the case of deck rat. >> help out here, deck rat. what globe should one be using when trying to >find >> any place where a left turn will not get one to a coast. > >Yeah but....he may not want to wait 6 weeks before he sees land. Any good one should do ya > help out here, deck rat. what globe should one be using when trying to find > any place where a left turn will not get one to a coast. so, which one shows a forever circle after turning left from anywhere? keep in mind, deck rat, what the word "left"means. >From: otnmbrd otnmbrd@earthlink.net >Date: 11/13/2004 3:40 PM Eastern Standard Time [quoted text clipped - 5 lines] >find >> any place where a left turn will not get one to a coast. Now Doodles, to quote you, I'm not here to do a google search for you, because you're too lazy to do the search yourself. As for your stupid "left", "right" games, have at it ..... looks to me like you're trying to figure out a word game to CYA. Later, Doodles otn > so, which one shows a forever circle after turning left from anywhere? keep in > mind, deck rat, what the word "left"means. help me out here, deck rat. googled for hours on end and still could not find a place on the planet where turning left would not get you to a coast. were you drunk when you claimed left would get you an endless journy? >From: otnmbrd otnmbrd@earthlink.net >Date: 11/13/2004 6:40 PM Eastern Standard Time [quoted text clipped - 12 lines] >keep in >> mind, deck rat, what the word "left"means. > help me out here, deck rat. googled for hours on end and still could not find > a place on the planet where turning left would not get you to a coast. were > you drunk when you claimed left would get you an endless journy? Time to throw a scenario into the mix. Lets say a friend of Jax's (yes..he has one) is sailing on a course due south in the Drake Passage. When he hits latitude 60 deg S, he gets a satellite call from Jax instructing him to 'turn left'. The friend takes this to mean a 90 degree turn to port..and is now sailing due East. Unless he changes his course, he'll sail forever...as no land mass exists on the 60 deg S latitude. So, the COMPLETE scenario is that one runs ******************EXACTLY**************** due east (not a fraction of a second less or more) one will never find land? That was a useful arguement to make in the context of a discussion about the necessity of a sextant (and associated gear) as a safety item on an ocean going boat just how????????? specious sophistry. >> help me out here, deck rat. googled for hours on end and still could not >find [quoted text clipped - 9 lines] >he changes his course, he'll sail forever...as no land mass exists on the 60 >deg S latitude. >Lets say a friend of Jax's (yes..he has one) is sailing on a course due >south in the Drake Passage. When he hits latitude 60 deg S, he gets a >satellite call from Jax instructing him to 'turn left'. The friend takes >this to mean a 90 degree turn to port..and is now sailing due East. Unless >he changes his course, he'll sail forever...as no land mass exists on the 60 >deg S latitude. ======================= It's been said that if you don't know where you're going, any direction will do. That is particularly applicable in Jax's case. >>Lets say a friend of Jax's (yes..he has one) is sailing on a course due >>south in the Drake Passage. When he hits latitude 60 deg S, he gets a [quoted text clipped - 9 lines] > > That is particularly applicable in Jax's case. Why do I get the feeling, Doodles is not going to be too happy with you two. OTN With the ocean sailing I've managed to complete, I don't every recall sailing in a straight line. It's always been a little bit north then a bit south or the opposite depending on with side of the equator one is located. Even at 60 south the changes are you will sight land, hopefully before you are overwhelmed by the seas. For example, check out Cape Horn. The fun point is, Doodles made an "absolute" statement which was/is flawed, then couldn't figure out where the possible flaw was. Your point is valid, but, in that area, you could also wander N-S of the line at all the wrong/right times and continually miss landfall. My major point of the whole exercise .... if you are doing serious offshore sailing, when the crap hits the fan, I don't care how many spare batteries you have or how many spare GPS units, .... you want to and need to, be able to revert to alternate means of saving yours and your crew's, butts, when the need arises..... and it very well may. When the chit hits the fan, you never know how you or others will react at any given time, and whatever you can do to make yourself and others feel more comfortable with the situation/conditions can more often help all deal more reasonably with a bad situation. otn > With the ocean sailing I've managed to complete, I don't every recall > sailing in a straight line. It's always been a little bit north then a bit > south or the opposite depending on with side of the equator one is located. > Even at 60 south the changes are you will sight land, hopefully before you > are overwhelmed by the seas. For example, check out Cape Horn. hey, deck rat, crawl back into the bathtub and play with your rubber ducky yacht. >From: otnmbrd otnmbrd@earthlink.net >Date: 11/14/2004 7:43 PM Eastern Standard Time [quoted text clipped - 21 lines] >> Even at 60 south the changes are you will sight land, hopefully before you >> are overwhelmed by the seas. For example, check out Cape Horn. Check the southern ocean out for yourself lady, then send a trip report, if you are able. Be advised, sailing in the roaring forty and below is very serious business. At that latitude, 60 south, one will either be overwhelmed by the sea or run into ice and below that ice is land, in most cases. Not sure who you're talking to here. Personally, I've never rounded the Horn and only been around the Cape about a half dozen times (running from Europe to Australia, New Zealand, i.e. never been to 60S or had the full benefit of the "Forties", so I'll leave those descriptions to those who have. At any rate, for the most part, Doodles is mostly correct .... for those who are doing offshore cruising in normal locales if you lose all means of navigation, turn left or right (heading North or South) and head East or West and you're bound to hit land. (We can all think of all the variables). otn > Check the southern ocean out for yourself lady, then send a trip report, if > you are able. Be advised, sailing in the roaring forty and below is very > serious business. At that latitude, 60 south, one