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  1. #61
    Join Date
    Apr 2003
    Location
    The Gareloch
    Posts
    3,802

    Default Re: Call yourself a navigator?'

    I took an interest in Astro at the start of Gulf War 1 “Shock and Awe”. I was on a jack-up rig, firmly stuck to the sea bed off southern Tunisia, when the GPS in the control room was convinced we were doing 17kts across the Libyan desert!
    Not having easy access to Almanacs and Sight Reduction Tables I developed my own Sun Almanac and tables.
    I have a plastic Davis mk5 plastic sextant, useful for practice; a 1939 Husun vernier sextant, a thing of beauty but hard to read; and a Freiberger drum sextant on board which I use to keep my hand in.

  2. #62
    Join Date
    Jun 2013
    Location
    Hopefully somewhere warm
    Posts
    9,394

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by vyv_cox View Post
    See post #10.
    Ta.
    So from a rather small sample size of on here, 1 incident over a few miles. Not too scared yet

  3. #63
    Join Date
    Feb 2006
    Location
    N Kent Coast
    Posts
    4,135

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by prv View Post
    Have you ever had your mobile phone die because of one? Do you know anyone who has? I don't.
    I don't have a smart phone, and don't know anyone who has been struck by lightening who carries one. However, there are some fabulous images on Google of phones that have, and it doesn't look like they worked afterwards.

    With some of the stories and reports of yachts struck by lightening, and recommendations of using ovens/tins for sensitive equipment, it is presumably not beyond the realms of possibility that a yacht could be rendered without electricity or electronics whilst at sea. If this is not the case then why do you and I put a backup in a tin? If your tinned GPS survived such an event you then come back to the problem of the ability to charge if everything else is fried.

  4. #64
    Join Date
    May 2007
    Posts
    37,288

    Default Re: Call yourself a navigator?'

    My back up GPS did a whole Atlantic crossing on one set of AAs, being fired up once a long enough day to get a fix.
    How many fixes do you need to get home? It's not hard to carry a few AA cells.
    I'd be more concerned about having some nav lights if charging became an issue.

  5. #65
    Join Date
    Feb 2007
    Location
    West London
    Posts
    2,052

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by AntarcticPilot View Post
    I can certainly do all the coastal navigation uses of a sextant; they are all pretty easy and in pre-GPS days were a very useful addition to the toolbox, being capable of greater precision than compass bearings. My favourite was distance off - after all, that's why charts carry the height of marks! I know the theory of celestial navigation, but have never had to use it in practice. However, my knowledge of spherical trigonometry is certainly adequate for the job, and the use of the sextant for coastal navigation means that I can handle a sextant.
    When doing distance off, do you take height of tide into consideration?

  6. #66
    Join Date
    Mar 2007
    Location
    Atlantic
    Posts
    20,449

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by prv View Post
    The expiry date for the pack is on my list, along with the equivalent pack in the grab-bag (for VHF, GPS, torches, and strobe), the food and water in the grab-bag, the main flares, the single flare in the dinghy, the lifejacket auto modules, and the lights on the lifejackets and Jon Buoy. So like everything on that list, the answer is "the winter before they expire".

    Checking the list, it seems the current pack is good to 2023. I must have replaced it a year or two ago, which will be why the box of technically-expired batteries in my study for non-critical items like TV remotes is particularly full at the moment.

    Since you ask

    Pete
    Very well organised. You would be surprised how many times Ive checked (along with the other billion things) before setting off to find all spare betteries on a boat are duff.

  7. #67
    Join Date
    Sep 2018
    Posts
    41

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by zoidberg View Post
    Worsley was the real hero on that trip IMHO...... but he wasn't a favourite of the King - just a 'tradesman'.
    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.

  8. #68
    Join Date
    May 2007
    Location
    Cambridge, UK
    Posts
    6,042

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by scotty123 View Post
    When doing distance off, do you take height of tide into consideration?
    I didn't, but probably should have for maximum accuracy. But in those days knowing your position to a cable was pretty good!

  9. #69
    Join Date
    May 2007
    Location
    Cambridge, UK
    Posts
    6,042

    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.
    The really clever bit was crossing the unknown interior of South Georgia to reach a whaling station on the north shore. Worsley's navigation was amazing, but that crossing was nothing short of miraculous.

  10. #70
    Join Date
    Nov 2016
    Posts
    2,460

    Default Re: Call yourself a navigator?'

    Quote Originally Posted by scotty123 View Post
    When doing distance off, do you take height of tide into consideration?
    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.

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