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    Re: Lightning at sea
    From: Courtney Thomas
    Date: 2004 Oct 15, 10:52 -0500

    George,
    
    What do you think of a fiberglass boat with iron ballast and aluminum
    mast, regarding lightning damage evasion ?
    
    How about bonding the aluminum mast to the iron [internal] ballast, then
    ballast to sea ?
    
    But, I ask....what then, in this case, would be the best procedure
    regarding the ballast to sea connection, as well as, mast to ballast ?
    
    Appreciatively,
    
    Courtney
    
    
    
    George Huxtable wrote:
    
    > 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---.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.
    > ================================================================
    >
    >
    
    
    --
    s/v Mutiny
    Rhodes Bounty II
    lying Oriental, NC
    WDB5619
    
    
    

       
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