will either be > overwhelmed by the sea or run into ice and below that ice is land, in most > cases. > Not sure who you're talking to here. Personally, I've never rounded the > Horn and only been around the Cape about a half dozen times (running > from Europe to Australia, New Zealand, i.e. never been to 60S or had the > full benefit of the "Forties", so I'll leave those descriptions to those > who have. I've been down there many times, across 66 S, in an icebreaker. The seas can easily get over 18m from trough to crest. I've been on the bridge and looked *up* at the crest of the following wave and the bridge is 5 decks above WL. Doodles, I suspect, has never gone anywhere out of the Northern Hemisphere and has the geography knowledge of a pre-schooler. PDW ... > Doodles, I suspect, has never gone anywhere out of the Northern > Hemisphere and has the geography knowledge of a pre-schooler. One of my favorite jaxisms is when he tried to tell us the location of Manhattan: "I live on an island in the Atlantic" >One of my favorite jaxisms is when he tried to tell us the location of >Manhattan: > >"I live on an island in the Atlantic" It happens to be the Center of The Universe. Only if the universe looks like a big set of butt cheeks. > >One of my favorite jaxisms is when he tried to tell us the location of > >Manhattan: > > > >"I live on an island in the Atlantic" > > It happens to be the Center of The Universe. Don't confuse the bathtub sailors, Garuda, with facts. >From: "Garuda" noname@invalid.com >Date: 11/14/2004 7:01 PM Eastern Standard Time [quoted text clipped - 5 lines] >Even at 60 south the changes are you will sight land, hopefully before you >are overwhelmed by the seas. For example, check out Cape Horn. I never do! Now, Doodles. If you keep making delivery trips as BR, mommies bound to let you graduate from splashing around in your bassinet, to the bigger splashing in the bathtub .... maybe, someday...... unlikely..... otn > Don't confuse the bathtub sailors, Garuda, with facts. > > help me out here, deck rat. googled for hours on end and still could not > find [quoted text clipped - 9 lines] > he changes his course, he'll sail forever...as no land mass exists on the 60 > deg S latitude. As much as I hate to say it, you are both right, in a sense. In your scenario, if you started on a heading of 180 degrees (ie Due South) then, when you hit 60 degrees latitude, changed your heading to 90 degrees, and kept it that way, you would indeed never hit land. This is plainly obvious. However, in order to keep your heading at 90 degrees, you would have to keep angling to the right a little bit. Around here, this is plainly evident on a road called "Zero Avenue". This road parallels the US/Canada border (the 49th parallel) to within a few meters. In order to keep the road from crossing into the US, the road has slight bends every few hundred meters or so. On the other hand, Jax is also right (God it hurts to say that). If you were on a heading of 180 degrees, and you made a 90 degree turn, then went perfectly straight, you would eventually run into land. If you did this, you would esentially be taking the great circle route to wherever you hit land. To see this first hand, take a globe, then try to find a circle with the same circomferance as the globe that does not touch land, anywhere. You will not find such a beast. Clear as Mud? :P Regards, Hans >bject: Re: Astra IIIB Sextant >From: Hans Johnson hjohnson@spork.sfu.ca [quoted text clipped - 8 lines] >the same circomferance as the globe that does not touch land, anywhere. >You will not find such a beast True, at the Equator. However, anywhere else, if you were steering 180, then turned left with the intention of steering a Great Circle, you would need the initial course to steer and the ability to know when and how much to alter course to maintain the Great Circle route. <G> Course, at 60S, if you were heading 180 and turned 180 left and followed that great circle you'd eventually hit land. Shen >>To see this first hand, take a globe, then try to find a circle with >>the same circomferance as the globe that does not touch land, anywhere. [quoted text clipped - 6 lines] > <G> Course, at 60S, if you were heading 180 and turned 180 left and followed > that great circle you'd eventually hit land. However, a Great Circle route is only somewhat "straighter" than a constant latitude route. Both are circles on a curved surface, the Great Circle only has the distinction of having the greatest diameter. Further, without a GPS or Sextant (and that is the scenario being discussed) one would be hard put to maintain a Great Circle, whereas a constant latitude could be maintained with a compass, and you can even correct for minor errors with a simple Sun sight that can be done without a sextant. Of course, jaxie's real claim is that a sextant is a worthless backup to a GPS because wandering aimlessly one will eventually hit land. This is from the doodle that claims a compass is worthless because unseen currents will instantly pull you off course. He also claims that understanding variation is a waste of time because no one ever uses a compass. Radar, is of course worthless, because no one should ever sail in fog. RDF was a hoax that was never actually used. The FAA is the ultimate authority on nautical navigation. And nylon line fails when stretched over 4%. This is from the same doodle who got lost off Cape Hatteras and wanted to turn back. The same doodle that called the Coast Guard in LIS because the engine was running "a little hot." >However, a Great Circle route is only somewhat "straighter" than a >constant latitude route. where the hell did you learn your math, jeffies. the stupid statement you made above wouldn't get you out of a high school math class, let alone enough college math course to qualify you for the degree in physics you claim. >>However, a Great Circle route is only somewhat "straighter" than a >>constant latitude route. > > where the hell did you learn your math, jeffies. the stupid statement you made > above wouldn't get you out of a high school math class, let alone enough > college math course to qualify you for the degree in physics you claim. What's your point jaxie? I guess non-euclidean geometry is another topic that is beyond your comprehension. jeffies, you add new dimensions to the term "idiot". What is incredible is that you think nobody notices. >From: Jeff Morris jeffmo@nospam-at-all-sv-loki.com >Date: 11/15/2004 9:22 PM Eastern Standard Time [quoted text clipped - 10 lines] >What's your point