Lightning Ground aboard Gray Eagle
a Tashiba 40

A few years ago I was anchored in Hamburg Cove, Connecticut. It was just after dinner 
and getting dark when the weather turned ugly. The cove is very protected with high 
bluffs on two sides. A serious summer thunderstorm rolled in and produced the worst 
lightning I have ever seen or heard. Reluctantly, I peered out the ports to watch the 
light show and could see the bolts hitting the trees just a hundred yards away. When 
it was over I felt lucky not to be a victim of the squall especially since I was 
sitting in salt water with a 50 foot aluminum mast challenging the All Mighty.

There have been many other close encounters with lightning and finally a hit when the 
boat was on our home mooring. Actually the boat five moorings away took it direct and 
we must have only experienced a minor hit. Most of the electronics were either 
serviced or replaced during the next two seasons as they either failed completely or 
had strange intermittent problems. What I found is that you do not have to get a 
direct strike in order to have damage and the damage may not be immediately noticed. 
It can take some time for all the stressed circuits and electronic components to 
finally give up the ghost. This is especially unfortunate because the insurance 
company wants to see the proof and not hear anything about, “I might have been hit by 
lightning two years ago”. This makes it impossible to submit a claim for lightning 
damage even though the total cost can be significant.  

With all this lightning experience in the ships log I started looking into how Gray 
Eagle was wired for lightning protection and what the latest recommendations are by 
ABYC and the (1-see below) NFPA 780. TaShing Yacht Building, Ltd. wired the Tashiba 40 
in accordance with the standards and understanding of lightning protection at the time 
which was 1987. This consisted of linking all the bronze through-hulls, rigging and 
tanks with green grounding wire. This circuit also serves as the bonding circuit and 
is connected to the zincs and one wire to DC negative battery. The new recommendations 
call for a strip of copper bolted to the hull measuring at least 4 feet long and at 
least ¾ inch wide and 3/16 inch thick with one end under the mast and the other near 
the engine. If these exact dimensions can not be accommodated the NFPA says the copper 
plate must have at least 1 ft sq area x 3/16 inch thickness. The copper strip must be 
below the water line at all times and most importantly located directly below the mast. 
Attachment must be with bolts equivalent to a #4 AWG cable. There are a few more 
details but this is basically what one must start with. 

On the Tashiba 40 it’s impossible to comply with the suggested 4 foot long strip of 
copper without tearing apart the interior to access the attaching bolts for the 
copper strip. Therefore an alternate shape must be worked out based on the best way 
to bolt through the hull in the area below the mast. After much planning I decided 
on a copper plate 33 inches long x 6 inches wide and ¼ inch thick. These dimensions 
slightly exceed the recommended area of 1 foot sq x 3/16 thick.  I arrived at the 33 
inch length by spanning the mast step fore and aft with one bolt in front of the mast 
step and one behind it on the port side. The copper plate uses two ½ inch diameter 
x 3 inches long, silicon bronze carriage bolts. Carriage bolts require the hole 
drilled into the copper plate to be squared off. This keeps the bolt from turning 
while torquing down the nut. Heavy duty silicon bronze nuts along with lock washers 
and big flat washer are used inside the boat to secure the assembly. The cable 
connection between mast and carriage bolts is made by drilling and taping a ¼ x 20 
thread in the mast just above the mast step and below the exit hole for the mast 
wiring. Two battery cables size 1Ø competes the grounding of the mast to the copper 
plate outside the hull. I feel the low ¼ inch profile will have very little impact 
on hydrodynamics compared to the traditional Dynaplate. When I installed the copper 
plate I used 3M, 5200 around the perimeter as an adhesive backup to the ½” diameter 
carriage bolts and to cut down on turbulence as water flows past the plate. Bedding 
with 5200 may be a little aggressive and I’m not sure I would use this product 
around the edges of the copper plate again as this plate will definitely have to be 
replaced someday and getting it off may be difficult. When bedding anything to the 
outside of the hull the bottom paint must be removed for good adhesion. So, please 
be sure to sand away the paint in the area where the copper ground will be. I held 
the plate up and used a magic marker to outline the plate and sanded down to the 
barrier coat inside the marks. 

In addition to using the copper plate as a lightning ground I will connect the 3 
inch wide copper foil from the single sideband radio and tuner and use it as a radio 
ground. This should establish an excellent connection with the water and improve 
radio transmit performance. Originally, there was no provision by the manufacturer 
for a radio ground.

In the end I hope to accomplish a safer boat when the lightning does show up again. 
On Long Island Sound, during the summer, I will definitely have the opportunity to 
test my work and will let you all know how it goes. 

Richard Cassano
S/V Gray Eagle, Tashiba 40
Oyster Bay, NY USA

REFERENCE:
(1) National Fire Protection Association publication “NFPA 780 Standard for the 
Installation of Lightning Protection Systems 1997 Edition”