Lightning never strikes twice – a turn of phrase often used to reassure someone who’s had a bad experience, implying that it won’t happen to them again.

However, as unlikely as it statistically may be that lightning will strike a specific object, the odds are exactly the same for it to strike the same object at some other time; both events being unconnected. This same principle applies to a lottery.

The odds of a specific number coming up are always the same, irrespective of whether it has come up before. So, it is just as likely that lightning will strike an object a second time as it was in the first instance.

• How to make a grounding system to keep yourself safe at sea
• Lightning strikes – advice from a marine surveyor

What is lightning?

In simple terms, lightning is an electrostatic discharge between two atmospheric parts or the atmosphere and the ground. We have lightning within a cloud and cloud-to-ground lightning. This charge imbalance can be the result of the rapid movement of air within a cloud and can exceed 1,000 volts per metre. If you then add moisture to the mixture, the cloud itself, fog or rain, this can facilitate the equalisation of this imbalance. This is lightning.

When lightning occurs, the path it travels along becomes incredibly hot. We have all experienced electrical wires feeling warm or even hot to the touch, because we had plugged too many devices in to the other end. The voltages here are in the hundreds and the current, measured in amperes or amps, and well below 100. The resulting watts (volts times amps) are thus in the low thousands; think of something along the lines of an electric heater.

With lightning the voltage differential can be in the hundreds of thousands and the current can range from 18,000A to 140,000A. The result of this are temperatures along the path of the lightning reaching 30,000°C. This heat creates a plasma that glows brighter
than the sun. The explosive nature of these occurrences is also the direct cause of thunder.

Lightning prevention theory

Ionic emission is nature’s way of neutralising a highly charged area, be it cloud, an object, or the ground surface area. In order for nature to neutralise a highly charged area (via lightning), there have to be three conditions present: a generally negatively charged thunder cloud, a generally positively charged surface area underneath it, and a path between the two charges.

The lightning rod, which uses the point discharge (corona effect) to attract the stepped leader of a thunder cloud, is constantly dissipating ions into the atmosphere. By multiplying the number of discharge points thousands of times, dissipators were developed to gather the static build up or electrical charge on an object and rapidly dissipate the charge into the atmosphere. The wind and circulation of air particles theoretically blow these accumulated ions into the atmosphere thereby neutralising the charge of the object. The ground charge therefore is never supposed to reach a high enough value to be attractive to a lightning strike.

Dissipators

Each yacht we’ve owned has come with a charge dissipator at the top of the mast. They look a little like a toilet brush. I’d always wondered what good they might do. They’re meant to lower the likelihood of a direct lightning strike by reducing the build-up of static ground charge and retarding the formation of ion streamers.

They’re also said to disperse ions (charged particles) to prevent lightning strikes, rather than waiting until one hits and dissipating through grounding which can still fry electronics and cause other severe damage.

The truth of the matter is that this may be the case in very low voltage situations in a laboratory but is by no means valid in a real-world scenario.

Bright flash overhead

One time we were sailing along at night in quite dense fog in a previous boat. We could hear thunder all around us and occasionally saw flashes of bright light to one side or the other. Then there was a bright flash immediately overhead with simultaneous thunder. We looked around and everything seemed OK. Little did we know, but nothing could have been further from the truth.

The first thing we noticed was that our running lights were not working. We went below and found that the cabin lights and instruments were all dead. We had no power. When the wind then died, we couldn’t start our engine, so we anchored. We had a small lamp aboard that had a candle inside and a Fresnel-style lens. It gave off a remarkably bright light, so we hung this aloft as our anchor light.

In the morning we called for help with our handheld VHF radio and were soon towed into the next harbour. Later that day a local mechanic determined that our house and engine batteries were dead. We’d been struck by lightning, despite the dissipator brush on our mast top. With the batteries replaced, we were under way with miraculously nothing else amiss.

