Lightning at sea is a scary thing. When the lightning bolts are fizzing down around you is probably not the best time to start speculating on the efficacy of your lightning protection measures, so give it some thought before you find yourself in that situation.
Let me say, first of all, that my boat has been struck by lightning, with me and crew aboard, and the impression I got was that there is nothing a mere human can devise to prevent or mitigate a strike – lightning is so all-powerful that it does what it wants to do whatever puny defensive measures we might take. But it’s probably a good idea to try anyway.
In my case no life was lost and the structural integrity of the boat wasn’t compromised. The masthead instruments were vapourised, the alternator controller burst into flames and started a fire in the engine compartment and all electronics were damaged – some reparably, others not.
So what can you do? If your boat has an aluminium mast electrically connected to the keel or other adequate grounding point, it will provide a zone of protection for a radius around its base equal to the height of the mast. This usually covers all of the boat, but some larger boats or those with short masts may have some unprotected areas peeking out of the zone – be aware of this.
People within this zone of protection are almost certainly safe from harm as long as they aren’t touching or standing close to metal components, particularly if that metal is connected to the lightning protection system. The absolutely worst place to be in a lightning storm is at the wheel with one hand on the backstay. The best place to be is below decks. Or at home.
If your boat has all the big bits of metal bonded together and to the boat ground you will have some protection from side flashes which occur when the lightning seeks alternative routes to ground. I wouldn’t personally elect to have the seacocks included in this matrix of interconnected metal bits but it’s not uncommon practice.
To protect electronics they must not only be switched off, they need to be disconnected completely, including from microphones and antennas. If they have plastic or aluminium cases they need to be put in a safe environment such as the oven or a steel box which will protect them from the huge magnetic fields caused by nearby lightning strikes.
There is a lot of useful, and scary, information regarding lightning protection in the American Boat and Yacht Council (ABYC) Standard E-4. I’m sure there are other competent sources as well. Probably a good idea to do some research.