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  1. #71
    Join Date
    May 2007
    Posts
    38,093

    Default Re: Call yourself a navigator?'

    If you measure your sextant angle between the high tide line and the light, it'll do.
    Best to use objects that are tall enough that a bit of tidal uncertainty doesn't matter.

    If you're near a headland, the height of tide where you are could be quite different from Her Maj's tide gauge in port...

  2. #72
    Join Date
    Jun 2019
    Location
    Cowes
    Posts
    594

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by zoidberg View Post
    Yes.

    The charted height of a light is given from the centre of the lamp's glazing to MHWS. Should you measure the angle subtended when the tide is low, you're measuring too large an angle. Add the difference between MHWS and tide height to the charted height of the light, to enter the relevant table in Reeds Nautical Almanac or one of the other, traditional 'navigational epitomes' such as Burtons.

    Should you choose not to do this, the 'Range Off' extracted from the table will be less than reality, suggesting you are closer to the light than you really are.

    Some racing navs, wanting to pass a hazard outside a specific 'Distance Off' - such as the reefs off The Lizard - would set the extracted Sextant Angle on the Sextant for, say, 3nm as extracted from the table - WITHOUT making a correction for Height of Tide - and provided the Limiting Vertical Sextant Angle does not exceed the extracted preset angle, they are confident of passing the hazard outside of the critical range.

    A similar technique can be applied when seeking to pass INSIDE an off-lying hazard. In this case, the Limiting VSA must be exceeded to ensure you pass close enough to the light to clear the offshore hazard..... via an inner passage.
    Now that's the kind of wrinkle you cant get from a book, unless Leccy's which I'm now going to google.

  3. #73
    Join Date
    Jul 2002
    Location
    Farnham, Surrey
    Posts
    21,397

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by zoidberg View Post
    Here's a rhetorical question for you.....

    Have you any idea how to use a sextant - and usefully plot the result - to give you a Horizontal Sextant Angle, a Vertical Sextant Angle, a Distance Off..... It's my understanding all Merchant Navy Cadets had to use the 'works sextant' to navigate in the Coastal Trade - headland to headland - doing just that, with the added benefit of a Dumb Compass for relative bearings.

    Antarctic Pilot's contact should be able to confirm, as no doubt can Minn, that these basic, fundamental skills had to be mastered ( mistressed? ) before beginning to peek at Heavenly Bodies.

    It's not difficult. Anyone who can rub two brain cells together should be able to do it - provided, that is, they can 'be bovvered'.....


    ( And you have me pondering your use of 'GHA' as a monniker. To me, that 3-letter algorithm translates as 'Greenwich Hour Angle'. Do I scent a trollery? )
    Yep. Can do all the above and sometime do to alleviate the tedium of a boring passage.

    Quote Originally Posted by Frank Holden View Post
    At 38 seconds in the OP's vid..... why is that bloke looking at his watch after getting a MerAlt? Checking if its Gin'o'Clock yet?
    . Just comparing his predicted local noon from the DR to the one he’s just measured? For all we know he was counting red motor cars in his head from the time of his noon sight.
    Semper aliud

  4. #74
    Join Date
    Jul 2017
    Posts
    243

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by zoidberg View Post
    Yes.

    The charted height of a light is given from the centre of the lamp's glazing to MHWS. Should you measure the angle subtended when the tide is low, you're measuring too large an angle. Add the difference between MHWS and tide height to the charted height of the light, to enter the relevant table in Reeds Nautical Almanac or one of the other, traditional 'navigational epitomes' such as Burtons.

    Should you choose not to do this, the 'Range Off' extracted from the table will be less than reality, suggesting you are closer to the light than you really are.

    Some racing navs, wanting to pass a hazard outside a specific 'Distance Off' - such as the reefs off The Lizard - would set the extracted Sextant Angle on the Sextant for, say, 3nm as extracted from the table - WITHOUT making a correction for Height of Tide - and provided the Limiting Vertical Sextant Angle does not exceed the extracted preset angle, they are confident of passing the hazard outside of the critical range.

    A similar technique can be applied when seeking to pass INSIDE an off-lying hazard. In this case, the Limiting VSA must be exceeded to ensure you pass close enough to the light to clear the offshore hazard..... via an inner passage.
    Like the inside passage around portland Bill looking at the uppermost window in line with the point of the white obelisk.

  5. #75
    Join Date
    Dec 2008
    Location
    tayvallich
    Posts
    2,979

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by PHN View Post
    When you google "Records of the Canterbury Museum Volume 32 2018" you can download Frank Worsley's navigational log book of the James Caird voyage from Elephant Island to South Georgia with comments and recalculations. Totally awesome to read step by step how he managed to reach South Georgia.
    many thanks for posting - great link :-)

  6. #76
    Join Date
    Jul 2008
    Location
    East coast UK. Mostly. Sometimes the Philippines
    Posts
    9,751

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by Ohlin Karcher View Post
    Now that's the kind of wrinkle you cant get from a book, unless Leccy's which I'm now going to google.
    Lecky’s Wrinkles is indeed where you will find it; he invented the vertical sextant angle and wrote the tables.

