"Roger Long" wrote in
:

The mast stanchion is essentially equal to a keel stepped mast. Since
my boat was originally a keel / centerboarder, the keel is large
volume and I doubt that Endeavour spent the money for a keel casting.
I'm quite sure the ballast is just stacked lead pigs in resin.
Lighting current going through that stuff would be like a bomb and the
high resistance at the bottom of the main conductor would create
extensive side flashing.

For reasons not evident on the crude drawing, any grounding plates
have to be outboard of the cabin sole. The Thomson paper says not to
let grounding conductors contact the hull but I have no choice if I am
to maintain the maximum radius recommended by other sources. The
reason for overkill on conductor and ground plate size is to
compensate for the tight conductor radius and need to run the
conductors close to the hull skin.

--
Roger Long




I've been involved in tower grounding (just a mast 1200' high with no
sails, if we can help it) for decades in broadcasting. The AM towers
are series fed, meaning they are insulated from ground but have two
trailer hitch balls a few inches apart (far enough so the 5, 10, 25 or
50KW transmitters don't make them arc in the downpouring rainstorms.)
Looking around Jim Hawkins' broadcast transmitter website, you can learn
a lot about lightning grounding from the professionals:
http://www.hawkins.pair.com/radio.html

Let's look at the feedpoint of WFAN/WCBS AM stations whos twin 50,000
watt transmitters across the river from NYC share one tower. (The RF
comes out of the building on that copper tubing with the rain loop in
it.)

The 900', i think, tower sits on a large brown ceramic insulator. The
ring around the outside of the insulator has a lightning gap to that
metal ball hooked to the ground plate on top of the concrete base.
radiating out from the base are large copper ground straps that hook to
another ring, to distribute the hundreds of thousand of amps of current
from the stroke, and there is a ring of ground rods driven to bed rock
with a pile driver around that ring, the ring attached to the top of
them.

The key to these grounding systems is their SHORT, STRAIGHT AS POSSIBLE,
LOW IMPEDANCE path. Lightning is not considered DC in these designs.
It is a PULSE of power with an amazing bandwidth in frequencies. The
length of the conductors increases INDUCTANCE, which raises the
impedance to the very quick pulse. Impedance is our enemy. As it
rises, so does the voltage drop across it, raising the voltage on the
upper end of it. Any sharp corners MUST be avoided as that makes a
little 1 turn COIL raising impedance to the pulse. Ground strap must
CURVE around a large radius, as discussed in the article, to reduce
impedance and pulse voltage drop.

By the way, most big AM stations use the Harris DX-50 solid state 50KW
transmitters now. They use 55KW of power from the power company to put
out 50KW of RF to the antenna...This is one of the 230 switching modules
that actually provide the RF power:
http://www.hawkins.pair.com/wabcnow/wabcn14.jpg
All the cooling it needs is a few big muffin fans through those little
heat sinks to put out a blowtorch of AM radio power! Amazing
technology.

If you wanna see more, look he
http://www.hawkins.pair.com/wcbs_wfan.html
Most people have no idea where the signal comes from. They think it
comes from the studio where the stupid talking heads live.

http://www.hawkins.pair.com/wado/wadotowrleg.jpg
Here's a similar base at WADO, formerly WNEW. It has two lightning
balls across the massive insulator. The odd looking intertwined rings
are called Austin transformers. The RF won't flow between the rings
because they are far apart and provide fairly good isolation. What DOES
go across between these coils is 60 cycle AC power, magnetically
coupled, that light the tower lights you see day and night for those
folks riding to their deaths in airliners.

See all the direct, large ground straps heliarc welded to the ground
system under the tower? Low impedance...low voltage....only a few
hundred volts of pulse in a stroke to this huge tower.

http://www.hawkins.pair.com/wor/wor_tower08.jpg
This picture of the base of WOR's towers shows "Johnny Bells" and the
lightning balls to ground. Lightning flows out to the edge of the bells
then jumps the gap to the balls.

================================================== =================

Your only hope is to BYPASS the lightning's current AROUND the
passengers and hull so it doesn't HAVE to jump THRU it, punching holes
in the expensive plastique. There are many "paths" to get the lightning
off the mast. The base of the mast must be connected to the sea as
short and direct as possible. From the above pictures, you can see how
big the conductor SHOULD be, but that's not very practical in most
boats. Keel stepped masts are easy. Copper straps to the inside of the
hull clamped between large stainless washers and 2 nuts where the
grounding block mounting bolts come through the hull. Coat it all in
your favorite sealant, but make sure you leave it where the sealant can
be shed so the grounding blocks can be replaced as they eventually will
be eaten.

I guess it's too ugly to expect painted straps down the OUTSIDE of the
hull to the same bolts UNDER the grounding blocks from the lower end of
the shrouds, another great path from mast to sea around the people, hull
and expensive electronics.

Backstay and Forestay ends also need grounding blocks underwater to
bleed off the charge around the ends of it.

Just dreaming....I've been knocked flat being between the backstay and
steering wheel in the way of lightning arcing between them. You'll
never forget it......so close.