Island Time 1,225 Posted February 11, 2015 Share Posted February 11, 2015 The phones will work as well. The GPS chip inside "talks" to the satellite (ok, well listens then), nothing to do with the cellular service. Some of the phones/tablets don't have a GPS chip, just rely on locations from the cellular towers - those ones won't work. Check you model specs... Kevin, a sextant is no use without tables. IMO, my 5 or more GPS units, Batteries (3 banks in boat, + removable/replaceable batts in the handheld GPS), solar power, wind power etc are VERY unlikely to be all gone at the same time. 3 of the GPS units are portable, and could be taken into the liferaft. All 3 are ip67 waterproof. Switched on to get a fix once or twice per day would give months of positions. I do carry a sextant and tables, but I'm rusty, and I'm sure the positions form this would be pretty bad to begin with! The sextant can get broken/dropped etc, and the tables can get wet/unusable as well. Nothing is 100% foolproof. Going to sea is a risk. So is being alive. Grant, sounds like you did the old MOT course that I did. Did you know that Coastguard have no record of these certs? At least not of mine.... Link to post Share on other sites
DrWatson 374 Posted February 11, 2015 Share Posted February 11, 2015 I was taught a neat method to estimate angles above the horizon using an outstretched arm and a handwidth, then fingers... Now you're not going to plot a position to mile accuracy, but you might get some idea of where you are as long as you still have a wrist watch, and some idea of the date. A surprising amount can be deduced from a compass, a wrist watch, and a guestimate of the angle. Link to post Share on other sites
Island Time 1,225 Posted February 11, 2015 Share Posted February 11, 2015 OK, here is another solution - there is an app for that!! Turn your phone into a sextant! If you have an android phone, https://play.google.com/store/apps/details?id=com.polyglotz.starstruck&hl=en it works surprisingly well! There may be a Iphone/Ipad app as well, I have not looked. Also, if you use opencpn, download and install the celestial nav plugin. Then using your phone (as above) or your sextant, take the sight, key the results into opencpn, and you position lines appear on the chart. No tables, no maths. Simple, and very useable... Link to post Share on other sites
grantmc 59 Posted February 11, 2015 Share Posted February 11, 2015 Grant, sounds like you did the old MOT course that I did. Did you know that Coastguard have no record of these certs? At least not of mine.... No IT I did my course just a few years ago. I used the home study course still advertised. Here is the current syllabus straight off the CBES web site: 1. Ocean Navigation The earth & the celestial sphere The PZX triangle The sextant Measurement of time Meridian altitudes Sun, star, & other sights Compass checking Satellite navigation systems Great circle sailing Meteorology Passage planning Communications 2. Oral Testing Sextant use & errors Seamanship & safety Stability Heavy weather handling Passage making Master's responsibilities Legal knowledge Just to add I also agree that much more than a sextant is needed to get a reasonably decent position fix. Any of which is subject to the risks associated with being on a boat. But then nothing is perfect. A few years ago a mate of mine was crossing an ocean and they were hit by lightning. Everything electrical fried. Sometimes sh*t just happens. We try to prepare as best we can. We do courses and train and practice. We buy heaps and heaps of stuff that we never actually want to use: boat epirb, personal epirbs, raft, spare gps, ais, handheld vhf, pyrotechnic and electronic flares. Even a cat 1 certificate is basically just mind insurance, it wont save you. Johny Wray's Ngataki wouldn't even get cat 5 today. Hell I don't ever even want to test my life jacket in anger. I can't help too but add I've been surprised how much discussion that has resulted from this topic. It's awesome to see people jotting down their thoughts. Too often all that we see are 'lookers'. Link to post Share on other sites
Island Time 1,225 Posted February 11, 2015 Share Posted February 11, 2015 Sorry GrantMc, I meant Grant KMM!! I guess KMM and I are just old Link to post Share on other sites
grantmc 59 Posted February 11, 2015 Share Posted February 11, 2015 Sorry GrantMc, I meant Grant KMM!! I guess KMM and I are just old Hey I'll take that as a compliment. Since the birth of my first grand kid I've always felt I was far too young to be a grandad. Link to post Share on other sites
Kevin McCready 83 Posted February 11, 2015 Author Share Posted February 11, 2015 Hoping to get an idea from the granddads and other old salts with more experience than me, how many days experience and practice did it take you to take decent sightings on the bouncy waves? Link to post Share on other sites
Island Time 1,225 Posted February 11, 2015 Share Posted February 11, 2015 Not sure about the others, but I always thought I was doing well if I got within 5 NM of my actual position with a sextant.... I bet others are better at it than me!! I've only done 20-30 sights at sea, GPS took over quite a while ago now! Lightning - The Malacca Strait has among the highest rate of strikes of anywhere in the world. We had several very frightening storms, and once had a simultaneous strike each side of the boat, within 20m. The whole boat shook, seemed to me to lift several inches and then fall. ^&*#( scary!! Fortunately no damage at all to us, but several other boats were hit. We do have basic lightning protection installed, I always wondered if it made a difference. Lightning is weird, and there seems to be no proven fail safe system to protect yourself. This could/should?? be another topic if people want to talk about it? Link to post Share on other sites
