Roger Long wrote:
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.



Roger, I believe your question is:

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.

The bend is pretty critical. By making a turn you create part of a
transformer otherwise known as impedance or the resistance to an
alternating voltage. The tighter the bend the higher the impedance.
Also the higher the frequency the higher the impedance. Since a
lighting strike typically has very high energy, high impedance
components you are well advised to make the radius as smooth as possible.

Paralleling the run may help or may not. Without doing much more
research I can't tell. The problem would be if the two runs create a
field that would counteract the flow in the opposing wire thus again
increasing the impedance.

Larger wire helps but maybe not as much as you would think. At high
frequencies the current only runs on the outside of the wire in
something known as "skin effect." That is why they recommend braided
wire, much more surface area. BTW skin effect is caused by the parallel
paths in a wire from one side to the other, so you see that it can occur
in even small wires.

At radar frequencies they use hollow wires known as waveguide. I have
seen waveguide melted because of resistance heating due to a small dent
that caused some local impedance.

Probably not the answer you were hoping for. Sorry.