jaxie? I guess non-euclidean geometry is another >topic that is beyond your comprehension. >Subject: Re: Astra IIIB Sextant >From: Jeff Morris [quoted text clipped - 22 lines] >to turn back. The same doodle that called the Coast Guard in LIS >because the engine was running "a little hot." ROFL I KNEW you couldn't last forever, before you'd HAVE to jump on the Doodles case!!! Hey Doodles!!! News Flash!!!! Just got word from "otn" that his puter may have finally given up the ghost! You may have some time to spout your nonsense, without worrying about him correcting your errors.Course, Jeff will be glad and I will enjoy, standing in, in his absense (sp?). Shen >>Subject: Re: Astra IIIB Sextant >>From: Jeff Morris [quoted text clipped - 33 lines] > > Shen Unfortunately jaxie has already hit bottom on this one and there's nothing left to say. Doodles once again proclaims that he has no understanding of navigation and is proud of it! The bizarre thing is that he seems unable to navigate even with a GPS! Its no wonder that he never is actually in charge on a boat. Jeff wrote: Its no wonder that he > never is actually in charge on a boat. I have a feeling, Doodles may be at least intelligent enough to realize he is not capable of the decision making required and realizes his place is best served in the Stewards Dept. Shen shen, I have been way beyond your capabilities since I was just barely old enough to notice that some girls were starting to wear training bras. >From: shen44@aol.com (Shen44) >Date: 11/15/2004 11:18 PM Eastern Standard Time [quoted text clipped - 9 lines] > >Shen >Subject: Re: Astra IIIB Sextant >From: jaxashby@aol.com (JAXAshby) [quoted text clipped - 3 lines] >shen, I have been way beyond your capabilities since I was just barely old >enough to notice that some girls were starting to wear training bras. ROFLMAO Shen jeffies, whatever are you smoking these days? you are talking as if your foot is nailed to the floor, just wandering in circles. >From: Jeff Morris jeffmo@nospam-at-all-sv-loki.com >Date: 11/15/2004 10:27 PM Eastern Standard Time [quoted text clipped - 46 lines] >that he seems unable to navigate even with a GPS! Its no wonder that he > never is actually in charge on a boat. Decide what you are buying it for. The cheapest Davis is sufficient in my opinion for any likely real world situation. If you want to be a hobbyist than decide on your budget and go from there. Celestial is not remotely a backup to GPS. At best it is a life boat skill. Jim Donohue > IMHO, the Davis Model 25 is an acceptable sextant in an emergency. The > plastic body is inherently less stable than a solid brass or aluminum [quoted text clipped - 32 lines] >> Thanks, >> R. >The cheapest Davis is sufficient in my >opinion for any likely real world situation. ================================ I agree. You are mostly looking for a training excercise and the plastic Davis will more than suffice. Carry a couple of extra pocket GPS units. In an emergency situation you will only be turning them on long enough to get a fix every few hours and the batteries will last a very long time in that mode. >Decide what you are buying it for. The cheapest Davis is sufficient in my >opinion for any likely real world situation. If you want to be a hobbyist >than decide on your budget and go from there. > >Celestial is not remotely a backup to GPS. At best it is a life boat skill. Lol, there you go again. Consider this: If you didn't have a functional gps for whatever reason, would you rather have a functional celestial system or nothing at all? Steve >>Decide what you are buying it for. The cheapest Davis is sufficient in my >>opinion for any likely real world situation. If you want to be a hobbyist [quoted text clipped - 7 lines] > > Steve <G> I believe "Celestial is not remotely a backup to GPS" was bait, thrown in front of me, so let's see if we can put this stupid argument to rest. GPS is a form of navigation which works 24hrs/day in all weather, offshore, nearshore, and inshore, with an accuracy of feet. I'm still getting reports of periods of down time, blackouts and occasional computer lock-ups, which render the system useless for periods ranging from minutes to .... break out the sextants. These are diminishing. Any idiot, including Doodles, can use the system, as long as they are able to plot and make use of the information given (here, Doodles will probably have a problem). All in all, to date, this is the best system to come down the pike. Celestial, however, is only an offshore, nearshore (sometimes) system of navigation which is only useful (normally) from about 1hour before sunrise to 1 hour after sunset in conditions where a celestial body can be observed (clear skies are NOT always needed) and it's accuracy depends entirely upon the skill of the navigator and can vary from "spot-on" to "oops, wrong continent". Now, can celestial be a "back-up" to/for GPS? Of course, within the parameters of what it is designed for and capable of doing. Will it always be available? No. Will it's accuracy be comparable? Rarely, but that accuracy is not needed in the offshore/nearshore conditions that celestial is used for. As I have said in the past, for those of you not doing any offshore work that exceeds two days from the nearest landfall, I wouldn't be too concerned with knowing and using Celestial, but if you are going to go more than that, I would seriously consider having the knowledge and capability to perform some basic celestial navigation. Trying to compare GPS to Celestial, is an exercise in stupidity. Saying that celestial is only good for "lifeboat navigation" is equally stupid. Both systems have their plusses and minuses, both systems have their drawbacks ..... the SMART sailor knows these and makes use of these and has the ability to use whatever is available, when the sh.t hits the fan. Hiya Jim otn PS Doodles, stay out of this, you don't have a clue as to what's being discussed <some snippage of well-put JAX baiting> >All in all, to date, this is the best system to come down the pike. Can't disagree with that, but I haven't thrown out my pelorus, either <G>. By the same logic, I have a depth finder AND a lead line. The lead line can retrieve bottom material, the nature of which is frequently charted. I've used it exactly once, but it's another nav aid. it's accuracy >depends entirely upon the skill of the navigator and can vary from >"spot-on" to "oops, wrong continent". Yep...like the violin it requires practice. Will it's accuracy be comparable? Rarely, but >that accuracy is not