Boat Shop Know-How With lightning, it’s respect and protect
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With lightning, it’s respect and protect

One thing that sure scares me is lightning.

From the time we’re kids we’re warned about sheltering under a tree during a thunderstorm, since trees are tall and can attract lightning. On the water — whether you’re in a power- or sailboat — you are the tallest thing out there.

Lightning is incredibly powerful, driven by as much as 30 million volts and carrying 100,000 amps. Its temperature can be five times as hot as the surface of the sun. My son-in-law, an engineer supporting the manned space program, has told me of NASA’s efforts to protect against lightning. He said to remember that lightning can go wherever it wants, when it wants. Not an encouraging thought.

1. The safest place to be at sea is below deck in a steel or aluminum vessel, because the electrical charge will travel along the outside to ground (water). I heard of the death of a yacht club commodore many years ago who was transiting the Erie Canal in a steel sailboat when a thunderstorm developed. In order to transit the canal in a sailboat you must drop the mast, and so it was on his boat. He stopped and ordered the crew below, but he came in contact with the mast as he started below. That was when the lightning struck.


2. One suggestion to protect from a lightning strike is to create a Faraday cage around the boat. The idea is to have a well-grounded metal structure with all metallic parts bonded so there is no electrical potential between those parts. This structure surrounds the boat, and though it may encourage a strike, the electrical charge should go to ground. That’s why a metal boat is safe. On other boats, you can bond the mast (usually aluminum) with the engine and other metal objects, including rails and arches, to a low-resistance path to the water through a grounding plate or grounding strip outside the hull. The plate should be at least 1 square foot. On powerboats or long-keeled sailboats, a copper strip about 1 inch by 12 feet along the keel may be better than a grounding plate, because it would provide a greater grounding area.


3. We used to secure a chain to the mast and let it drag in the water until someone suggested that the bond was probably not good enough and that there were better conductors than galvanized steel. We then flattened heavy battery cable, secured it to the mast, and led it over the side to the water. Other times we mounted a lightning rod and bonded it to the metal objects on board and to ground. The idea is that lightning will travel, we hope, along the most direct route to ground, so there should be no sharp turns in the system. With everything on board bonded, it’s at the same potential and, therefore, “invisible” to the lightning, so there is nothing to discourage it from traveling direct to ground.

4. Another theory we put into practice was to secure copper “bottle brushes” to each of our carbon fiber masts to help disperse the buildup of static electricity, which could attract a leader strike. Our boats have always been bonded; however, I’ve been told by some “experts” that not bonding the boat is, in fact, safer because there is no low-resistance path to the water to attract lightning. Their theory holds that with such a great area of ground (water) surrounding your boat, the boat is an almost infinitely small target. I don’t know which theory is correct, but I know the conductor you use should be as heavy gauge as you can handle. I also know that lightning scares the hell out of me.

5. I have seen two boats that have been struck by lightning. One was a wooden schooner, the other a fiberglass sloop. The schooner had a huge hole blasted from its bilge; the sloop was a sieve along its waterline. No one was aboard either boat, and both, of course, had sunk.


6. As I said, lightning scares me, and it should frighten you, too. You can’t control lightning, but you can keep a weather eye out for conditions that can spawn it and, hopefully, avoid it. I hate to be discouraging, but lightning can strike when there are no storm clouds in view. Still, the water around is big, and you are small. Being a bit of a fatalist, I believe that when “Big Six” calls, you go.

7. In the meantime, enjoy the summer.

 

This article originally appeared in the July 2009 issue.

 


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