Lightning Protection 101 - SailNet Community
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Lightning Protection 101

The pizza man finally came walking down the dock carrying our large pepperoni and mushroom.

"Hey!" He said.

"Hey." I responded, glad my pizza was finally here and wishing I'd called one of those "30 minutes or it's free" places.

"Nice night." The pizza man added. "Last time I delivered a pizza here, it was so rough I felt sea sick just walking down the dock."

"Hmff!" I mumbled, anxious to just pay and get my pizza.

"Boy, a boat like this you could sail to Australia!" Quipped the pizza man, as my dinner visibly got colder.

"You sure could." I replied weakly, thinking as long as you didn't starve to death first.

"Man, that sure is a tall mast. What do you do in a lightning storm?" He asked with genuine interest.

"Well, that's a good question." I answered. Resigned now to cold pizza, I added, "Sometimes even the experts don't agree, but this is what we've been able to gather".

Lightning strikes have always been a concern to boaters (and apparently now pizza deliverymen), but before we can learn to deal with it, we need to understand a little bit about its complicity.

When clouds containing heat and moisture pass over water during a thunderstorm, negative electrons are stripped from the clouds leaving them with a positive charge. The surface of the water beneath the clouds then acquires a negative charge to counter the positive charge above. Unfortunately for us sailors, these charges tend to gather on the highest objects, like masts and rigging. As these charges build and become stronger, the potential for a lightning strike increases. So, what can we do to protect ourselves?

The mainstream approach for dealing with lightning centers on two ideas. First, install the best lightning ground possible throughout your boat to properly handle a strike if it occurs, then try and prevent a strike altogether through the use of a lightning rod or an ion dissipater.

When a lightning strike occurs, the lightning seeks a path of least resistance. A mast and rig that's properly grounded allows this potentially dangerous bolt to follow a pre-determined "safe and direct" route through your boat to a point outside of your hull. (Either an external keel or a grounding plate affixed to the hull.) The key is to keep the lightning moving in a straight line toward your ground and away from crew members. This path needs to be established from the top of your mast to your grounding plate or keel. Since lightning sometimes takes parallel paths, all of your shrouds and stays should be grounded as well. ABYC recommendations call for #4 wire minimum from your mast, and #6 wire from the each shroud. If your mast is aluminum, you can start the wire at its base, since the mast itself is a great conductor. Lightning doesn't like to turn corners, so keep your runs as straight as possible. If you must curve your grounding wires, use as wide of an arc as possible. Tie in your engine and other large pieces of metal within six feet of your path.

If you fail to provide this "safe and direct" path to ground, lightning may find its own route, often without regard for human life. This may be through crew members grabbing a shroud, or a helmsman at the wheel, either of which could prove fatal. Through-hulls have been known to explode, and holes have literally been blown in hulls that lacked proper grounding. Sometimes even vessels with properly grounded rigs get struck and damaged, but the statistics strongly favor grounding.

Lightning rods are sometimes added to the top of a properly grounded boat. The idea is that the rod itself begins the "safe and direct" path to ground. Furthermore, the tip of the rod is pointed, so that the building charge of electrons will be gradually bled off from the mast head and neutralize the positive charges above. If this "bleeding off" of electrons occurs in adequate amounts, an impending strike could be prevented altogether. If not, a lightning rod with a proper ground should still reduce the severity of and damage from a strike, if it occurs.

An ion dissipator is another way to lessen the charges that may build at the top of your mast. A dissipator is placed at your masthead, and allows a small amount of the charges to bleed off, reportedly as fast as they occur. As opposed to lightning rods which taper to a single tip, dissipators have many points from which to "bleed off" the electrons and dissipate the charge. Like lightning rods, though, you still must provide proper connections to ground for dissipators to function properly. If you sustain a direct hit with a dissipator and proper ground in place, the dissipator should act as a lightning rod, and safely channel the strike to your ground.

"So basically" I summarized to my new friend, the pizza man. "In our case, we already have a lightning rod that was installed on Serengeti, but I think I like the idea of the dissipator better, so we plan to replace it with one. We have a ground system installed, but it appears to be undersized, so we'll make some changes and bring the wiring up to spec, and we carry a lucky rabbit's foot, just in case."

By this time the pizza guy had heard a little more than he had bargained for. We settled up the tab, and as he started back down the dock, he stopped, turned to me and said, "It's pretty complicated, isn't it? I think I'm glad you've got the lucky rabbit's foot."

The Cone of Protection

"Don't worry, we're within the cone of protection," we'd say to one another years ago while racing in our J-22. We really didn't know what we were talking about as we continued the downwind leg during the thunderstorm, but we felt better thinking there was some magical cone that would protect us from a lightning strike.

Years later, I now know this theoretical cone is known technically as the "Faraday Cage."

This "Faraday Cage", or cone of protection, is a concept that has comforted sailors for years.

Although providing no guarantee, the cone of protection theory asserts that a well-grounded lightning rod or mast will provide a zone of safety about its base with a radius equal to the height of the mast. For a sailboat, this would create a secure zone over the entire boat under most circumstances.

It's important to remember the "Faraday Cage," although widely accepted, is a theory not an absolute. Lightning can and does take parallel paths, jumps and strikes where it pleases, and still remains highly unpredictable as well as a heated topic of controversy, even among the experts. Crew members should minimize time on deck, and avoid bridging any metal objects (mast,stays,wheel,stove,radio,etc.) during a storm. Also, it's prudent to don rubber boots.

Sue & Larry is offline  
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