needed in the offshore/nearshore conditions that >celestial is used for. Unless you are taking a range or figuring out the distance to shore by sighting a charted tower or mountaintop, but I basically agree...it's an offshore skill. but if you are going to go >more than that, I would seriously consider having the knowledge and >capability to perform some basic celestial navigation. That's the plan. >Trying to compare GPS to Celestial, is an exercise in stupidity. Saying >that celestial is only good for "lifeboat navigation" is equally stupid. >Both systems have their plusses and minuses, both systems have their >drawbacks ..... the SMART sailor knows these and makes use of these and >has the ability to use whatever is available, when the sh.t hits the fan. Couldn't agree more. So far, no opinions on the Astra IIIB itself, however. I've seen the Davis, and while adequate, I want the accuracy of a metal sextant to aid my learning curve. R. >Couldn't agree more. So far, no opinions on the Astra IIIB itself, >however. I've seen the Davis, and while adequate, I want the accuracy >of a metal sextant to aid my learning curve. I bought an Astra direct from China the year they hit the market. It was always more accurate than the person using it, though you could likely say the same about the Davis. The reservations I have about the Astra concern areas that I would bet have been addressed long since. For instance, it was a handmade instrument in the truest sense of the word. The small bolts (icluding the ones for adjusting the mirrors) were actually hand made with roughlymore or less square heads. They were not standardized, so a wrench was included that was sized specifically for the heads on that sextant. Obviously, spare parts were not going to be available. It didn't have the satisfying "feel" you get from a top of the line instrument, but it certainly served me well over the years. I glanced in passing at one at the boat show this month. It looked nicer than mine, but I didn't pick it up for a closer look. I'd recommend Celestaire as a vendor. ____________________________________________________________ Glen "Wiley" Wilson <usenet1 SPAMNIX at world wide wiley dot com> To reply, lose the capitals and do the obvious. Take a look at cpRepeater, my NMEA data integrator, repeater, and logger at http://www.worldwidewiley.com/ Thanks for these comments. I am aware it's not the best out there, but I suspect it's better than plastic. People seem to be holding on to the better makes, to judge by Ebay. R. >>Couldn't agree more. So far, no opinions on the Astra IIIB itself, >>however. I've seen the Davis, and while adequate, I want the accuracy [quoted text clipped - 23 lines] >Take a look at cpRepeater, my NMEA data integrator, repeater, and >logger at http://www.worldwidewiley.com/ At the end of my celestial class we did beach shots off Malibu. We used something like a dozen different instruments about half Davis and the rest up to very expensive German Brass. There was no correlation of the accuracy of the resultant fixes with the instrument used. The conclusion of the group was that a fresh setup Davis was a good as the more expensive alternatives. Note that at least three of the shooters were experts. They did much better than the rest of the group...but with any instrument. The most interesting question is whether the bulk of the all metal instruments helps in weather on the deck of a small boat. Our instructor, an ex Navy and Annapolis instructior was not secure in an answer. The Davis is clearly easier to handle while hanging from rigging but lacks the inertia of the heavier metal instruments. My active experience has all been with the Davis so I can't really offer an opinion. I don't however think that GPS backup is a likely candidate for the much more expensive instruments. Hobby however certainly is. About a year ago I did an informal survey of about 25 boats crossing the Pacific. Virtually all had a sextant on board. None had actually made a shot in the last six months. You want to guess what kind of star shots you get from a guy who has not done one in 6 months? So as far as long range cruisers are concerned the answer is in.... Jim Donohue > <some snippage of well-put JAX baiting> > [quoted text clipped - 37 lines] > > R. >At the end of my celestial class we did beach shots off Malibu. We used >something like a dozen different instruments about half Davis and the rest [quoted text clipped - 10 lines] >inertia of the heavier metal instruments. My active experience has all been >with the Davis so I can't really offer an opinion. I have used both Davis and brass, but not aluminum. To me, the index drift of the plastic is the principal practical objection, and is mainly a problem if the sextant is heated or cooled significantly during a series of sights. The lower heat conductivity of the plastic means it can get different temperatures in different parts of the instrument, which distorts it. The inertia of a brass sextant is its least endearing quality when bouncing around on a small boat, IMHO. In gentler seas, it may help one to hold it steady. Rodney Myrvaagnes NYC J36 Gjo/a "WooWooism lives" Anon grafitto on the base of the Cuttyhunk breakwater light >I have used both Davis and brass, but not aluminum. To me, the index >drift of the plastic is the principal practical objection, and is >mainly a problem if the sextant is heated or cooled significantly >during a series of sights. The lower heat conductivity of the plastic >means it can get different temperatures in different parts of the >instrument, which distorts it. This is my principal objection as well, and as I plan higher latitude cruising as well as the usual tropical routes, it could be an issue should I make a plastic Davis the "in-house" model. >The inertia of a brass sextant is its least endearing quality when >bouncing around on a small boat, IMHO. In gentler seas, it may help >one to hold it steady. That's what I'm hearing...thank you. R. The answer is in....What? Perhaps you need Jax to help you out! >The answer is in....What? Perhaps you need Jax to help you out! here ya go. celest nav has gone the way farm horses pulling plows. >here ya go. celest nav has gone the way farm horses pulling plows. Or like Speedos on aging blowhards? Up the furrow? Comments interspersed: > At the end of my celestial class we did beach shots off Malibu. We used > something like a dozen different instruments about half Davis and the rest [quoted text clipped - 3 lines] > alternatives. Note that at least three of the shooters were