Re: Lightning & Boats
David T. Minasian (antlion@concentric.net)
Fri Jun 26 22:43:51 1998
Geetings Martin,
Being a sailboat man myself I have always been greatly concerned about
the potential danger of lightning. My mainmast is 52' and aluminum. It
would seem to be an open invitation to lightning.
I have done some research on the subject and pondered the different
theories on protection and the most prominent conclusion one can draw
from the available resources is that lightning is, above all, totally
upredictable. Therein lies the problem of finding guaranteed
protection. There really is none. The only proper action is to minimize
the chances of a strike.
>From the description of Sea Dart, she probably did not suffer a direct
strike, only a splinter discharge from a direct hit nearby. However,
although your antennas were destroyed,you should do a complete check of
everything electrical aboard including your alternators, engine
instruments, right down to the last light bulb. Voltages in even a
related discgarge can do enormous but less obvious damage. By the way
most marine insurance covers lightning.
As to alternative methods of protection, I have most seriously
considered two schools of thought; total grounding and slow static
discharge. A third school, of course, is to do nothing which is
certainly viable given the very small liklihood of ever being hit as
demonstrated by statistics.
Total grounding would involve tying all high structures together with
heavy copper wire and leading as directly as possible, no curves or
sharp turns, to a ground plate underwater. The argument here is that
you couldn't use heavy enough wire to take the millions of volts in a
direct hit. The counter argument to this argument is that, although the
wire could not carry 100% of the voltage, most of the voltage is
conducted safely to ground by an ionized column of air conforming to the
path of the wire. And then an argument to that states that total
grounding increases the effective height of your vessel by two and a
half times and that increases your chances of being hit. It all makes
sense but the argument goes on.
I do favor the slow static discharge theory. In this case, the theory
is that a lightening strike is a "contract" between the two poles,
positive and negative or cloud and ground. Each party plays a part in
the making of this contract. The cloud builds up enormous electrical
potential due to the dynamics going on within the storm cell and it will
ultimately have to releive itself. The ground cooperates by being
equally anxious to absorb the energy from the cloud. Both are endevering
to select the best point for the exchange to take place and conclude the
contract. An instant before the strike, literally thousands of points
send tiny discharges from the ground up (also known as St. Elmo's'
Fire), and although any one of these discharges could result in a path
of lightning to ground, the release of ground potential dimenishes its
attractiveness to the cloud and will likly be passed over.
Therein lies the theory of placing at the boats highest point, a brush
like device containing many copper or bronze sharp bristles. As the cell
nears, and that particular location has the potential equal to the
electrical energy building within the cloud, the premature release of
potential energy from the "brush" will reduce the chances of the boat
being the target.
In all honesty, though I favor the latter approach, I am still a member
of the third school. I have done nothing yet, but I do plan to install
the brush hopefully before the chickens escape the coop.
Dave Minasian
SSS 104
Wayne NJ
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