Lightning protection

Thunder and lightning season approaches – are you ready for it?

The Met Office tells us: Thunderstorms develop when the atmosphere is unstable – when warm air exists underneath much colder air. As the warm air rises it cools and condenses forming small droplets of water vapour. If there is enough instability in the air, the updraft of warm air is rapid and the water vapour will quickly form a cumulonimbus clouds. Typically, these cumulonimbus clouds can form in under an hour.
As the warm air continues to rise, the water droplets combine to create larger droplets which freeze to form ice crystals. As result of circulating air in the clouds, water freezes on the surface of the droplet or crystal. Eventually the droplets become too heavy to be supported by the updraughts of air and they fall as hail.
As hail moves within the cloud it picks up a negative charge by rubbing against smaller positively charged ice crystals. A negative charge forms at the base of the cloud where the hail collects, while the lighter ice crystals remain near the top of the cloud and create a positive charge.
The negative charge is attracted to the Earth’s surface and other clouds and objects and when the attraction becomes too strong, the positive and negative charges come together, or discharge, to balance the difference in a flash of lightning (sometimes known as a lightning strike or lightning bolt). The rapid expansion and heating of air caused by lightning produces the accompanying loud clap of thunder.

Lightning at sea is a scary and, occasionally, dangerous 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.

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 repairably, 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.

If your boat has all the big bits of metal bonded together and connected to the boat’s earth you’ll 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. It’s probably a good idea to do some research before we get much further into lightning season.