NavList:

   

Reply

Lisa Fiene raises interesting questions about lightning.

Because lighning strikes so randomly, it's hard to study it scientifically.
So there's still a lot of folk-wisdom and superstition, since the days when
it was attributed to Jove hurling his thunderbolts about.

The experience I can claim is many years of designing control systems for
high-voltage  particle accelerators, running at many millions of Volts, so
I have some familiarity with aspects of sparking and the damage it can do.
But that's as-nothing compared with the voltage and the power behind even
an ordinary lightning discharge, so I know little more about protection
than the next man. Don't consider me to be a pundit: in this field, there
are none! Commonsense is what counts.

I'm glad to say that I have never experienced the sort of storm Lisa refers
to, of lightning with a gale. In the waters of UK / North France we can get
summer lightning displays, which seem quite severe when you're underneath
them, often associated with rather calm conditions (though with sudden
gusts). In 30 years of cruising, we have ridden through quite a few of
these, some under way, some at overnight anchor. Never yet, so far, has my
boat been struck.

The feeling you get when there's lightning about, and there's just your
sharp metal mast protruding above the sea with nothing else about, is that
if it's going to strike anywhere, it's going to strike that mast. It
doesn't seem to work like that, though. We have frequently seen strikes
that have chosen to go straight down into the sea, perhaps no more than 40
metres away, rather than go for our mast. Being a devout atheist, I can
hardly attribute it to divine intervention on my special behalf!

But if it's going to hit your boat anywhere, it's almost certainly going to
be the top of the mast. If your mast is in Jove's sights, there's nothing
you can do to stop it. The current will pass through your boat on its way
to the water. All you can do is to provide a path that will create least
damage.

As for Lisa's steel vessel, I doubt if she has any reason to fear
structural damage. Presuming that her boat has no wooden step at the foot
of the mast, then the enormous currents of many thousands of amps that will
inevitably flow can be expected to pass without causing the local heating
that is usually the cause of structural damage. Down the tubular mast,
across the metal deck, down the sides of the hull to the water; that's a
really low-resistance path and Lisa should think of herself as being pretty
immune from such damage. Because the mast-deck combination is such a good
conductor, there's no incentive for the current to take a more destructive
path down the rigging wires. And within the metal cabin, there's no
personal danger at all, except perhaps from cabling to the masthead and
pulpit.

A fibreglass or wooden vessel is in quite a different category. If we
assume a metal mast, that's likely to be able to carry the current from any
lightning-strike without being damaged. But what happens at the foot of the
mast? If it's stepped at deck level, the current has to somehow find a way
across the insulating deck, and down the insulating hull-sides, to the
water . Its path may be initiated through the salty conducting film that
usually builds up on surfaces exposed to the sea, and as the current builds
it can then cause instant local heating and charring along that path, even
(especially in wood) a local explosion as its water-content turns to steam.
I think such severe damage is (fortunately) rather rare.

On the other hand, there's an inviting path from the masthead, down the
rigging-wires, and down the wet hull-sides, which bypasses the insulation
of the deck. The steel shrouds are likely to be burned out by the
concentrated current flowing through a thin wire (just like the element of
an electric fire being subject to an overvoltage) if they carry the whole
current of a strike. This is a special hazard at anchor, when there's a
promising path down the forestay, bow-fitting, and anchor chain. It's worth
trying to deter current flow down the rigging wires, then.

My strategy is to encourage current-flow down the mast instead. I do this,
in an electrical storm under way or at anchor, by taking some surplus
length of anchor chain and draping it round the foot of the mast and over
the side into the water, in a number of loops. I have heard others
pooh-pooh this idea, on the grounds that the many oxide-coated surfaces
between the links make a chain a rotten conducor. So they would, indeed, if
you measured it with a resistance meter. But the voltages in lighting are
so great that they will (in my estimation) break down and spark-over such
interfaces, and provide a useful current path. Such a chain, under such
high voltages, will not be following Ohm's law!

You might say that this technique has worked, in that I have never yet been
struck, but of course it's not intended to reduce the chance of strike,
just to minimise damage if you are struck.

Bill came up with a similar suggestion-

"I know of some sailors that will carry heavy duty automotive jumper cables,
and attached one end to the rigging? and let the other end drag in the water
when electric storms come up.  Any feeling of whether this is
safe/practical, or would help?"

I think this would be a useful thing to do, probably better than my chain,
but not quite as Bill suggests, when he says "attach one end to the
rigging". That's exactly what you are trying to avoid, providing an easy
path down the rigging wires. Attach it well, to the metal MAST.

