NavList:

From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Bob Goethe
Sent: 09 March 2015 17:07
To: francisupchurch@gmail.com
Subject: [NavList] Re: Status of Celestial Nav in 2015

Dear Francis,

>>My fundamental question is therefore "is a a sextant, (plastic or metal) and a quartz watch less prone to lightning strike damage than electronics?<<

In a word:  yes.  Definitely yes, as pertains to the sexant.

I think the sextant is unlikely to suffer any damage at all if your boat is struck by lightning.  It is not really a part of any circuit connecting the thundercloud to ground, so direct current passing through it is not an issue. 

    If you were standing on a golf course, so that YOU are the highest
    point for 100 yds around, and holding your sextant over your head,
    for some obscure reason, then the sextant could be part of the
    path for the direct current on its way to ground.  In THAT case,
    your Tamaya sextant might melt along with the soles of your 
    Nike shoes.

     But this is not the situation that pertains on a sailboat or power yacht.

The direct current of lightning is most often passing from the masthead thence down the electrical cables to your instruments, energizing everything that is connected to your boat's electrical grid along the way, and then out through the keel/propellor shaft/hull.  I understand that a lot of boats in Florida, home of daily thunderstorms as it is, have grounding systems to a plate attached to the outside of the hull.  The hope here is that they can provide a path to ground that does NOT involve the VHF radio or chartplotter.

     The account I quoted in an earlier post suggested the lightning struck
     near the stern of the boat.  Somehow it still managed to pass through
     the boat's electrical system on its way to somewhere.  That aspect
     of the account mystefies me completely.

Lu Abel asked if people themselves were likely to survive a lightning strikes that savages the electrical system of a boat, and the answer to that is "probably yes".  Lightning is unpredictable at all times, but fundamentally your mast/electrical system/keel provides a better path-to-ground than a person does. 

If you Google:

    Lighning sailboat EMP

...then you will turn up a ton of references.  One of the consistent themes will be that there is a "cone of protection" that extends down from the masthead at a 45° angle.  Any people inside that cone are safe.  There are better paths-to-ground available within this cone than the human body.

     Friends, I know we are passing beyond the normal subject boundaries
     for a navigation list...but a discussion around the role of celestial as
     a backup system to GPS leads here almost inevitably.

I turned up one account where a boat was struck while under way.  The electronics/electrics (save for the diesel engine and starter system, which seems to be pretty durable in all these accounts) were fried.  The man's daughter was steering the boat at the time, and the iPod in her pocket ended up with scorch marks on the screen...but all the people were just fine.

As devastating as EMP can be for integrated circuits, it is a non issue for us humans, per se.  Lightning doesn't induce significant currents in us.  It was not until the advent of vacuum tubes that anybody even noticed the existence of EMP.  Even so, it really didn't come to the foreground among civilians until solid state circuits became common.  By being able to respond quickly to induced current at ever lower voltages, and incorporating wires the diameter of a human hair, modern electronics has become ever-more-susceptable to EMP and melted circuits.

Basically, *all* EM waves induce current.  AM/FM radio waves are of low enough intensity that the electrical currents they induce in your antenna need an amplifier circuit to make anything useful of them.  Lightning produces a burst of radio waves intense enough that the circuits in an iPod (or EPIRB) are themselves a sufficient antenna to conduct a destructive current even without an amplifier.

>>Presumably, my plastic back-up Davis 3 and 3 quartz watches in proper Faraday cages (i.e, not just wrapped in foil, but wrapped in neoprene rubber, then metal box) should survive?<<

"Absolutely yes" regarding your Davis sextant, and equally yes if it was a metal Astra IIIb or Tamaya sextant. 

In fact, these don't need the protection of the Faraday cage at all.  

Even a metal sextant is fine as long as it is not part of the electrical connection between thundercloud and ground.  Nobody ever leaves their sextant propped on the top of the SSB radio during a storm...and there is really no other scenario I can visualize that could leave a sextant vulnerable to high voltage.

"Yes" also when it comes to your 3 quartz watches.  What you have described with electronics-then-neoprene-then-metal-box should be perfect as a Faraday cage.  Just use some string or duct tape or something to ensure that the neoprene doesn't unwind and let a watch physically touch the metal box.  You want zero breaks in your insulating material.  You also want zero open holes in the metal box.

You are more thorough than me in terms of the number of quartz watches you carry, and storing them in a proper Faraday cage.  I may need to emulate you in this.

What I did last summer was to bring a mechanical wristwatch ($24 from Singapore, via Amazon.ca) along with me, and record its rate of gain every day I was on board. 

But a quartz watch that would go in a Faraday cage need be no more expensive than my mechanical pocket watch, and will be vastly more accurate.  I like your way of doing this instead of my way. 

There are not a lot of metal boxes in my world.  What kind of metal box do you use?  And from where does one purchase neoprene?

>>I'm happy to just do it because  I enjoy doing it. <<

I think THIS is key, Francis.  The chances of being hit by lightning are only 3 in 1000 if you are in Florida, less than that in BC, Canada.  It just seems to me that if I ENJOY sextant navigation ANYway, then I might as well ensure that I have a working timepiece on board in the (unlikely) event that GPS ever becomes unavailable.

As an aside, this link outlines a real world scenario in which GPS did indeed become unavailable:

http://www.techweekeurope.co.uk/news/mod-halts-gps-jamming-after-safety-complaints-42074

If you weren't even recording your positons in a logbook...just using the chartplotter to keep a visual of your positions...then you could indeed become as disoriented as these Scottish fishermen seem to have been.  If they were using sound practices in navigation to begin with, I doubt if the loss of GPS would have been worth mentioning by them.

At need, naval ships typically, I believe, jam GPS in a 50 mile radius around them.  Depending on how they are distributed, a dozen ships could render GPS unusable across 6,000 square miles of ocean.  Evidently, this is pretty much an annual exercise for NATO.  I imagine these sorts of things pop up in the Pacific as part of the ANZUS alliance exercises.

http://www.newscientist.com/article/dn20202-gps-chaos-how-a-30-box-can-jam-your-life.html?full=true#.VP2IF7k3Ocx suggests that the Navy has inadvertently jammed GPS even in a place like downtown San Diego.

>>If so, maybe this is the main truly logical reason for cel nav as a back-up?<<

I think so.

In 35 years of yachting, I have never once needed to use a manual bilge pump.  But it is a trivially easy thing to keep a handle for the bilge pump available...and the consequences of NOT having one if you needed it could be catastrophic...so I would not go to sea without one. 

In the same way, it is an easy thing (particularly if you enjoy it, as I do!) to carry a sextant.  I would never consider sailing out of the sight of land without having one onboard.

Bob