experts. They > did much better than the rest of the group...but with any instrument. Take an individual skilled with a sextant, put him/her on a firm beach and give them a number of different sextants (adjusted) and their sights should all be reasonably close. The main point .... the individual may not like the particular sextant, but because of their skill, they will easily compensate for the shortcomings (to them) of the particular instrument. > The most interesting question is whether the bulk of the all metal > instruments helps in weather on the deck of a small boat. Our instructor, > an ex Navy and Annapolis instructior was not secure in an answer. The > Davis is clearly easier to handle while hanging from rigging but lacks the > inertia of the heavier metal instruments. My active experience has all been > with the Davis so I can't really offer an opinion. The same answer does not fit all individuals at all times. Some prefer a lighter sextant to a heavier one. Having played with some lightweight metal sextants at sea and hefted some plastic ashore, my personal preference is the weight of my Plath (my sense is that because of it's weight, it's less jerky) Generally, what you have is what you get used to and what you end up preferring. > I don't however think that GPS backup is a likely candidate for the much > more expensive instruments. Hobby however certainly is. As stated previously, would agree (if I read this correctly). No need to spend a great sum on something which will be rarely used, as long as what you buy is not total junk. > About a year ago I did an informal survey of about 25 boats crossing the > Pacific. Virtually all > had a sextant on board. None had actually made a shot in the last six > months. You want to guess what kind of star shots you get from a guy who > has not done one in 6 months? I'm still getting reports of ships requiring regular celestial practice because of "oops" related to GPS. As for accuracy, I haven't done a celestial fix in about 15 years, but my bet is that my sight accuracy would still be quite close .... the problem would be in doing the calculations, correctly. Back in the day, I took about a 6 year hiatus. When I got back into celestial, my accuracy was spot on after the first day. > So as far as long range cruisers are concerned the answer is in.... .....the individual otn >I'm still getting reports of ships requiring regular celestial practice >because of "oops" related to GPS. Conjectural "oops" or actual "oops?" Rodney Myrvaagnes NYC J36 Gjo/a "WooWooism lives" Anon grafitto on the base of the Cuttyhunk breakwater light "oops" in the sense of lost/no signals (for whatever reason) and/or equipment failure (two points on equipment failure 1. I know of no ship using a single GPS receiver, so I find this puzzling. 2. These units run 24/7, 52 weeks a year). >>I'm still getting reports of ships requiring regular celestial practice >>because of "oops" related to GPS. [quoted text clipped - 4 lines] > > "WooWooism lives" Anon grafitto on the base of the Cuttyhunk breakwater light I've had a JRC GPS and an expensive Ashtech 3D GPS die recently. The JRC was due to water ingres at the cable junction and we fixed it. The Ashtech, dunno, it recovered all by itself so one suspects a firmware problem of some sort. Since we had another 3 GPS units aboard it didn't matter except for the 50m offset to the fwd unit when the aft one died. PDW > "oops" in the sense of lost/no signals (for whatever reason) and/or > equipment failure (two points on equipment failure 1. I know of no ship [quoted text clipped - 9 lines] > > > > "WooWooism lives" Anon grafitto on the base of the Cuttyhunk breakwater light Interesting comment on the Davis vs. real life. Maybe I should buy a Davis to learn on and then make a call when ready to sail off. >At the end of my celestial class we did beach shots off Malibu. We used >something like a dozen different instruments about half Davis and the rest [quoted text clipped - 13 lines] >I don't however think that GPS backup is a likely candidate for the much >more expensive instruments. Hobby however certainly is. Well, more like knowing St. John's Ambulance is a "hobby", I suppose: assuming you never have to use it, it's still good to know how to splint a limb. >About a year ago I did an informal survey of about 25 boats crossing the >Pacific. Virtually all >had a sextant on board. None had actually made a shot in the last six >months. You want to guess what kind of star shots you get from a guy who >has not done one in 6 months? I would guess a crappy one. The idea would be to use the thing regularly---otherwise it's semi-pointless--and the rationale is the mental discipline, the sense of getting a further "feel" for navigation I find is much less looking at an LCD, and to improve one's technique. As I still have an interest in astronomy, I would like to try star sights, as well. >So as far as long range cruisers are concerned the answer is in.... Hmm...I wish when they swallow the anchor they would discount them to deserving younger sailors, but that doesn't seem to be the case! R. > Interesting comment on the Davis vs. real life. Maybe I should buy a > Davis to learn on and then make a call when ready to sail off. That is how I started out. Used the Davis 25 for about a year to practice and figure out if I really wanted to learn the arcane art. Then I graduated to the Astra IIIB. The Astra made the experinece much more enjoyable and the results a good bit more accurate. Celestial is becoming an obsolete art but so is sailing. We do it for personal satisfaction more than practical use and it definitely gets you closer to your environment. You get a real feeling of fulfillment when you get a fix using the natural world that is within a few seconds of what the GPS is saying. The key is to take sites often and reduce them manually. You need to build up your speed and get where you can handle the reductions automatically. You will never build up the understanding with a computer and if you don't have the batteries to keep the GPS running you probably don't have enough for the computer either.  