After that experience, which we were told was the result of a static electricity charge, we acquired a steel cable with a big lump of zinc at one end. We attached this to one of our shrouds while our boat was at anchor or on her mooring, lowering the zinc into the sea. Roughly a year later, we went for a sail the day after a thunderstorm. Before we set off, we pulled the wire with the zinc out of the water, the zinc was gone. The steel wire had melted through where it had entered the water. Yet again lightning had struck in spite of the dissipator.

Keel damage to the Blackwells’ yacht caused by lightning

It was time to seek professional advice. The consensus was that our mast wasn’t grounded to our lead keel. But as it turned out, it was, and well done at that, so no worries there. We picked up a new wire with a zinc and all was fine for another year or two. One fine sunny Saturday morning after a night of torrential rain we arrived at our sailing club to a scene of great excitement. The harbour staff had just phoned in a report of a huge lightning strike that had shaken the hut on the pier and terrified its occupants. The target once again was our boat, despite being moored in the middle of a large number of yachts, some with much taller rigs.

We piled into a launch and motored out to our boat, which was swinging peacefully on her mooring. Aboard, we found chaos. All our electronics were fried – their innards melted. Radio, radar, chartplotter, instruments, all gone – as were the handheld devices. Much of the shipboard wiring had also melted.

Batteries and engine were amazingly unscathed. Thankfully, there was no water in the bilge, so we shrugged it all off, called our insurance company and went sailing. That evening, with the water being nice and warm, I decided to check the hull for fouling – something I did with a mask and snorkel a few times each season. Diving down to the bottom of our keel I ran my hand from forward to aft and as I reached the aft end there was suddenly nothing. My hand went right up into the keel. The trailing edge was just two thin, loose flaps of fibreglass with nothing inside. The lightning had passed through the keel and blown the fairing right out of the bottom!

We removed the so-called dissipator but will never be able to say that this was the reason we were spared after that. As we said at the outset, the chances of a direct hit are infinitesimally small – much like a lotto win. Needless to say, when we moved on to our next boat the dissipator came off the first time I climbed the mast.

Top tips to prevent lightning strikes onboard

Head for safety

When storm clouds gather, head for shore. The worst place to be is on the open water where your boat is the tallest lightning rod around. Drop anchor in shelter if you can.

Beware of metal

The cockpit is one of the most dangerous places in a yacht because of its metal parts. If you have one, engage the autopilot rather than steering by hand. During a lightning strike, high voltages could zap a skipper if, for example, he or she has one hand on a metal steering wheel and the other on the metal engine controls or the lifelines.

Avoid the mast down below

When below, stay away from the mast-to-keel area. That’s the primary route of the lightning seeking a place to exit.

Stay away from the water

Avoid any connection between yourself and the water. Your body is a better conductor than air, so lightning will think your body is the easier route making you a human lightning rod. If you enter the water, electrocution is highly probable if lightning strikes nearby.

Go below

If you do see lightning too close for comfort, go below. That gets you away from the metal in the cockpit. You also don’t want to be at the mast during the lightning storm. The places on deck to avoid most are directly beneath the mast or the boom.

Try a little radio

An old boater’s trick is to turn on an AM (not FM) radio to listen for static. Small, cheap portable battery powered radios are best. The louder the static the closer the storm. It will also indicate if there’s an electrical charge building around your boat.

Drop the sails

Get sails down early and furled to avoid the sudden, intense winds at the leading edge of a squall, which will be associated with strong winds and waves, heavy rain, and even hail.

Don’t get in the tender

Being on a large yacht with a mast is much safer than being in a small unprotected yacht, so choose to stay on the bigger boat rather than hightailing it to land in the dinghy.

Get your boat inspected

If your boat’s been hit by lightning have it hauled out for inspection immediately. Many insurers cover a short-haul, and some do not charge a deductible against this coverage. Seeing where the lightning came out is critical and can help avoid a catastrophic failure down the line.

Why not subscribe today?

This feature appeared in Practical Boat Owner magazine. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a subscription to Britain’s best-selling boating magazine.

Subscribe, or make a gift for someone else, and you’ll always save at least 30% compared to newsstand prices.

See the latest PBO subscription deals on magazinesdirect.com