Lightning

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Thomson’s system works by creating, in effect, a Faraday cage around the boat and its occupants. This invention is named after Michael Faraday, who in 1836 discovered that an enclosure of conducting materials shielded its contents from electrical effects and could be used to protect against lightning.

Starting from the top of a fiberglass (or carbon fiber) vessel, Thomson’s system uses one or more air terminals (lightning rods) attached to other system components with heavy-gauge cable. Thomson looks for existing metal structures, such as a Bimini frame and deck rails on a powerboat or the metal bow and stern pulpits or toe rails on sailboats. In the ideal scenario, these structural conductors are as far outboard as possible and make at least one circuit around the outside of the boat. They are then bonded to a series of Siedarcs, electrodes made of copper rod, honed to a point and potted with epoxy in a through-hull fitting, installed at intervals just above the waterline.

Thomson developed the Siedarc after years of forensic examination of lightning-struck boats showed that the area above the waterline was frequently hit by side flashes — high-voltage sparks that cause damage, injury and possibly death. Thomson’s system calls for at least two grounding strips beneath the water rather than a traditional metal plate. He urges that these grounding strips be attached high on the hull, closer to the waterline than the keel, but he thinks these hunks of metal play a less important role than previously believed.

“The idea behind the ground plate has been that it somehow conducts current into the water and then down to China — that’s been the concept,” Thomson says, launching into an analogy. “On a battery you have a plus terminal and a negative terminal. Hook a wire up to the positive and a wire to the negative and put something like a light in the middle. Charges flow from the battery through the light, give it energy and then go back to the battery again. That’s an electric circuit. Lightning, when it hits the boat, is effectively a wire to the charge in the clouds, through the boat to the water. But what’s the return path from the water to the cloud to make a complete electric circuit? It makes a huge difference when you realize that this circuit is not going to China; it’s not even going into the ground. The current goes back into the sky, but the way it gets there is from a charge on the surface of the water.”

Trying to force this massive charge out through a ground plate at the bottom of the vessel is a bad idea because water is a medium that impedes the process, like impedance in an electrical circuit.