Note this article appeared (with photos!) in the Oct 2004 Practical Boat Owner magazine. You can get a back copy by visiting their web site ybw.com

The Strike

In early July 2003, I was motoring my 32ft Eclipse catamaran (see PBO 449) into the Pamlico Sound, a fresh water lake on the East coast of the USA. For several days it had been very hot and humid and there had been regular afternoon thunderstorms. That particular day had started cooler and fresher, but at 2pm the familiar anvil clouds appeared. Up ahead I could see a rain squall and so, as another one was developing to the west, I decided to wait for them both to pass. But as I edged out of the channel prior to anchoring, there was an enormous "crack" right above me, and a strong smell of ozone (close up, lightning doesn't "thunder", it "cracks"). I didn't need the shower of melted masthead fittings falling on me to know that I had been hit by lightning!

Clearly I had a major problem, so I anchored to recover from the shock (pun intended!) and assess the damage. Most obviously, the 4 stroke outboard was now running irregularly and would not restart after I stopped it. As expected I found I had no working instruments, radios or Navtex, while the ST2000 autopilot had a hole blown in the casing and was completely dead. I also found that my main steering compass now had a 30 degree error.

Going inside I saw that most of the circuit breakers on the switch panel had melted. None of the cabin lights worked. I found the filament bulbs had simply blown, while all the halogen lights had exploded (glass was everywhere) and the LED lights had completely vapourised. The main saloon dimmer switch and cockpit PIR security light had also failed. All my digital clocks were flashing error messages, including the ones on my personal organiser and electronic barograph. Investigating further, I found that the CD player and Isotherm fridge had also stopped working. Finally, scorched headlining under the stanchions gave me a clue to the route I think the lightning took - down the cap shroud, across to the lifelines and then out through the rudder stocks. A good thing it wasn't raining and that I wasn't holding onto anything metallic!

Some local fisherman had seen what had happened and motored over to see if I was OK. They said the strike had looked very dramatic, generating huge sparks off the mast head. They hoisted me up the mast, and fortunately I could see no rigging damage, although the wind speed, tricolour light and VHF aerial were all totally destroyed. But at least I was still alive and still had a sailing boat. Luckily it was an easy couple of miles sail into the nearest harbour. But it made me think: I could have been sailing offshore with the autopilot on, and navigating using the radar and chartplotter. Then "bang" and I would have had nothing. Not even a clock or a compass - even Columbus was better equipped than that! That's why I now always carry a sextant and a clockwork clock.

Fortunately the next day I found an outboard mechanic to fix the engine. Once he had replaced the CDI unit it started first time, but a week later the stator failed which meant another expensive repair. Although outboard engines are more at risk from lightning, most modern diesels have electronic control systems, so are also likely to fail after a strike. I learnt that it's also common for apparently unaffected items to fail several days later. For example, it was some time before I realised that my year old batteries were no longer holding a charge and so needed replacing.

I slowly began the expensive business of replacing all the destroyed gear. A portable GPS and hand held VHF for starters, while I found a fishfinder a great echosounder in the shallow murky waters of the US east coast. A new log and wind instruments could wait. The other major item was the fridge, the repair of which cost more than the original unit!

Like most English cruisers, I hadn't given lightning protection much thought before I left the UK, mainly because there are so few thunder storms in northern Europe. Indeed lightning protection isn't even a requirement in the RCD. But that's not the case elsewhere and it's clear that you will be at risk in many popular cruising areas.

Lightning Protection

Since being hit by lightning is a real threat, what can we do to protect ourselves and our boat? Well, first off, there is no such thing as a lightning-proof boat, only a lightning-protected boat. Lightning protection is a hugely complex subject. I have discovered that there's widespread disagreement as to the best practices, so this can only be a brief introduction based on my very limited understanding. To find out more I recommend visiting www.marinelightning.com, the web site of Ewen Thomson who helped me extensively with the preparation of this article. You may also want to look at Seyla Marine Inc's web site, www.strikeshield.com as they supply a number of lightning protection systems.

The essential components for such a protection system are an air terminal, a main conductor and a good "earth." The air terminal is a blunt (not pointed) rod mounted at the masthead and should be about 150mm above any other fitting. Some people claim the so called "dissipators", which look like chimney sweep brushes at the masthead, prevent strikes. But they don't, and thus aren't worth fitting. The conductor should be a copper wire of minimum 6mm (1/4in) diameter (that's thick!), run as straight as possible to the ground and crimped, not soldered, where necessary.

