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Old 06-13-2001
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Lightning Worries

In the Tip of the Day section on your website there was a suggestion that to avoid getting hit by lightning, you should connect a short length of chain to the chain plates and hang it overboard into the water. This is something that has been of great concern to me; do you have any scientific or other empirical evidence that this works?

Mark Matthews responds:
Thanks for the question. There's no body of scientific knowledge to support or disprove it that particular technique. Lightning is the result of the difference, or potential, between positive and negative electrical charges, and these differences can be measured in hundreds of thousands of volts. The positive charges are usually higher in a cloud formation while the negative charges are lower, resulting in electrical current movements within a cloud, between clouds, or from cloud to ground. There is also evidence that the potential can be from ground to cloud, meaning that the positive charges are accumulating on objects on the ground below the cloud. What happens is that the negative charges seek their positive counterparts. Because air is a poor conductor of electricity, this buildup of potential searches for a conductive path between the positive and negative charges until this energy is released in a sudden flow as lightning.

It may provide some solace to recognize that lightning is not a predictable event. Lightning seems to strike taller objects, over shorter ones, but this isn’t always the case as the Cal 22 owner a couple of docks down from me found out. Lightning searches for a conductive path to ground. Berthed in a marina in which his mast was dwarfed by the larger boats around his, the strike hit his boat and then proceeded to fry the shorepower hook up, which for whatever reason in that case offered the most immediate ground.

If you find yourself underway in a lightning storm, don’t touch metal items, especially two different metal items. Try to keep inside, don’t dangle arms or legs overboard, and unplug all electronics you’d like to use in the future.

There are several theories of how to best prevent lightning strikes. And the operative word here is theoretical. Lightning rods are one technique. Basically the idea is the lightning rod is the highest point on the boat, and connected to the boat’s grounding system, allows a strike to ‘safely’ pass to the ground. This may or may not be the case. As I mentioned before, lightning is notoriously unpredictable, capable of bouncing around, and will not always strike the highest point.

You might want to have a look at Nigel Calder's Boatowners Electrical and Mechanical Manual by Nigel Calder (which is available in the SailNet Store) as he has very informative chapter on lightning protection and wiring prevention. Calder cites an incident where a ketch-rigged boat had the mizzenmast struck and then the charge travelled the length of the anchor chain which then crumbled allowing the boat to drag onto the beach. There's no easy explanation for this behavior. 

So, connecting chain to the chainplate and hanging it overboard will offer a lower-resistance path to the ground, but there's no guarantee that the strike won't blast down another shroud and melt a hole in the boat. Generally speaking, by not offering a path to the ground, you invite lightning to make its own path, with far more damaging consequences.

One other alternative is lightning dissipators, which are sold as a means to prevent lightning strikes in the first place. The theory is that the hundreds of points (they look like a metal brush mounted on the top of the mast) prevent a static electrical charge from building up. For more information, I’d refer you to a couple articles on our site. Check out Sue and Larry’s Lightning Protection 101 and their Zone of Protection. Kathy Barron’s Lightning Strike and our buyers’ guide on Lightning Protection, which profiles some safety products on the market, can also be helpful. 

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