MuzzaB 14 Posted February 12, 2015 Share Posted February 12, 2015 In answer to the OP's question... I can't recall, because it has been so long since I used my sextants, but I do recall the learning process well. I remember starting out by standing on a beach with a clear horizon, then standing on the boat at anchor with a clear horizon. Remember than land on the horizon is OK for these practice shots - it just needs to start below the horizon. Then I started taking shots during coastal cruises in calm water and in less settled water. Each time I worked the shot to get practice. I tried different positions on the boat and different points of sail with different types of motion. It doesn't matter if you cannot get two sights for a fix - it is just practice. Eventually I was taking sights with confidence. This was probably over the course of about a year - or at least one summer seasons. And I remember the first time I was navigator offshore (no GPS back in those days). I was absolutely amazed when on the 5th day land appeared where it was supposed to be, and even more amazed to find as we got closer and could identify the land, that were were pretty much just where we were supposed to me. The previous 48 hours had been rough and the sights shaky through gaps in the cloud. I think the crew was more amazed than me. Before long, sun sights are old hat. Star sights were worth pursuing. Ah - those were the days. I'd still always take a sextant - but I'd also have multiple GPS receivers. Link to post Share on other sites
David T 0 Posted February 12, 2015 Share Posted February 12, 2015 Navigational accuracy when using a sextant requires several things; skill in using the sextant to take a sight, the sextant itself being relatively free of error (in terms of physical features), an accurate time when taking a sight, and correct calculations (e.g. using sight reduction tables) to plot a position line from a DR position. In my experience all of these are important. To check the sextant, you could get several readings from a known position in calm water (or on land) to check the sextant accuracy. You need to be able to see the part of the sea horizon closest to the sun to do this. Work out the readings from these sights. One sextant I used had a constant error which put me about 20nm closer to the sun than I actually was. Once I knew this my sights were more accurate. Using morning, noon and afternoon sights allows triangulation which is more accurate than a single sight. Remember to adjust the position lines for the vessel's distance travelled between sights. Use of the sextant itself requires a bit of skill, especially if your vessel is bouncing around. I used to brace myself across the hatchway entry so the upper body can stay more or less upright. Swing (rotate) the sextant back on forth slightly to check you get the smallest angle between the sun and the horizon when taking a a sight and then record this angle and the exact time. Taking a sight with a sextant is one of those skills that is probably becoming much less common over the years as GPS systems replace manual methods. Hope this helps. Link to post Share on other sites
Kevin McCready 83 Posted February 12, 2015 Author Share Posted February 12, 2015 Wow, awesome DavidT, Thanks so much. It puts me in mind that someone should invent a nice sextant training app. It would be linked to a GPS and tell you the angle you should expect and your margin of error in nm. Link to post Share on other sites
erice 732 Posted February 12, 2015 Share Posted February 12, 2015 for the iphone try astronav lite ... which will allow you to play with taking sun angles...hint instead of sighting into the sun without filters try just looking the other way and minimising the shadow + sight calc Link to post Share on other sites
rigger 47 Posted February 12, 2015 Share Posted February 12, 2015 Before long, sun sights are old hat. Star sights were worth pursuing. Ever tried a noon sight of Venus with a sun sight for a cross? Was amazed how easy it was to find Venus in daylight (pre worked) but hard to hold long enough to nail on the meridian. Only managed it a few times... Link to post Share on other sites
Kevin McCready 83 Posted February 12, 2015 Author Share Posted February 12, 2015 Hey Rigger. Sounds like fun. Am I misunderstanding though? Don't you bring it down to the horizon not meridian? Link to post Share on other sites
Black Panther 1,581 Posted February 13, 2015 Share Posted February 13, 2015 I used to get consistently inside 1 mile. The more you do the better you get. What about the sextant that guy invented that clips onto your glasses? I thought that was cool. Here Link to post Share on other sites
Island Time 1,225 Posted February 13, 2015 Share Posted February 13, 2015 BP, that thars good shootin! I bet it was not a plastic sextant like mine?? But then, your old as well, and had way more practice that me!! Link to post Share on other sites
Black Panther 1,581 Posted February 13, 2015 Share Posted February 13, 2015 Was that a compliment?? It was a Zeiss. For the person asking about adjusting it, you can get it very accurate with a good horizon and a star. Point it at the horizon set at zero and rock it, then try to minimize the amount the horizon "splits". Then do the same with a star, if it is zeroed accurately the two images won't separate. Also turn it uspide down so you can see the arc and the reflection of itself - same thing. Link to post Share on other sites
Island Time 1,225 Posted February 13, 2015 Share Posted February 13, 2015 of course bp how could it be anything else? Link to post Share on other sites
erice 732 Posted February 13, 2015 Share Posted February 13, 2015 so those of you who carry a sextant as a back up way of establishing position in the event of being unable to use gps do you also carry a log to trail behind the boat to establish distance covered for dead reckoning between sites when astro nav is blocked by clouds? http://www.trademe.co.nz/motors/boats-marine/parts-accessories/navigation-devices/auction-845703596.htm fair enough if you already have one but bothering to track 1 down and carry it seems a bit too much nowadays Link to post Share on other sites
Black Panther 1,581 Posted February 13, 2015 Share Posted February 13, 2015 Never had a log or a speedo or anything for many years. Each day around midday you go "hm, yesterday I was going fast in the afternoon, slowed down towards evening, then stopped for a bit, then it filled in around midnight and I've been doing quite well since - say 125 miles". Surprisingly accurate. Link to post Share on other sites
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