If you have a metal mast, you can use it as the main conductor with a cable from the mast step to ground. A wooden mast, and very definitely a carbon one, should have a copper conductor from the mast head. In fact, even with a big conductor, a carbon mast may not survive a strike. Carbon is a better conductor than epoxy, which means that when struck the mast will heat up unevenly and thus may generate enough stress to blow the mast apart. So just think what will happen to an unprotected carbon hull! The best possible ground is a bare metal hull, but as the epoxy paint forms an insulating barrier, even a steel boat will need to fit a grounding plate.

Despite being an ocean sailor, I was hit when sailing in fresh water. In part that's because, unlike the sea, fresh water is not a good conductor and so lightning strikes in fresh water are far more vicious than at sea. So when fitting a ground plate, reserve it solely for grounding a lightning strike and assume it's for fresh, not salt, water. Thus you will need several square feet of copper sheet, although, partly because lightning dissipates along the edges of a plate, a long narrow strip fitted fore and aft is better than a square one. Unfortunately you can't fair it into the hull because sharp edges work best at dissipating the discharge into the water. Don't use the sintered earth plate from your SSB. It is deliberately made porous and can explode if hit by lightning. Using the keel sounds an attractive idea, but again the paint acts as an insulator. It's also probably unwise to use the propeller as an earth as massive currents could run through the engine if you do. Don't use your seacocks since in some cases lightning has blown them apart.

When fitting your grounding plate(s) err on the side of safety, particularly as one of the other problems (especially if hit in fresh water) is damage caused by sideflashes. These are sparks that form between the lightning protection system and ungrounded conductors or the water. These side flashes are potentially the most hazardous as they often blow holes through the hull.

But as I said earlier, even with a good grounding system, the electronics are still vulnerable. To protect these you can use surge protectors on every circuit, but as you can't sensibly test out the system you have to assume the worst. I think it is safer and more reliable to completely disconnect all your electric circuits, for, as I found to my cost, simply turning them off at the circuit breaker is not enough.

For that reason I no longer have any built-in instruments and my fridge and watermaker have their own plugs and sockets. I can also physically disconnect all the masthead cables, including the tricolour and steaming lights. Fortunately the only proven defence against lightning damaging your electronic equipment is actually very simple. It is to store it all in a "Faraday Cage" which is a fancy name for a metal box. I use the oven and pressure cooker, but you could be really prepared with a well grounded large steel box. I have heard that wrapping electronics in aluminium kitchen foil may also work.

Now when lightning threatens, I put my chart plotter, VHF, radar, camera, computer, etc in the oven. Then I disconnect the engine from the batteries and hand steer if at sea. Having to sail into harbour and navigate "properly" seems a small price to pay if it means saving my electronics and engine.

Finally, unlike you, I have already been hit, so there is one more thing I do when lightning threatens. I hide under the bed clothes. And maybe that's the lasting legacy of my lightning strike. One year on, my boat has been fully repaired, but (like the shell shock victims of WW1) the psychological effects of the worst day of my life still haunt me.

What Causes Lightning

Most lightning strikes occur in the late afternoon when moisture laden air has warmed sufficiently to have formed huge cumulus nimbus clouds, often reaching over 10 miles high. As the moisture rises, it chills and forms ice. These ice crystals rub against each other to create static electricity with the upper portion of the cloud developing a positive electrical charge, while the lower level becomes negative. Furthermore, since opposite charges attract as the cloud moves over the sea, it drags beneath it a concentration of positive charges which "infect" all that it passes over. These positive charges are desperate to meet the negative ones in the cloud and so always concentrate at the highest point available, which in our case is the masthead.

Lightning actually occurs when the differences between the positive and negative charges becomes great enough to overcome the resistance of the insulating air - in fact it's a bit like a giant spark plug. This built up energy may result in a discharge of over 100 million volts and increase local temperatures to 30,000 deg C.

But that's not the end of the story. All sparks create radio signals (indeed Marconi's original radio was a simple spark transmitter), so obviously a lightning strike generates huge signals. And these, together with the static charge accumulation resulting from non bonded conductors, means nothing electrical can survive a strike, certainly not sensitive, low voltage electronics. So, even if you have a good path to ground and can keep the main discharge out of the boat's interior, you will probably still have damage. Even nearby boats that are not hit directly can suffer electronics damage from induced effects.

Finally, don't believe the "lightning never strikes twice" nonsense. In Florida I saw a J24 that had been hit 4 times. We were in a Cheasapeake marina when a large cruising yacht on the next berth was hit. It had been hit in exactly the same spot the year before

What are the Chances of Lightning Striking Your Boat?

Insurance company statistics are one place to look. I contacted Pantaneous UK who are one of the biggest insurers of ocean cruisers. They have 5000 boats on their books. 300 made a major claim last year, of which 14 were due to lightning. But having said that, I only met 2 other English yachts sailing the east coast of the USA in 2003. Both had also been struck by lightning, which is a 100% hit rate.