Lightning and Sailboats

Clark November 22nd, 2017

You can read many authoritative treatises about boats and lightning, and they’re all full of crap. There is only one thing we know for sure about lightning: It is unpredictable.

An acquaintance recently introduced me to a fellow sailor, saying, “You got run over by a container ship. He got struck by lightning.”

I reflexively asked, “Did it zap all your electronics?”

“No, it blew a 2 x 3 foot hole out the side of the boat and she sank in twenty seconds.”

He was sailing solo in the vicinity of Drakes Bay, Costa Rica, also very close to Isla del Caño, one of the most lightning-struck places on earth. His boat was a cold molded 60-footer, with a nearly 100-foot aluminum mast. Being a conscientious guy, he did what the books say and connected the base of his aluminum mast to his external lead keel with some large gauge cable.

Did the lightning do what the books say and politely conduct itself down the mast, through the cable, and out the keel? Nay, it jumped from the base of the mast to a monel water tank that was 2 x 3 feet in cross-section.

He heard a kerblong, the noise the water tank makes when it’s full, as the lightning struck. When he went down the companionway to investigate the water was already four feet deep, and his bilge was four feet deep, meaning the water was actually eight feet deep. He grabbed his ditch bag, then went for the life raft, which was in the forepeak, but the water was already so high that it triggered the hydrostatic release on the life raft, inflating it, and wedging it hopelessly in the forepeak. If there is a lesson to be learned from his story, it is to keep life rafts with hydrostatic releases on deck.

In the end he escaped with his ditch bag, some water, the clothes on his back, and his surfboard, which he paddled through the night, and into the next day, before reaching shore. Among the wreckage was a piece of 2 x 3 foot cold molded wood, confirming the whole lightning blew the monel water tank out the side of the boat hypothesis. (I might hypothesize a bit more, that the lightning violently boiled the water in the tank, and the resulting steam blew the tank apart?) A day or two of walking and paddling down the shoreline, a remote vacation house, a chicken bus to the nearest town, a few phone calls, and he was soon on a plane back to the US. The boat wasn’t insured.

His is the worst sailboat-lightning story I’ve ever heard, but I’m sure there are some that are never told.

While I was in Costa Rica I knew eleven boats that got struck, ranging from minor damage to total destruction of all things electrical and electronic.

Once I watched as lightning struck the boat anchored right next to me. It vaporized the tri-color light on the top of the mast, traveled down one of the outer stays, and jumped from the stay to the lifeline. Where it jumped from the stay it left a puddle, as they say in welding parlance, a puddle just big enough to take out two of the wires in the 1 x 19 wire rope. My dad flew down a week or two later, and brought them their new tricolor and enough wire rope to replace the stay. They were lucky, but I was even luckier, because I was just a hundred feet away and didn’t get hit.

It looked something like this:

On another boat the strike found its way to the anchor chain and didn’t do any apparent damage, but the lightning strike electropolished the anchor chain and anchor to a mirror finish. I could see my reflection in the fluke of the anchor, which was galvanized but looked like shiny stainless.

On the boat that had a total loss of all things electronic and electrical they just did a quick and dirty replacement of the engine battery, starter, and alternator, then bought a handheld GPS and VHF to get them across the Pacific. Total electrical replacement took place later in New Zealand.

At that point I’d had the bejesus scared out of me by lightning, read the books, connected my aluminum mast to my external lead keel with some large gauge cable, and got one of the bottle brush static dissipators for the top of the mast. The so-called lightning experts often scoff at these dissipators (”You can’t dissipate all the ions in the whole ocean off the top of your mast!”) but none of the eleven boats struck had dissipators, and none of the boats with dissipators were ever struck. Coincidence? Maybe-probably.

After my mad scramble to “lightning-proof” my boat, I never saw lightning like in Central America again. I was in the break of the monsoon in Southeast Asia, spent months in Southern India, a whole circumnavigation with lots of lighning, but nothing ever compared to Central America where, well, eleven boats got struck that I knew. There were probably many more I never heard about, meaning it’s a significant danger down there.

So what shall we do? Everyone agrees that connecting the mast to the keel is a good idea, or somehow otherwise providing a low resistance path for lightning to get from your mast to the sea. And this lightning ground should be connected to the bonding system, so that the charge can be conducted, rather than jump, to various thru-hulls, and hull appurtenances.

A common misconception is that lightning, and electricity in general, will take the path of least resistance. Wrong. Electricity will TAKE ALL AVAILABLE PATHS. This is a good thing to keep in mind with grounding systems and all manner of wiring aboard. So even if we provide lightning with a low resistance path to ground, will it take it? Maybe. Will it take it, and also take other routes? Probably, and we’re talking about lightning here, a phenomenon not generally described in harmless or insignificant quantities.

I was going to close with a flippant remark about the “cone of protection,” the 45-degree zone below your mast, which you always read about in discussions about boats and lightning. I was going to suggest that during an electrical storm you stand in the cone with a golf club over your head while I cower in the quarter berth, but the National Lightning Safety Institute beats me to the punch with their dispelling of the Cone of Protection Myth.

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1 Comment »

Comment by Richard Cassano
2017-11-23 08:22:43

A few years ago I watched a TV interview with a scientist in the mid-west who was researching lightening on a government grant. The interviewer asked the scientist if he has learned anything new or has any guidelines for lightening. Scientist replied, “not at all, there are no rules we can make regarding lightening”. So much for my wire brush on the top of the mast, however, I was hit with a side strike before putting it up and never hit again after installing it. The boat on the mooring next to me in Oyster Bay took the full hit.

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