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    Re: Lightning at sea
    From: Phil Camera
    Date: 2004 Oct 15, 17:28 +0000
    George's ramblings are pretty much right on based on what I have learned.  I'd like to share some of my comments to his comments.

    > Because lighning strikes so randomly, it's hard to study it scientifically.
    > So there's still a lot of folk-wisdom and superstition,

    Actually lightning has been studied extensively and the knowledge base is huge and all information points to that one can not predict nor prevent it. 

    > Don't consider me to be a pundit: in this field, there
    > are none! Commonsense is what counts.

    Besides being crew on a racing sailboat (20 + yrs incl. Navigator) I am also a licensed Amateur Radio Operator.  I have four tall towers with mulitple antennas at my home and am well versed and protected against both direct strikes (rare) and induced voltages from nearby strikes (very common).  If one wants to learn more, I'd suggest the following two websites.  Polyphaser and ICE make the lightning and grounding equipment that some of us hams and commercial stations use to protect their gear.  Remember that the police and fire dept, AM & FM & TV stations don't disconnect in a storm.  They systems must be able to take the voltage and keep on working.  Lot's of this info can be applied to boating; you're surrounded by the perfect ground and that's all lightning or induced voltage wants to do, is to get to ground as fast as possible. 





    > 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!


    No not divine intervention it's just that your mast is probably well connected to ground and therefore your constantly bleeding off static charge.  Believe it or not, there's also a lightning bolt that jumps up from ground to meet the sky bolt coming down.  If your "system" is bleeding off that charge buildup, then the another area, maybe on the surface of the water, will have a higher potential to ground than your mast and lightning will prefer that.  Things get real fuzzy on this subject but experience has proven it to be true.

    > But if it's going to hit your boat anywhere, it's almost certainly going to
    > be the top of the mast.

    No necessarily, see above.

    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.
    > True, true.  My towers are setup to constantly bleed off charge but if they do take a direct hit, that energy will be shunted right into the ground and away from my house and equipment. 

    And within the metal cabin, there's no
    > personal danger at all, except perhaps from cabling to the masthead and
    > pulpit.

    That's called the Faraday Shield effect, like staying inside your car in a lightning storm.

    > 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 .

     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. 

    All the boats I've sailed on have had the mast and/or the rigging connections wired to the metal keel or the grounding plate built into the hull of the boat.  Don't all boats have this plate?  An alternate could be grounded to the prop shaft.

    > 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.

    Yes, bad idea.  Lightning will follow the path of least resistance and the chain sounds like a high resistance path.  Some energy will leak this way but most "may" find another path to ground.


    Such a chain, under such
    > high voltages, will not be following Ohm's law!

    Captain, you can't changes the laws of Nature!!!  Ohms Law will still be valid, period.

    > "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?"

    This sounds a little better than the chain.

    > 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.
    > >>
    > 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.
    > >

    What about the grounding plate built into the hull?  I always thought all boats had these.  Good discussion though.  One could use some of those arrestors made by the companies (and there are other companies I'm sure) mentioned in the web links above.

    > 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.
    > >

    Isn't this why we all carry our sextant with us or at least a kamal and timepiece 'cause one never knows when the electronics may fail!

    > 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.

    Induced current and voltage from a nearby strike is a big problem and the one that should be guarded against.  It's what I worry about in the ham radio towers and antennas.  A strike a mile away can induce enough voltage to zap out electronics.  Personally lightning hit and blew up a neighbor's tree.  It induced over 30 amps of current in the electrical wiring in my barn, located 300 ft away (Blew a hole in the side of the 30 amp fuse protecting that circuit.)  My towers and antennas, as close as 200 ft. away, kept on ticking without a scratch.  Just my two cents.  Phil Camera, Lockport, IL


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