NavList:

Good morning, Francis (or in the UK, good afternoon).

>>More scary, your link mentions you can buy a GPS jammer for £20!  <<

Indeed!  Could be a problem for us all if very many people start carrying these things around in their cars.

The North Koreans have mounted big jammers on the backs of trucks and then will knock out all GPS in South Korea for a week or two at a time, just for the sheer joy of it.  They are kind of like an whimsical adolescent that spray paints the walls of buildings downtown...except they have ballistic missiles and nukes.

http://gpsworld.com/massive-gps-jamming-attack-by-north-korea/

Whether they develop the means and the motivation to launch a big jammer into orbit over the Pacific basin is something one can only speculate about.  One might think the Americans et al would treat that as an act of war, and that this would warn them off...but you simply can't count on the North Koreans to be completely rational.

Part of the response to their behavior by South Korea is to build out an eLoran system as a backup to GPS.  Britain is also doing this.  The US is putting all its eggs into the GPS basket.  I have no idea what we are doing in Canada....

But regardless, eLoran is only of use on dry land or in coastal waters.

>>Keep practicing those sun sights, that’s what I say!<<

I think so!!  For myself, I prefer not to risk the safety of boat and life on the continued rationality of the North Koreans and the Iranians...and particularly insofar as I simply ENJOY celestial navigating anyway. 

As I ponder the fate of Scottish fishermen and others who put 100% of their faith in GPS alone for navigation (possibly not even keeping fixes in a log as a basis for doing ded reckoning if the need arises), I think back to the words of Clint Eastwood in the first "Dirty Harry" movie:  "Do you feel lucky?  Well...do you?"

>>I also have a relatively  cheap Rotary Chinese made  mechanical watch .Auto wind, water proof to 50m and  if you wear it continuously,(presumably gives constant temperature)  keeps very constant rate +25secs per day. <<

Since you talked about protecting three digital watches in a Faraday cage, I have pondered what ought to be the prudent sailor's strategy for timekeeping.  

Although I have discussed Faraday cages with a fellow in my church who holds a PhD in electrical engineering...and know that they SHOULD work, even if the lightning strike I experience is "the big one", I think I am going to be a suspenders-AND-a-belt kind of guy.  Protect one or more digital watches in a Faraday cage/box, and ensure that I have rated a mechanical watch aboard as well.  It is cheap-and-easy insurance.

>>Re Faraday cages: I use ex army aluminium mess tins, very cheaply available in UK. <<

Thanks.

We just need to ensure that we have no breaks in our metal enclosures.  If the lid of the tin doesn't fit snugly, then you might want to wrap your neoprene in aluminum foil, so you still get those electronics>insulator>conductor layers working for you. 

If there are openings (if you were using a wire mesh for your Faraday cage, or a lid of a metal box that doesn't quite fit) that can be OK.  You will notice that the mesh in the door of your microwave oven has openings large enough to see the food inside.

But the shorter the wavelength, the smaller your mesh has to be.  My engineer friend didn't know what the wavelength of lightning's EMP might be, but thought it might be "pretty broad ranged."  So keeping a tight seal in your lid is probably a good idea.

>>How goes mechanical watches in a lightning strike?  <<

I think they are pretty robust in this respect.  A lightning strike is a transitory event.  It can do a lot of damage to electronic devices for which 1 millisecond is a long time, but there is not much that can happen to mechnical devices (like watches and sextants) as long as they stay clear of the direct path of the current.

Bob