SignatureGlenn Ashmore I'm building a 45' cutter in strip/composite. Watch my progress (or lack there of) at: http://www.rutuonline.com Shameless Commercial Division: http://www.spade-anchor-us.com >Celestial is becoming an obsolete art but so is sailing. An excellent point. We do it for >personal satisfaction more than practical use and it definitely gets you >closer to your environment. It's been described as the slowest, most expensive way of getting somewhere while endangering one's life, excepting that it's the best way to travel ever invented <G> You get a real feeling of fulfillment when you >get a fix using the natural world that is within a few seconds of what the >GPS is saying. I believe sailing can lead to a greater integration with the natural world. Not to go all Moitessier on people, but getting a fix from stars, moon and sun is part of that. Getting power from wind and sun and towed generators is another aspect of "sailing lightly over the earth". The jury's still out on Solomon Technologies and that fuel-cell boat engine firm, but that's a different thread. R. >> Interesting comment on the Davis vs. real life. Maybe I should buy a >> Davis to learn on and then make a call when ready to sail off. [quoted text clipped - 15 lines] >have the batteries to keep the GPS running you probably don't have enough >for the computer either. I disagree on building understanding. On a trip south of Nova Scotia about 20 years ago my wife decided to try celestial. We were on a friends boat and he had a Davis. She studied the algorithm in the back of the Nautical Almanac, and programmed it into a Lotus 123 spreadsheet. She already understood the reduction process before she ever took a sight. Then she went out and tried the sextant for the first time, sun sights on relatively smooth seas. All shots were within 2 miles. If you just follow the steps in a table-based reduction, you could do CN for years without ever understanding what you were doing. Rodney Myrvaagnes NYC J36 Gjo/a "WooWooism lives" Anon grafitto on the base of the Cuttyhunk breakwater light > <snip> > [quoted text clipped - 3 lines] > > R. Sorry, can't help you there. The majority of my sextant time is with a Plath. BTW, my Plath has been saltwater soaked more times than I care to remember. Take off the scope and dunk it in a bucket of fresh water (not always possible on small cruising boats, but concentrate small amounts of fresh on moving parts and mirrors)... it still looks like it did the day I bought it .... well a few scratches in the paint, but mechanicals are fine and mirrors are original (bought 1965). Two final points... 1. The biggest drawback to good accuracy on a small boat, is seeing a good horizon, not necessarily the motion (big ships are not as stable a platform as some would think). 2. You don't need perfect conditions for a sight. On many cloudy days you can take a sunline through the clouds or stars/planets through openings .... they don't always work out, but as your skill level increases, the times they do, improves. otn >Sorry, can't help you there. The majority of my sextant time is with a >Plath. Lucky you. Seems to be the sextant to beat. >Two final points... >1. The biggest drawback to good accuracy on a small boat, is seeing a >good horizon, not necessarily the motion (big ships are not as stable a >platform as some would think). I've spent more time in salt water on big ships than small, and I agree entirely. >2. You don't need perfect conditions for a sight. On many cloudy days >you can take a sunline through the clouds or stars/planets through >openings .... they don't always work out, but as your skill level >increases, the times they do, improves. That's why basic, but accurate...or should I say consistent...is what I'm looking for. Maybe if I start hitting garage sales of old sailors... R. > Maybe if I start hitting garage sales of old sailors... > > R. <BG> You may get lucky, but unlikely. Mine will become a "family heirloom" BTW, I got mine in Panama for the outrageously high price (at the time) of $195.00 . otn > Couldn't agree more. So far, no opinions on the Astra IIIB itself, > however. I've seen the Davis, and while adequate, I want the accuracy > of a metal sextant to aid my learning curve. Can someone explain why a metal sextant would inherently be more accurate than a plastic one? I saw one mention of thermal expansion/contraction, but CoE for plastic ranges all over the place, from considerably less than metal to considerably more. Or is it simply that the specific models/brands available in plastic are lower-end, lower-accuracy devices? Thanks! KeS >> Couldn't agree more. So far, no opinions on the Astra IIIB itself, >> however. I've seen the Davis, and while adequate, I want the accuracy [quoted text clipped - 4 lines] >expansion/contraction, but CoE for plastic ranges all over the place, >from considerably less than metal to considerably more. It is the thermal conductivity. The metal ones distribute heat more uniformly and quickly.Thus their geometry stays about the same as they expand and contract. Most evident when the sun is shining on it. Rodney Myrvaagnes NYC J36 Gjo/a "WooWooism lives" Anon grafitto on the base of the Cuttyhunk breakwater light > >Can someone explain why a metal sextant would inherently be more > >accurate than a plastic one? I saw one mention of thermal [quoted text clipped - 4 lines] > uniformly and quickly.Thus their geometry stays about the same as they > expand and contract. Most evident when the sun is shining on it. Thanks much. KeS >> Couldn't agree more. So far, no opinions on the Astra IIIB itself, >> however. I've seen the Davis, and while adequate, I want the accuracy [quoted text clipped - 11 lines] > >KeS I';d like to hear about those low expansion plastics. Which ones are you thinking of? Brian W > I';d like to hear about those low expansion plastics. > Which ones are you thinking of? > > Brian W http://www.edl-inc.com/Plastic%20expansion%20rates.htm http://www.engineeringtoolbox.com/34_95.html KeS >> I';d like to hear about those low expansion plastics. >> Which ones are you thinking of? [quoted text clipped - 6 lines] > >KeS These references show brass 10.4 bronze 10 ppmDegF and the pure plastics at 20+ to 140 ppmDegF [The second reference shows epoxy at 10 ppmdegF It is usually shown as 20+ by the way, so this might represent glass-filled too] It is quite true that filling any plastic with glass fiber (at 5 ppm degF) pulls those high plastic expansivities well down. With enough glass filler they compete with the metals. But only then. Still, if you crumble marble chips into epoxy resin you can come up with respectable expansivity numbers at home too! Brian W > >> I';d like to hear about those low expansion plastics. > >> Which ones are you thinking of? [quoted text clipped - 22 lines] > > Brian W I'm not sure who you're arguing with. I just asked why plastic sextants were considered more subject to expansion errors. I got an immediate, helpful