  7. #77
    Join Date
    Sep 2008
    Posts
    2,458

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by GHA View Post
    Question is still interesting though, has it actually happened to anyone here - long enough to require a sextant or be lost?
    Before the days of GPS I never saw anyone do Astro for real on a boat. Decca seemed to be deeply unreliable.

    So I think coastal sailors managed just fine without GPS Astro.

    Offshore sailors like Joshua Slocumb got all the way around the world without working out accurate positions. I knew an old boy from the days of WW2 who said they regularly crossed the Atlantic without once getting any useful position due to cloud. So even offshore sailors could cope.

    Before the advent of smart phones but well into the GPS era myself and two friends did a short coastal trip (Hamble to Poole IIRC) without any electronics at all. I didn't show it, but I was quite concerned when the viz closed in a bit outside Poole - I'm so used to a a constant metre metre perfect position it felt weird not to know *precisely* where we were. In 1990 I wouldn't have given it a second thought, it would have been the norm, not the exception.

    My conclusion is people managed pretty well without Astro OR GPS and our obsession with a precise position (Astro or GPS) is just a modern symptom of being used to always having one. ETA arrivals of car journeys are similar IMHO. I used to just say "I'll be there in two or three hours." Now I say "Sat nav says 8:31pm" and it's usually scarily correct. I'd probably get a bit jumpy if I didn't have an estimate of ETA to minute resolution!

    Regarding spoofing. A problem on large shipping but not really on a small vessel because you'd have to spoof in such a way as to get the COG and SOG right or it will be obvious. So someone in real time needs to be generating positions that look credible. Worthwhile doing that to crash an oil tanker or warship but not a yacht. (Unless the yacht was being a bit irresponsible with their autopilot, but that's not a navigation issue.)
    Last edited by Mark-1; 14-08-19 at 08:18.

  8. #78
    Join Date
    Feb 2007
    Location
    West London
    Posts
    2,861

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by Mark-1 View Post
    Before the days of GPS I never saw anyone do Astro for real on a boat. Decca seemed to be deeply unreliable.

    So I think coastal sailors managed just fine without GPS Astro.

    Offshore sailors like Joshua Slocumb got all the way around the world without working out accurate positions. I knew an old boy from the days of WW2 who said they regularly crossed the Atlantic without once getting any useful position due to cloud. So even offshore sailors could cope.

    Before the advent of smart phones but well into the GPS era myself and two friends did a short coastal trip (Hamble to Poole IIRC) without any electronics at all. I didn't show it, but I was quite concerned when the viz closed in a bit outside Poole - I'm so used to a a constant metre metre perfect position it felt weird not to know *precisely* where we were. In 1990 I wouldn't have given it a second thought, it would have been the norm, not the exception.

    My conclusion is people managed pretty well without Astro OR GPS and our obsession with a precise position (Astro or GPS) is just a modern symptom of being used to always having one. ETA arrivals of car journeys are similar IMHO. I used to just say "I'll be there in two or three hours." Now I say "Sat nav says 8:31pm" and it's usually scarily correct. I'd probably get a bit jumpy if I didn't have an estimate of ETA to minute resolution!

    Regarding spoofing. A problem on large shipping but not really on a small vessel because you'd have to spoof in such a way as to get the COG and SOG right or it will be obvious. So someone in real time needs to be generating positions that look credible. Worthwhile doing that to crash an oil tanker or warship but not a yacht. (Unless the yacht was being a bit irresponsible with their autopilot, but that's not a navigation issue.)
    Wasn't it all 'aim for a landfall' (not a specific harbour), then turn port/starboard to get to your destination?

  9. #79
    Join Date
    Sep 2008
    Posts
    2,458

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by scotty123 View Post
    Wasn't it all 'aim for a landfall' (not a specific harbour), then turn port/starboard to get to your destination?
    Something that was remarked on the other day. Nobody ever says landfall any more! We're used to being on the button every time. Where's the fun in that.

    My recollection is there was always a clue that got you were you were going and on the rare occasions you missed everyone just shrugged and said "Ok, we'll go to Alderney instead of Cherbourg" or whatever.

    Plus the dirty little secret that light pollution often allows any major town to be identified 40 miles away in the Channel area and cloud helps that rather than hindering.

    I think the real days of adventure were when boats had tiny Auxiliary engines at best and no GPS or even Decca. Then you literally could fail to get anywhere near where you wanted to be. (I'm glad to say I don't recall those days, although with a small bilge keeler with a manky outboard I do exclusively choose my destination long after I've departed although I usually have somewhere I'm hoping to get to!) I also suspect people who had to get to work on Monday morning didn't make as many long weekend trips as those of us with 50hp inboards do in the modern world!
    Last edited by Mark-1; 14-08-19 at 09:05.

  10. #80
    Join Date
    Jul 2008
    Location
    East coast UK. Mostly. Sometimes the Philippines
    Posts
    9,751

    Default Re: Call yourself a navigator?'

    I most certainly have seen a small(ish) boat navigated accurately with a sextant, a deck watch, a Walker Excelsior log and a lead line, for several thousand miles including little known coastlines.

    https://comlay.net/tilman/voyages/1974-spitzbergen/

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