Jere Lull wrote:


No, no, No, NO, *NO*! Find something to occupy those idle hands that
will likely add positive survival probability.

The thing I've learned in this very interesting thread on via Google is that
lightning will follow every path; not just the path of least resistance.
There is so much current that even a small fraction can do enormous damage.
When it gets to the end of an ungrounded conductor, it's going to go
somewhere. The approach of leading it down the stays might work with a
non-conductive mast that didn't have any wiring in it but, as Ian points
out, when the largest conductor on the boat just ends either right above the
heads of people huddled inside (in the case of a non-conductive support
pillar) or at a non-grounded keel, bad things are going to happen.

There seems to be an inconsistency in the historical fear level of marine
lightning and current statistics. I suspect this is due to two main
reasons. First, up until about 40 years ago, the typical vessel had a
wooden mast with outside chainplates that lead near the waterline. This is
far from effective protection but may actually be as good as can be obtained
with secondary grounding of a metal mast. Second, boating in Florida and
other high strike probability areas has become vastly more common in the
same time period.

Most of the strikes I have heard of anecdotally in this part of the world
have only resulted in electronics wipe out. I've never heard of a sinking or
fatality in a sailboat in New England. Strikes clearly vary in intensity.
Some would probably sink a boat with a 4" diameter solid copper conductor
running to 50 square feet of ground plate. There is a huge probability
factor at work here.

The Sea Grant study

http://www.thomson.ece.ufl.edu/lightning/

showed that 75% of Florida boats struck in salt water suffered no hull
damage, and less than 10% had watertight integrity breaches, a large
proportion of which were survivable. These translate into pretty good odds
for a boat operating in northeast waters or only in Florida during the dry
season.

In view of the difficulties doing anything clearly effective on my boat, I'm
now tending towards your quoted statement. There is a big element of "fun
for it's own sake" in these projects. I enjoy watching weather and
thunderstorms and that enjoyment would be increased by a lower anxiety level
about a strike. However, similar money and effort spent on similarly
interesting projects would probably increase the overall safety of my boat
more than grounding the mast.

If lighting wants to go in a straight line to large masses of metal, my mast
is probably somewhat grounded anyway. There is a lot of lead down there and
the keel is quite wide. Side flash would probably go down into that large
mass and spread out below the top of the encapsulation. There would still
be major flashes around inside the boat but it sounds like there would be
with any expensive and complex grounding plates I added as well. My
grounding scheme might well just attract the charge towards the thin part of
the hull. Damage down in the encapsulation would probably be major but
there would be so many paths that it probably wouldn't result in
catastrophic leaks. Leaking would take care of any fire that resulted in
the ballast area.

A small blood clot in a heart artery is probably an order of magnitude
greater in probability than a boat sinking strike in this part of the world
so perhaps I should just have two asprin and call back in the morning.

--
Roger Long