answer, said thanks, and am done. I answered your question to be polite, but don't really care to debate the issue. KeS >I'm not sure who you're arguing with. I just asked why plastic sextants >were considered more subject to expansion errors. I got an immediate, >helpful answer, said thanks, and am done. I answered your question to >be polite, but don't really care to debate the issue. > >KeS Which reminds me - in feedback from newsgroup users from earlier years, I recall that there was often, on balance, a somewhat 'depressing' reaction to group participation. This was both because there is always someone who knows more, sometimes much more than one does on some given subject, but also because there is usually a specimen of the lunatic fringe at hand, who would argue the toss relentlessly, regardless of the merits of one's position. I see examples of both types here, but really, the joint interest keeps this group quite cohesive in my view. It helps to remember this is only a newsgroup where people hopefully with compatible interests chatter. One's manhood is not involved.... Brian W >there is usually a specimen of the lunatic fringe at hand, who >would argue the toss relentlessly, regardless of the merits of one's >position. you got that right. over-the-knee, shen, old thom, rhys sees monkey, dougies, jeffies and other come to mind. not a single coherent thought among em. they laugh hysterically telling unsupecting newbies how to hit the rocks midnight in a flat calm. The problem with accuracy is that you'll never get a nice steady sight from a small boat. With all the pitching and rolling, even on a calm sea, you'll be lucky to get a close approximation. The additional precision of a fancy sextant is lost on pretty much anything smaller than a carrier or large ship. SignatureKeith __ "If you get to thinking you're a person of some influence, try ordering somebody else's dog around." - Will Rogers > >> Couldn't agree more. So far, no opinions on the Astra IIIB itself, [quoted text clipped - 12 lines] > > KeS >would you rather have a functional >celestial system or nothing at all? > >Steve there is not an iota of difference when the sky is cloudy. On a recent trip, a crew who wanted to practise his celestial had to wait 5 days for his first sights. >>would you rather have a functional >>celestial system or nothing at all? > >there is not an iota of difference when the sky is cloudy. On a recent trip, a >crew who wanted to practise his celestial had to wait 5 days for his first >sights. It's nice and sunny here right now. Steve as it was on the sixth day, seven hundred miles later. >>>would you rather have a functional >>>celestial system or nothing at all? [quoted text clipped - 7 lines] > >Steve >>Decide what you are buying it for. I am buying it to develop the skill of celestial navigation. The cheapest Davis is sufficient in my >>opinion for any likely real world situation. That's what my father-in-law says. I think a Davis plastic model is a great addition to the "go bag" for the liferaft, and probably a lot cheaper than a submersible GPS. If you want to be a hobbyist >>than decide on your budget and go from there. Budget of $500 U.S. <G>...depending on the exchange rate. >>Celestial is not remotely a backup to GPS. At best it is a life boat skill. Can't agree there. I would say that GPS is a back-up to celestial in cases of reasonably good sight opportunities while offshore. Consider that GPS can be turned off, while the sky...so far...cannot. I would use GPS naturally as both a back-up and to confirm times, lat/lon, etc., but I believe it's foolish to rely exclusively on one form of nav aid when others are available. Have you ceased to consult your compass because you have that function on a GPS? >Lol, there you go again. Consider this: If you didn't have a >functional gps for whatever reason, would you rather have a functional >celestial system or nothing at all? Exactly. R. >I would say that GPS is a back-up to celestial in >cases of reasonably good sight opportunities while offshore. gps time to show location: immediate celest time to show location: upwards of an hour gps accuracy: a few feet celest accuracy: 2,000 to 20,000 times that uptime for gps: typically 3,600 seconds/hour, 24 hours/day, 365 days/year uptime for celest: maybe 50% longest known time for gps down: a day, maybe longest know time for celest down: 40 days and 40 nights Now Doodles, I TOLD you to stay out of this discussion because you didn't have a clue .... your second Doodlism is a prime example. otn >>I would say that GPS is a back-up to celestial in >>cases of reasonably good sight opportunities while offshore. [quoted text clipped - 16 lines] > > you seem confused again by all those big words, over the knee. drunk so early in the evening? >From: otnmbrd otnmbrd@earthlink.net >Date: 11/12/2004 7:56 PM Eastern Standard Time [quoted text clipped - 25 lines] >> >> Now, now, Doodles. What part didn't YOU understand? Second Doodlism: "celest time to show location: upwards of an hour" A star sight (3 stars) from the time you start taking sights to final plot ... 10-15 minutes .... in the middle of the Pacific, making 6k .... whoopdee Doo, Doodles. Doodles, take some advice. Stay out of discussions on navigation, charts, boat handling, Rules of the Road, except to ask questions. otn > you seem confused again by all those big words, over the knee. drunk so early > in the evening? [quoted text clipped - 26 lines] >>> >>>longest know time for celest down: 40 days and 40 nights just like to over-the-knee to forget the details. such as doing the paperwork. dumb squat thinks *taking* the sight ______ gives _____ you the position. yuk. >From: otnmbrd otnmbrd@earthlink.net >Date: 11/12/2004 8:42 PM Eastern Standard Time [quoted text clipped - 41 lines] >>>> >>>>longest know time for celest down: 40 days and 40 nights ROFLMAO Which is it, Doodles "reading comprehension problem" or "meds need adjustment" Read .....time you start taking sights to final plot...10-15 minutes. ROFLMAO Unlike you, Doodles I know what's involved intaking and PLOTTING are star sight. Once again, Doodles, stay out of the discussion .... it's WAY beyond your meager levels of intelligence and understanding. otn ROFLMAO !!!!!!!!!!!!!!!!!!!!!!!!! > just like to over-the-knee to forget the details. such as doing the paperwork. > [quoted text clipped - 15 lines] >> >>otn over-the-knee, you are drunk. go sleep it off and report back in a couple three days. >From: otnmbrd otnmbrd@earthlink.net >Date: 11/12/2004 10:11 PM Eastern