Dear cruising friends.
Being an
Electrical Engineer, with first hand experience of
lightning strikes aboard my 43'
Privilege catamaran, I am happy to share some ideas in the matter of
lightning.
Assuming your
mast is aluminium, it is in itself a very efficient lightning rod towering 10 to 30 meters above your ship and the sea surface.
It is actually too efficient as it will attract lightning in a several hunded meters radius around the
boat. Were it not for the mast, lightning might have struck the sea nearby with little harm.
( If the lighning strike is close enough, it may still kill your
VHF spliter by induction in the mast cables)
Assuming your shrouds are
steel, ( i.e.: not non conductive Dynema)
One of your chain plates (of top
shroud or of back stay, one is enough) should be connected with a heavy gage copper wire or copper band to a bolt of the iron
keel (in case of cast iron
keel on a mono hull).
In the case of a
fiberglass encapsulated lead keel isolated from the sea,
or in the case of a
catamaran where "keels" are non conductive
fiberglass fins, the copper wire or copper band should be connected to a 1 square foot metal (copper) "sea-plate" attached outside the
hull well below the waterline so that it remains submerged when the
boat heels or the sea is rough.
(This is proprerly done on
Privilege catamarans)
When ligning strikes your mast (as it happened to my catamaran), the kilo-amps will flow to the sea, mostly via the iron keel or sea-plate)
I say mostly, because, depending on the energy of the lighning strike,
the kilo-amps will find additional ways to reach the sea through the
engine and sail drives (killing the starter
motor and alternator), the regrigerator's
keel cooler (killing the refrigerator's compressor), or worse through the
steel armature of the plastic pipe between the
bronze through
hull and the engine's colling
water pump.
In this worst case scenario, the steel armature is vaporised and the pipe severed, leaving one (or several) disconnected throughhulls open to flooding, beyond the capacity of
bilge pumps ( which are probably dead after lighning anyway).
Without a a
chain plate connected to a keel bolt or sea-plate, the kilo-volts and kilo-amps will find their own way to the sea : Expect the worse: hull punctured + fire aboard.
So priorities after the strike are ;
1- Is the crew safe, ( no cardiac massage required )
2- Is there fire aboard (that schould be easily ascertained)
3 -Are we sinking :
Lift all floor board and investigate the bilges and
engine hold seriously)
Having found no one hurt, no fire and no sinking... you are not done yet
At the mast top you have a
wind sensor, a
VHF antenna and
navlights, all three duly connected by
electric cables down the mast into the boat.
They will all be reduced to charcoal by lightning.
Through these 3
cables the kilovolts (50 Kv, 100 Kv..) descend into the boat on a rampage, toasting all your
electronics, (AIS, VHF,
Chartplotter,
Radar Auto pilot's computer,
inverter,
generator, randomly shorting your wire looms, killing
bilge pumps, starters,
electric winches..etc..)
In the
cabin we found shards of glass on the
bed sheets .. from exploded light bulbs in the ceiling !
We had 30 000 $ dammages.
"Luckily", an
Ipad fitted with
Navionics was spared as it was not connected for
charging . It afforded us navigating means as we sailed on "by hand" toward
Tahiti.
The only possible defense against the Kilo Volts rampage is to fully isolate the
boats electric
network from the cables in the mast and the mast itself.
In all production
boats, the connectors are hardly accessible, and will not be disconnected if and when a chance of lightning is perceived.
To encourage zelous disconnection of mast cables, all cables coming from the mast should be led to an easily accessible "switchboard" where all cables can be disconnected with minimum 1 inch air gaps with simple gestures.
Yet there is no garanty this will suffice. Think of the KiloVolts going down the forestay, to the pulpit, the navlight in the pulpit, the 12 v cable of the navlight.... .
And when you feel safe , say back in some marina, and take stock of dammages with the help of some professional electrician , you are still not done yet :
In your home ashore, 220 V or 110 volts will immediatly turn an
electrical fault into a shortcircuit tripping a breaker.
Not so aboard your 12 Volts ship! The fault will simmer and burn slowly, and 2 weeks after the strike a pump'
motor which still worked fine after the strike will be reduced to charcoal, the starter motors will die , etc....
We were voyaging in the Pacific when lightning struck. Procuring the spare
equipment and repairing our catamaran took 3 months.
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