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  1. #1
    Join Date
    Jan 2009
    Posts
    354

    Default Sailing with gps...

    Over the years (only ten!) Ive tried to keep my nav skills up by chart work , cocked hats, lee way etc. I do enjoy it. However,last week we had to sail at nite using gps with way points. Two of us calculate the way points independently to spread the risk /or not take all the blame, which ever way you see it!! Anyway it was a very foggy 4 hr passage at nite . We were solely on gps...Last week the european gps system was turned off (with warning) which left me wondering or feeling a little more vulnerable on total reliance on gps. Even my cycle computer has dual Glonass (?) and gps! Do modern chart plotters have this? ta Nik

  2. #2
    Join Date
    Aug 2011
    Location
    On the Celtic Fringe
    Posts
    14,129

    Default Re: Sailing with gps...

    Yes they do. Good to hear others are keeping their traditional skills up to date.

  3. #3
    Join Date
    Sep 2001
    Location
    Home - Sothampton, Boat - Gosport
    Posts
    10,018

    Default Re: Sailing with gps...

    Before GPS, if the fog came in, your only possibility was dead reckoning. Assuming you kept a proper log, with regular notes of your position, even if the GPS went down, you would have been far better off than pre-GPS, because your last accurate position would have been no more than an hour old.

    Of course, if having the GPS tempted you out into the fog, where without you would have gone to the pub, that's another conversation!
    Steve
    Never believe anything until it has been officially denied.

  4. #4
    Join Date
    May 2007
    Posts
    37,244

    Default Re: Sailing with gps...

    Quote Originally Posted by Stemar View Post
    Before GPS, if the fog came in, your only possibility was dead reckoning. Assuming you kept a proper log, with regular notes of your position, even if the GPS went down, you would have been far better off than pre-GPS, because your last accurate position would have been no more than an hour old.

    Of course, if having the GPS tempted you out into the fog, where without you would have gone to the pub, that's another conversation!
    Before GPS, there was Decca, RDF, contour lines on the chart, radar, as well as DR.
    People also sailed with the mindset of not needing to know where they were to the nearest metre.

  5. #5
    Join Date
    Sep 2001
    Location
    Home - Sothampton, Boat - Gosport
    Posts
    10,018

    Default Re: Sailing with gps...

    True, but everybody and his dog has GPS now. RDF on a small boat contained a large measure of wishful thinking and Decca was far from universal. There was many a yottie left the Solent for Chrbourg and heaved a large sigh of relief when he saw the Cotentin Peninsular in front of him - and a good few that missed to one side or the other!

    Contour lines are good things to keep an eye on and will keep you from bumping into most of the hard bits even with GPS. A plotter will show you where you are in relation to the land, but it's all to easy to misread a lat/long from the GPS, especially when tired or in bad weather
    Steve
    Never believe anything until it has been officially denied.

  6. #6
    Join Date
    Sep 2016
    Posts
    320

    Default Re: Sailing with gps...

    GPS is a mature technology which has not had a global outage in over 20 years*; by contrast Galileo is not yet fully operational.

    I suspect that for most of us the highest risk is that the primary receiver gives up, these days that is easily mitigated by the shear number of devices carried that have GPS chips: phones, VHFs, laptop dongles, tablets etc. Even if you do lose GPS entirely, are you any worse off than if you hadn't been relying on it in the first place? I recently did a 130Nm crossing of the North Sea from Harwich to Ijmuiden, in good visibility it is now possible to do this without ever being out of sight of a fixed object. If visibility drops then you only need to know where you are to the extent that you avoid bumping into things, I think most of could sail for quite a long time on dead reckoning before the error became large enough to be a problem. The advice given to the Walker children in We Didn't Mean to go to Sea still applies: get offshore and stay there.

    *I cannot find any decent evidence of local/regional outages in that period either with the exception that coverage at high latitudes is known to be limited.

  7. #7
    Join Date
    May 2007
    Posts
    37,244

    Default Re: Sailing with gps...

    There have been local (ish!) incidents of jamming by the military, including in UK waters.
    There have been 'spoofing' events closer to Russia.
    Although GPS is highly reliable, it should not be taken to be 100%, particularly with a typical civilian receiver.
    There are loads of people who, like me, have seen inaccurate positions given by GPS receivers, and that gets worse the more you put between the antenna and the sky.

  8. #8
    Join Date
    Sep 2001
    Location
    Home - Sothampton, Boat - Gosport
    Posts
    10,018

    Default Re: Sailing with gps...

    ISTM that it comes down to my Rule 1 of boating: If you've only got a plan B you're already in trouble.

    GPS is a wonderful thing. To know where you are to within a couple of metres in the middle of an ocean is something our parents could only have dreamt of. To rely on it (or any other single system) to the exclusion of good sense and precautions is heading towards a Darwin Award application, just like the idiots who drive into rivers 'cos their car GPS says there's a road.
    Steve
    Never believe anything until it has been officially denied.

  9. #9
    Join Date
    Jul 2003
    Location
    River Itchen, Southampton
    Posts
    6,954

    Default Re: Sailing with gps...

    Quote Originally Posted by Stemar View Post
    Before GPS, if the fog came in, your only possibility was dead reckoning. Assuming you kept a proper log, with regular notes of your position, even if the GPS went down, you would have been far better off than pre-GPS, because your last accurate position would have been no more than an hour old.

    Of course, if having the GPS tempted you out into the fog, where without you would have gone to the pub, that's another conversation!
    I agree with every word of this.

    If doing a trip of less than 1 tide I don't even bother to get the charts out anymore. I just put the destination in the plotter, put the pilot on track and let it figure out the cross track for me.

    If there is a significant change of tidal flow on my trip I plot it traditionally and steer a heading. Even then I don't bother plot my position on the chart - but I do write it down regularly. That gives the starting point you talk about if I need it.

    However I make sure I go out a few times a year with the plotter off and do it "properly" so I don't forget how. I do know where my hand bearing compass is.

    There is no doubt that I miss the fear/excitement of arriving at a distant shore in the dark, having used dead reckoning, trying to convince myself that the light is flashing 3 as I expect and not 2 as I can see. But I also miss wagon wheels the size of bin lids.

  10. #10
    Join Date
    Sep 2016
    Posts
    320

    Default Re: Sailing with gps...

    There are many single points of failure on a boat that have far more severe consequences than GPS ceasing to work*. Some of these we mitigate against, but many we do not. If you were to perform an FMEA on sailing then I doubt that this risk would come close to the top.

    For those who say that they do not rely solely on GPS, what do you do to mitigate? In what way do you act differently to those of us who trust it?

    *e.g. rudder, steering mechanism, cap shrouds, backstay, skin fittings, lots of things in the engine, single (cockpit mounted) display of the log reading, only one person who knows what they are doing.
    Last edited by TLouth7; 25-07-19 at 08:17.

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