Standard Time [quoted text clipped - 31 lines] >>> >>>otn dumb squat compares fractions of a hour to fractions of a micro-second and doesn't notice the difference. Maybe over-the-knee has been spending too much time in whore houses the last several years with his lady love(s)? >From: otnmbrd otnmbrd@earthlink.net >Date: 11/12/2004 8:42 PM Eastern Standard Time [quoted text clipped - 41 lines] >>>> >>>>longest know time for celest down: 40 days and 40 nights > dumb squat compares fractions of a hour to fractions of a micro-second and > doesn't notice the difference. > > Maybe over-the-knee has been spending too much time in whore houses the last > several years with his lady love(s)? Nother stupid post, Doodles otn only to one who doesn't know the difference between hours and micro-seconds. >From: otnmbrd otnmbrd@earthlink.net >Date: 11/12/2004 10:46 PM Eastern Standard Time [quoted text clipped - 10 lines] > >otn 1. Never used the full horizon set-up, and my understanding is that it helps with sights from a platform with a bouncing or jerky motion. 2. Unless you can find a good used Plath, etc., I wouldn't spend the money for something which may or may not get used when the "cheaper" version will get the job done, if needed ..... unless, of course, money is no object. >To any celestial navigators still out there: > [quoted text clipped - 19 lines] >world (I have two handhelds now, will likely add a chartplotter to the >bluewater cruiser...) For getting fixes on a sailboat, a plastic sextant like Davis Mk 10 will get you sun sights within 2 miles if the seas aren't too bad, and you won't do much better with a better sextant. But, the metal sextant won't keep changing its index error as it absorbs or loses heat, so if you are taking a twilight round of star and planet sights in limited time, you may be able to take more sights with the metal one. And, there is a distinct esthetic pleasure in using a good sextant. The Astra has a good rep. I don't know if it is as good as my Kelvin Hughes, which itself is not a C Plath, but mine is a lot more fun to use than the Davis. As for learning practical mathematics: Just following steps to do sight reduction from tables doesn't teach anything about math, or the underlying spherical geometry. You can make lessons out of it if you go through the geometry, and work out what you are actually doing when you take a sight. If it is only a backup in case all the GPS sats in view fail at once, a $25 lifeboat sextant is plenty. Rodney Myrvaagnes NYC J36 Gjo/a "WooWooism lives" Anon grafitto on the base of the Cuttyhunk breakwater light Celestial is definitely in the "hobby" category. Its is rather unlikely that you would ever need celestial because of GPS failure. Consider: an electrical disaster that wipes out all of your GPS's will also wipe out your accurate clocks, such as your GPS and radio. My advice: get a Davis and learn the art. If you enjoy it, invest in a better sextant. If it doesn't do much for you, you'll still have a good functioning sextant that might help you in a lifeboat situation. One problem with a high quality sextant is that you won't want to bring it out in anything other than perfect conditions. A Davis you would be willing to bring in adverse situations where you really need the practice. You might also use the Davis to practice piloting techniques for measuring distance off shore. Actually, before GPS, I used a sextant a lot for horizontal angles - that's a skill you might actually use in your travels. BTW, I'm not sure how much a sextant would help you in a liferaft. Are you planning on having a sail? My father-in-law actually spent 18 days in a lifeboat, having been on the Oklahoma when she was torpedoed (for the second time) at the end of WWII. They did bring a sextant, charts, and a good watch when they abandoned ship, but the emergency radio fell overboard and sank! They were able to sail to Trinidad, but being afraid to make a night entrance they stood off. In the morning they found they had drifted past the harbor and ended up going several more days to Aruba. > To any celestial navigators still out there: > [quoted text clipped - 22 lines] > Thanks, > R. >My advice: get a Davis and learn the art. If you enjoy it, invest in a >better sextant. If it doesn't do much for you, you'll still have a good >functioning sextant that might help you in a lifeboat situation. Thanks for the advice. >One problem with a high quality sextant is that you won't want to bring >it out in anything other than perfect conditions. A Davis you would be [quoted text clipped - 3 lines] >sextant a lot for horizontal angles - that's a skill you might actually >use in your travels. I use it now after a fashion with a watch and a pelorus to take running fixes from shore marks, which are plentiful and clearly charted in my part of the world. OF COURSE, I can (and do) use the GPS, but I enjoy figuring out to a reasonable point of accuracy these things by other, non-electronic means. >BTW, I'm not sure how much a sextant would help you in a liferaft. Are >you planning on having a sail? Haven't gotten that far yet, but probably would have a liftraft for super-ugly conditions and a Portabote with sail rig if we merely sink <G> My father-in-law actually spent 18 days >in a lifeboat, having been on the Oklahoma when she was torpedoed (for >the second time) at the end of WWII. They did bring a sextant, charts, [quoted text clipped - 3 lines] >found they had drifted past the harbor and ended up going several more >days to Aruba. Heck of a story. Thanks. R. The Astra is a very good sextant at a very good price. Its inherent accuracy is better than you can actually achieve on a small boat, so you aren't giving anything away by using a $500 Astra versus a $1000+ Plath. Any metal sextant is more stable, and more inherently accurate, than a plastic one - the plastic ones tend to be temperature-sensitive if you let them heat up in the sun. I think the Astra is right in the 'sweet spot' for an occasionally-used navigation tool that fits into the backup or hobby category. At around $500 new (as you say), it's probably a better deal than, say, a used Tamaya at the same price, since you don't know whether the used one has been abused or is significantly out of adjustment. Checking and adjusting a sextant is kind of a specialized skill, and while it's nice to know how to do it and to have the tools, it'd be nicer to be able to trust the sextant right out of the box. These days it's quite difficult to fin |
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