"Roger Long" wrote in message
...
This winter's major project is to add some serious lightning protection to
"Strider". What I have now is probably sufficient to increase the odds
of being alive to climb into the dinghy and watch the boat sink but I'd
prefer to sail home. It's not a subject that comes up often for a
designer of metal vessels so I've been look around the web and learned:

The ABYS standards of 1 sq. foot of ground area and 8 GA conductors are
marginal and highly suspect.

Probably nothing feasible is going to protect a plastic boat in fresh
water. Although I'm generally in salt, I'd like to be ready to go up some
rivers.

Conductors should have a minimum 8" radius bend.

I've got a metal mast support strut that has sufficient through bolts to
the mast deck step to make it electrically continuous. This lands on a
wide, internal ballast keel. I plan to run flat copper straps about 1/16"
x 1/2" (approximate cross section of 4 ga wire) from this up each side to
6" x 24" bronze ground plates on each side of the hull. These will be
about 1/16" thick and through bolted to the hull at each corner. Inside,
there will be straps under the bolt heads in an "X" pattern with the strap
from the mast strut lead to the center. There will also be a 4 Ga wire
or strap from the engine block to one of these plates to help protect the
engine bearings.

Comments welcome on this conceptual plan which will also include other
secondary bonding additions as recommended by ABYC.

Here's my main question for someone who understands high voltage better
than I do:

I only have 6" under the cabin sole. How critical is the 8" bend? Can I
compensate for the tighter radius by increasing the conductor cross
section? How much? The turn is more than 90 degrees because the straps
have to run back up the hull deadrise about two feet to where I can locate
the plates and through bolts. I don't think putting the plates on the
keel sides is feasible.

Another question:

Is the standard metal rod VHF antenna at the top of the mast with the
typical metal can on a bracket riveted to the mast a sufficient air
terminal or should I add a dedicated rod?

I have no illusions about having any electronics working after a strike on
a 32 foot boat but replacement of my minimalist outfit wouldn't break me
financially. I'd just like to be alive with a working engine and
watertight boat.


To understand lightning you've got to understand why it strikes. You need to
"see" the world how lightning sees the world.

You get an unbalanced electrical charge between the clouds and the water.
Like all current, lightning takes the path of least resistance. This may or
may not have anything to do with height of mast. Height of mast and material
of mast and grounding of mast all have little to do with how lightning
"sees" the best path to ground.

The purpose of all the bonding and grounding is to bleed off charge slowly
before it builds up to a great enough extent to arc from above the boat. A
properly grounded GRP sailboat will be seen by lightning potential directly
over it as sitting in a valley. One not grounded at all will be seen as
sitting atop a hill. Again, it's not distance I'm talking about but
resistance in the path to ground. An ungrounded boat does not bleed off
potential. It spikes it up.

The trouble with grounding your boat properly is the slow bleeding off of
the charge imbalance can actually, in some cases, prepare a lower resistance
path to ground as charge is already moving that way. While it may thwart
lightning strikes from directly above, should lightning strike at a 45
degree angle, for example, it may well strike a well-grounded boat and not
strike an ungrounded boat anchored several boat lengths away.

But, a properly grounded boat will suffer less damage as the voltage will
not build up as much because the current (amperage) will flow easier to
ground. An ungrounded boat, though less likely to be "seen" by lightning
will, if struck, suffer more damage because the voltage will build higher
and the resultant amperage will also be higher.

My boat is totally ungrounded and in over 25 years has been struck only
once. I was aboard and all kinds of bad stuff happened. There was a zzzzzzt
and a huge crash simultaneously. Then a series of gun shots as ball
lightning went arching and dancing around everywhere inside. I was forward
lying in the v-berth. One ear on the pillow and the other up. It was
temporarily deafened for a couple days in the upper ear and had extensive
ringing but the hearing came back. I think I was knocked unconscious for a
couple of minutes. My hair smelled burnt.

Every wired electronic device but one GPS was cooked. Solar panels were
fried from the inside out. The wiring harness was cooked. All the lights
burned out. A charred hole the size of a walnut was blasted out of the GRP
ceiling where the cabin light wires were encapsulated. The backstay where it
split was burned in two on the port side where current arched from it to the
outboard motor the foot of which was in the water acting as a ground. The
current jumped from the backstay to the tiller handle and cooked the wiring
and throttle cable - burned them to a crisp. The plastic fuel tank had black
marks on it but wasn't burned through. The ignition was not ruined, however,
probably because most of it is under the metal flywheel. The base-loaded VHF
antenna at the mast head was an empty shell. The mast wiring inside was all
cooked. The depth sounder transponder was fried. Bottom paint was blasted
off the outside of two of the bronze thru hulls. The keel bolts above the
GRP floors were blackened. The boat was full of smoke. A bottle of brandy in
the undersink galley locker had burst and was burning in the bilge. The
Adler Barbour fried. The secretary at a nearby marina saw the strike. She
said is was very large and lasted seconds hitting again and again.

It cost over 4,000 dollars not counting my labor time to repair it all.
However the good news is the cast iron keel took the brunt of it with paint
and epoxy coating blasted off in several fist-sized places. The hull at the
water line took no damage. No leaks as a result. No hull damage. There was
some deck damage at the foot of the mast were the current actually passed
through to get to the wood enclosed steel post that supports the deck
stepped mast foot and then to the keel bolts.

The choices as I see them are these:

a) Bond and ground your boat well and be prepared to be hit more often but
with less damage.

b) Don't bond and ground and your boat will be struck less often but with
more damage.


b) has been and will continue to be my choice as proper grounding and
bonding can only really be done during the construction process. Once a boat
is built too much stuff has to be torn out to do the job right.


Wilbur Hubbard