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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@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.
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