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    Re: Lightning at sea
    From: George Huxtable
    Date: 2004 Oct 15, 22:34 +0100

    Phil Camera wrote, in response to my comment-
    
    >> 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.
    
    =============
    
    Response from George-
    
    Well, I'm quite familiar with the notion of a stream of positive ions
    that's drawn from pointed objects when there's a high voltage-gradient,
    that in severe cases gives rise to "St Elmo's fire" (which I've never
    seen), and which can help to initiate the downward strike. But I can't see
    the logic in Phil's claim (if that's what he is claiming) that supplying a
    conductive path to the water from the boat will in some way decrease the
    gradient, and decrease the ion stream. Seems to me it should work the other
    way round. Things have indeed got fuzzy there.
    
    When Phil says "it's just that your mast is probably well connected to
    ground and therefore your constantly bleeding off static charge.", that
    just ain't so, in my case. I have no anode or grounding plate connected to
    my electrical system, and the engine block is insulated from the prop shaft
    by a flexible connector. My fibreglass boat has been that way for 35 years,
    and the bronze prop. is 24 years old, living in salt water. I have no plans
    to change it.
    
    Phil adds-
    
    >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.
    
    Please would Phil, or perhaps Henry Halboth, explain further, in the case
    of a non-metal keel, about this "grounding plate, built into the hull of
    the boat", which seems similar to what I've suggested in answer to Courtney
    Thomas' recent query about connecting to his mast. I presume that this
    contacts the water outside the hull: is that correct?. Is it a US
    requirement for fibreglass craft? How big is it supposed to be, in terms of
    area exposed to water and size of metal connection through the hull? What
    material is it made of, and how does it fit in with protection against
    electrolysis?
    
    Here in the UK, if craft have a through-hull electrical connector, its
    primary pupose is corrosion protection, and it's made of Zinc, and corrodes
    away. In the days of the Decca navigator, an immersed plate was sometimes
    necessary, to provide a low-noise "ground" at the low frequencies of Decca
    operation. But we have no requirement for such an immersed electrode (that
    I'm aware of), and many fibreglass vessels, such as mine, rely on
    insulation from the surrounding sea, mainly for keeping down electrolytic
    corrosion.
    
    However, In spite of its drawback of adding onother hole in the hull, I
    accept that a stout connection to the sea will have distinct virtues, in
    providing a path to ground which might otherwise pop a hole in the hull at
    a weak point such as an echo-sounder.
    
    Henry Halboth implies, I think, that the grounds of all electrical
    equipment should be connected in "star" pattern to a single ground plate,
    and I agree, that would have distinct advantages over the common
    arrangement (as in my craft) of "chaining" the ground connection  from one
    unit to another. It would ensure that large currents flowing into the
    ground system did not cause large differences in the potentials, between
    the ground points of one unit and another.
    
    I suggested "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."
    
    Phil replied-
    >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.
    
    I really doubt that it would be a high-resistance path, under kiloAmp /
    kiloVolt conditions, when the oxide layers, link-to-link, would break down
    immediately. But I have no evidence to back that up.
    
    To my suggestion-
    
    "Such a chain, under such high voltages, will not be following Ohm's law!"
    
    Phil Camera responded-
    
    "Captain, you can't changes the laws of Nature!!!  Ohms Law will still be
    valid, period."
    
    I have news for Phil Camera. Ohm's law applies only to those conductors
    that Ohm's law applies to. Try a filament lamp, a Zinc Oxide resistor, a
    semiconductor junction, a Zener diode, an electric arc. Ohm's law applies
    to none of these. Nor does it apply to a contact between two oxide-corroded
    metal surfaces.
    
    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.
    ================================================================
    
    
    

       
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