==========

If you have a wooden mast, then your protection against current flow down
the rigging wires would be to fit a copper strip lightning conductor, as
you find on a building, from top to bottom, and then ideally extend it to
reach sea-water somehow. The Admiralty, in the days of wooden masting,
initiated a research project, and that was their recommendation.

==========

On an insulating vessel, with a deck-stepped mast, how safe are you against
electric shock down below? That's a question I have asked myself when the
thunder has woken me in my bunk, at anchor, the chain-plates being bolted
through the hull no more than a few inches from my berth. I don't know, but
I do my best to keep away from any metal fittings that intrude from
outside.

==========

What about a keel-stepped metal mast on an insulating vessel? That might be
a bit of a worry, in my view, unless you have a metal keel, which is
electrically bonded, through the hull, to the mast. It would seem unwise,
in my view, to make this bond via a keel-bolt, in that the last thing you
would want is to damage a structural keel-bolt. Unless you have such a
bond, then there's a great concentration of electrical stress at the mast
step, and it's easy to imagine how a severe strike could puncture a hull
there.

==========

Protecting electrics.

I doubt if there's any way to protect your masthead electrics in the event
of a strike. What about the rest?

Just think about the negative 12-volt line that runs around the boat,
starting at the battery, linking one unit to another by a "ground"
connection. Usually, that will be a heavy connector, with no easy way to
deliberately break or disconnect it. Consider (for example) the VHF
antenna. The coax downlead has a  a heavy outer conductor, which may or may
not be linked electrically to the metal masthead itself, but is unlikely to
be carefully insulated from it. When a strike occurs, it's likely to reaise
that coax to the transient voltage at the masthead, which could be many
kilovolts. This is fed down to the VHF transceiver, then through its power
supply to the battery negative, perhaps the engine block, propshaft,
propellor, anode to the sea. Or perhaps to the sea via the echosounder
sensor, or the through-hull log, where the insulation is locally weak.

If the main strike finds its way to the sea by such a path, then you can
probably write off the whole lot. But even if the main current path is
elsewhere, the extreme voltages that can occur, for no more than a second
or so, between masthead and sea, imply that the voltage along this
(nominally) "ground" wire vary considerably, from one instrument to
another. That wouldn't matter much, if there was only the one connection,
that "ground" wire, to each item. But of course all the electrical gear has
other connections. If it's just a lamp, then no more than the positive
power lead (which might have been disconnected by switching off),
otherwise, input cabling from a sensor, output cabling to a display or
speaker.. More and more, all this stuff gets linked together with a web of
wires. (Not so much on my boat, by the way, which has minimal
instrumentation). And as I see it, it's transient voltage differences,
between one wire and another, that give rise to the fragility, because
semiconductor junctions can break down from overstresses of 20 volts or so.

For simple stuff such as lamps, simply switching them off in an electrical
storm may suffice. For the rest, you can do a lot by unplugging, especially
every lead that goes up the mast and perhaps pulpit and pushpit. If you
pull a plug out, it's not necessary to shift it yards away: an inch is
quite enough. I've heard of strategies of isolating equipment, then putting
it in an oven, or wrapping in Aluminium foil; these strike me as absurd
overkill, though they will certainly do no harm.

Is it worth bothering, in an electrical storm? If you're near home, losing
your electrics may not be too serious; you just have to replace it. On
ocean passage, it could be another matter.

When you go home and leave your boat unattended, at anchorage or pontoon
(and most craft spend more time that way than in use) do you take any steps
to protect it? Most don't, I guess. I don't, anyway.

Many years ago, I was launching a dinghy from a ramp, when there was a
lightning strike into the water, perhaps 50 metres away. I was holding the
dinghy's rigging at the time, and felt enough of an electrical shock to
make me jump. Presumably, the low cloud base formed one plate of a
capacitor, the land-and-water another, and the lightning had instantly
discharged that capacitor, to some extent. The rigging was picking up some
fraction of the sudden step in voltage gradient. It made me think: if that
rigging had been some sort of antenna for a receiver, that voltage step, if
it could more-than-tickle me, could easily destroy the input stage of an
amplifier. So perhaps it's not necessary for the boat to be struck to
suffer damage; a nearby strike may do the trick.

Sorry to have rambled on with these musings. No doubt others will disagree
with some of them, but that'll do no harm.

George.


================================================================
contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
================================================================

   

Reply