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Re: Lightning at sea
From: Courtney Thomas
Date: 2004 Oct 15, 10:52 -0500
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@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. > ================================================================ > > -- s/v Mutiny Rhodes Bounty II lying Oriental, NC WDB5619