To bring a direct lightning strike down through the boat from the air
terminal requires a robust conductor or conductors leading vertically
in a straight path to ground.
Copper is the preferred metal, with a wire gauge of No. 6 or even
better, #4 AWG stranded copper.
An aluminium mast can be the conductor provided that there is a direct
connection to the air terminal and a substantial wire connection to the
grounding plate at the bottom.
A wooden mast cannot be considered as a conductor however, metal
shrouds and stays could be used as the down conductors providing that
there is a continuous connection down to the chainplates and these are
attached to the grounding electrode.
However, stainless steel is not an especially good conductor.
And on a traditionally rigged vessel the shrouds are likely to have
‘parcelling’, ‘serving’, deadeyes and lanyards which effectively act as
Alternatively a metal sail track could be considered as the conductor
but again it would need be electrically connected to the air terminal
and to the grounding plate.
Generally with a wooden mast there should be a separate conductor,
possibly a #4 AWG or larger tinned, copper, wire running down the mast.
All conductors and connections should be kept as straight as possible.
So we now have an air terminal or terminals connected to the grounding
wire, this now should then be connected to a ground plate or terminal
to complete the lightning protection circuit.
grounding plate is
considered to be the "exit terminal" of the
lightning protection system.
The grounding plate or grounding strip is installed externally on the
hull so that it is immersed in the water.
And it should be connected directly to the down conductor with as
little bend or deflection as possible.
This "exit terminal" should according to the ‘experts’ be of copper,
copper alloy, stainless steel, or aluminium with an area of at least
300 square mm.
Ideally it should be a strip with sharp corners or points rather than a
smooth or round edged plate and it should never be antifouled, painted,
caulked or faired into the hull.
And the grounding ‘strip’ should run from directly below the lightning
protection conductor or mast, toward the aft end of the boat.
All of which would seem to rule out the suggestion that the exit
terminal could be an external ballast keel connected via a keel bolt.
It has also been suggested that the grounding plate should be augmented
by the use of perimeter "grounding terminals", long strips at the water
This is the contention of another ‘expert’ who believes that lightning
prefers to go to the surface of the water rather than down into it via
the ground plate.
On wooden boats it is also recommended that an equalization “bus bar”
be installed as part of the lightning protection, on the inside of the
hull parallel to the exterior
grounding strip and electrically connected to it at each end.
So if having all that spiky metal on the outside of the hull isn’t
enough you’re gona need even more inside.
This “bus” is there to connect to the boat's engine and any other large
metal objects above and below decks and those perimeter “grounding
terminals” to the Lightning Protection system to protect against side
So what are these side flashes that require all this bonding and
auxiliary grounding electrodes in the lightning protection system?
Apparently there are two types,
internal side flashes that leap between any two metal
or inside the boat
and external side flashes that arc directly between an
conductor and the water.
They are more likely to occur when the path to ‘ground’ is insufficient
to carry the charge.
This can create a chain of side flashes from one metal item to the next
as the lightening tries to find its way to ground.
It would also seem (according to one set of statistics) that
side-flashes are more likely to occur on a boat in fresh water that one
in salt water regardless of the lightning protection fitted.
To minimize side flashes, all large metal objects such as tanks,
engines and even such things as winches, seacocks and stoves should be
connected to the lightening protection system by copper connections of
least #6 AWG.
The larger items such as the engine block are better being connected
directly to the grounding plate or the lightening protection system’s
None of these connections should be routed alongside the boat's wiring
as this can lead to high voltages being inducted into the boat's wiring.
However, the negative terminal of the DC electrical system should be
bonded to the lightning protection bus.
I am perfectly aware that the majority of Wooden Boat aficionados are sensible folk. However, I need to point out that I am an amateur wooden boat enthusiast simply writing in order to try to help other amateur wooden boat enthusiasts. And while I take every care to ensure that the information in DIY Wood Boat.com is correct, anyone acting on the information on this website does so at their own risk.